RCU Archive on lore.kernel.org
 help / color / Atom feed
* [PATCH v3] selinux: cache the SID -> context string translation
@ 2019-11-08  8:39 Ondrej Mosnacek
  2019-11-08 14:09 ` Stephen Smalley
  2019-11-10 21:03 ` Paul E. McKenney
  0 siblings, 2 replies; 4+ messages in thread
From: Ondrej Mosnacek @ 2019-11-08  8:39 UTC (permalink / raw)
  To: selinux, Paul Moore
  Cc: Stephen Smalley, rcu, Paul E. McKenney, Michal Sekletar

Translating a context struct to string can be quite slow, especially if
the context has a lot of category bits set. This can cause quite
noticeable performance impact in situations where the translation needs
to be done repeatedly. A common example is a UNIX datagram socket with
the SO_PASSSEC option enabled, which is used e.g. by systemd-journald
when receiving log messages via datagram socket. This scenario can be
reproduced with:

    cat /dev/urandom | base64 | logger &
    timeout 30s perf record -p $(pidof systemd-journald) -a -g
    kill %1
    perf report -g none --pretty raw | grep security_secid_to_secctx

Before the caching introduced by this patch, computing the context
string (security_secid_to_secctx() function) takes up ~65% of
systemd-journald's CPU time (assuming a context with 1024 categories
set and Fedora x86_64 release kernel configs). After this patch
(assuming near-perfect cache hit ratio) this overhead is reduced to just
~2%.

This patch addresses the issue by caching a certain number (compile-time
configurable) of recently used context strings to speed up repeated
translations of the same context, while using only a small amount of
memory.

The cache is integrated into the existing sidtab table by adding a field
to each entry, which when not NULL contains an RCU-protected pointer to
a cache entry containing the cached string. The cache entries are kept
in a linked list sorted according to how recently they were used. On a
cache miss when the cache is full, the least recently used entry is
removed to make space for the new entry.

The patch migrates security_sid_to_context_core() to use the cache (also
a few other functions where it was possible without too much fuss, but
these mostly use the translation for logging in case of error, which is
rare).

Link: https://bugzilla.redhat.com/show_bug.cgi?id=1733259
Cc: Michal Sekletar <msekleta@redhat.com>
Signed-off-by: Ondrej Mosnacek <omosnace@redhat.com>
---

Changes in v3:
 - add rcu@vger.kernel.org and Paul McKenney to Cc for review of the RCU
   logic
 - add __rcu annotation to the cache entry pointer (sidtab.c now passes
   sparse checks with C=1)

Changes in v2:
 - skip sidtab_sid2str_put() when in non-task context to prevent
   deadlock while avoiding the need to lock the spinlock with
   irqsave/-restore (which is slower)

 security/selinux/Kconfig       |  11 ++
 security/selinux/ss/services.c | 138 +++++++++++++++----------
 security/selinux/ss/sidtab.c   | 179 +++++++++++++++++++++++++++------
 security/selinux/ss/sidtab.h   |  58 +++++++++--
 4 files changed, 294 insertions(+), 92 deletions(-)

diff --git a/security/selinux/Kconfig b/security/selinux/Kconfig
index 5711689deb6a..35fe8878cf1c 100644
--- a/security/selinux/Kconfig
+++ b/security/selinux/Kconfig
@@ -85,3 +85,14 @@ config SECURITY_SELINUX_CHECKREQPROT_VALUE
 	  via /selinux/checkreqprot if authorized by policy.
 
 	  If you are unsure how to answer this question, answer 0.
+
+config SECURITY_SELINUX_SID2STR_CACHE_SIZE
+	int "NSA SELinux SID to context string translation cache size"
+	depends on SECURITY_SELINUX
+	default 256
+	help
+	  This option defines the size of the internal SID -> context string
+	  cache, which improves the performance of context to string
+	  conversion.  Setting this option to 0 disables the cache completely.
+
+	  If unsure, keep the default value.
diff --git a/security/selinux/ss/services.c b/security/selinux/ss/services.c
index 3a29e7c24ba9..b6dda5261166 100644
--- a/security/selinux/ss/services.c
+++ b/security/selinux/ss/services.c
@@ -91,6 +91,12 @@ static int context_struct_to_string(struct policydb *policydb,
 				    char **scontext,
 				    u32 *scontext_len);
 
+static int sidtab_entry_to_string(struct policydb *policydb,
+				  struct sidtab *sidtab,
+				  struct sidtab_entry *entry,
+				  char **scontext,
+				  u32 *scontext_len);
+
 static void context_struct_compute_av(struct policydb *policydb,
 				      struct context *scontext,
 				      struct context *tcontext,
@@ -716,20 +722,21 @@ static void context_struct_compute_av(struct policydb *policydb,
 }
 
 static int security_validtrans_handle_fail(struct selinux_state *state,
-					   struct context *ocontext,
-					   struct context *ncontext,
-					   struct context *tcontext,
+					   struct sidtab_entry *oentry,
+					   struct sidtab_entry *nentry,
+					   struct sidtab_entry *tentry,
 					   u16 tclass)
 {
 	struct policydb *p = &state->ss->policydb;
+	struct sidtab *sidtab = state->ss->sidtab;
 	char *o = NULL, *n = NULL, *t = NULL;
 	u32 olen, nlen, tlen;
 
-	if (context_struct_to_string(p, ocontext, &o, &olen))
+	if (sidtab_entry_to_string(p, sidtab, oentry, &o, &olen))
 		goto out;
-	if (context_struct_to_string(p, ncontext, &n, &nlen))
+	if (sidtab_entry_to_string(p, sidtab, nentry, &n, &nlen))
 		goto out;
-	if (context_struct_to_string(p, tcontext, &t, &tlen))
+	if (sidtab_entry_to_string(p, sidtab, tentry, &t, &tlen))
 		goto out;
 	audit_log(audit_context(), GFP_ATOMIC, AUDIT_SELINUX_ERR,
 		  "op=security_validate_transition seresult=denied"
@@ -751,9 +758,9 @@ static int security_compute_validatetrans(struct selinux_state *state,
 {
 	struct policydb *policydb;
 	struct sidtab *sidtab;
-	struct context *ocontext;
-	struct context *ncontext;
-	struct context *tcontext;
+	struct sidtab_entry *oentry;
+	struct sidtab_entry *nentry;
+	struct sidtab_entry *tentry;
 	struct class_datum *tclass_datum;
 	struct constraint_node *constraint;
 	u16 tclass;
@@ -779,24 +786,24 @@ static int security_compute_validatetrans(struct selinux_state *state,
 	}
 	tclass_datum = policydb->class_val_to_struct[tclass - 1];
 
-	ocontext = sidtab_search(sidtab, oldsid);
-	if (!ocontext) {
+	oentry = sidtab_search_entry(sidtab, oldsid);
+	if (!oentry) {
 		pr_err("SELinux: %s:  unrecognized SID %d\n",
 			__func__, oldsid);
 		rc = -EINVAL;
 		goto out;
 	}
 
-	ncontext = sidtab_search(sidtab, newsid);
-	if (!ncontext) {
+	nentry = sidtab_search_entry(sidtab, newsid);
+	if (!nentry) {
 		pr_err("SELinux: %s:  unrecognized SID %d\n",
 			__func__, newsid);
 		rc = -EINVAL;
 		goto out;
 	}
 
-	tcontext = sidtab_search(sidtab, tasksid);
-	if (!tcontext) {
+	tentry = sidtab_search_entry(sidtab, tasksid);
+	if (!tentry) {
 		pr_err("SELinux: %s:  unrecognized SID %d\n",
 			__func__, tasksid);
 		rc = -EINVAL;
@@ -805,15 +812,16 @@ static int security_compute_validatetrans(struct selinux_state *state,
 
 	constraint = tclass_datum->validatetrans;
 	while (constraint) {
-		if (!constraint_expr_eval(policydb, ocontext, ncontext,
-					  tcontext, constraint->expr)) {
+		if (!constraint_expr_eval(policydb, &oentry->context,
+					  &nentry->context, &tentry->context,
+					  constraint->expr)) {
 			if (user)
 				rc = -EPERM;
 			else
 				rc = security_validtrans_handle_fail(state,
-								     ocontext,
-								     ncontext,
-								     tcontext,
+								     oentry,
+								     nentry,
+								     tentry,
 								     tclass);
 			goto out;
 		}
@@ -855,7 +863,7 @@ int security_bounded_transition(struct selinux_state *state,
 {
 	struct policydb *policydb;
 	struct sidtab *sidtab;
-	struct context *old_context, *new_context;
+	struct sidtab_entry *old_entry, *new_entry;
 	struct type_datum *type;
 	int index;
 	int rc;
@@ -869,16 +877,16 @@ int security_bounded_transition(struct selinux_state *state,
 	sidtab = state->ss->sidtab;
 
 	rc = -EINVAL;
-	old_context = sidtab_search(sidtab, old_sid);
-	if (!old_context) {
+	old_entry = sidtab_search_entry(sidtab, old_sid);
+	if (!old_entry) {
 		pr_err("SELinux: %s: unrecognized SID %u\n",
 		       __func__, old_sid);
 		goto out;
 	}
 
 	rc = -EINVAL;
-	new_context = sidtab_search(sidtab, new_sid);
-	if (!new_context) {
+	new_entry = sidtab_search_entry(sidtab, new_sid);
+	if (!new_entry) {
 		pr_err("SELinux: %s: unrecognized SID %u\n",
 		       __func__, new_sid);
 		goto out;
@@ -886,10 +894,10 @@ int security_bounded_transition(struct selinux_state *state,
 
 	rc = 0;
 	/* type/domain unchanged */
-	if (old_context->type == new_context->type)
+	if (old_entry->context.type == new_entry->context.type)
 		goto out;
 
-	index = new_context->type;
+	index = new_entry->context.type;
 	while (true) {
 		type = policydb->type_val_to_struct[index - 1];
 		BUG_ON(!type);
@@ -901,7 +909,7 @@ int security_bounded_transition(struct selinux_state *state,
 
 		/* @newsid is bounded by @oldsid */
 		rc = 0;
-		if (type->bounds == old_context->type)
+		if (type->bounds == old_entry->context.type)
 			break;
 
 		index = type->bounds;
@@ -912,10 +920,10 @@ int security_bounded_transition(struct selinux_state *state,
 		char *new_name = NULL;
 		u32 length;
 
-		if (!context_struct_to_string(policydb, old_context,
-					      &old_name, &length) &&
-		    !context_struct_to_string(policydb, new_context,
-					      &new_name, &length)) {
+		if (!sidtab_entry_to_string(policydb, sidtab, old_entry,
+					    &old_name, &length) &&
+		    !sidtab_entry_to_string(policydb, sidtab, new_entry,
+					    &new_name, &length)) {
 			audit_log(audit_context(),
 				  GFP_ATOMIC, AUDIT_SELINUX_ERR,
 				  "op=security_bounded_transition "
@@ -1255,6 +1263,23 @@ static int context_struct_to_string(struct policydb *p,
 	return 0;
 }
 
+static int sidtab_entry_to_string(struct policydb *p,
+				  struct sidtab *sidtab,
+				  struct sidtab_entry *entry,
+				  char **scontext, u32 *scontext_len)
+{
+	int rc = sidtab_sid2str_get(sidtab, entry, scontext, scontext_len);
+
+	if (rc != -ENOENT)
+		return rc;
+
+	rc = context_struct_to_string(p, &entry->context, scontext,
+				      scontext_len);
+	if (!rc && scontext)
+		sidtab_sid2str_put(sidtab, entry, *scontext, *scontext_len);
+	return rc;
+}
+
 #include "initial_sid_to_string.h"
 
 const char *security_get_initial_sid_context(u32 sid)
@@ -1271,7 +1296,7 @@ static int security_sid_to_context_core(struct selinux_state *state,
 {
 	struct policydb *policydb;
 	struct sidtab *sidtab;
-	struct context *context;
+	struct sidtab_entry *entry;
 	int rc = 0;
 
 	if (scontext)
@@ -1302,21 +1327,23 @@ static int security_sid_to_context_core(struct selinux_state *state,
 	read_lock(&state->ss->policy_rwlock);
 	policydb = &state->ss->policydb;
 	sidtab = state->ss->sidtab;
+
 	if (force)
-		context = sidtab_search_force(sidtab, sid);
+		entry = sidtab_search_entry_force(sidtab, sid);
 	else
-		context = sidtab_search(sidtab, sid);
-	if (!context) {
+		entry = sidtab_search_entry(sidtab, sid);
+	if (!entry) {
 		pr_err("SELinux: %s:  unrecognized SID %d\n",
 			__func__, sid);
 		rc = -EINVAL;
 		goto out_unlock;
 	}
-	if (only_invalid && !context->len)
-		rc = 0;
-	else
-		rc = context_struct_to_string(policydb, context, scontext,
-					      scontext_len);
+	if (only_invalid && !entry->context.len)
+		goto out_unlock;
+
+	rc = sidtab_entry_to_string(policydb, sidtab, entry, scontext,
+				    scontext_len);
+
 out_unlock:
 	read_unlock(&state->ss->policy_rwlock);
 out:
@@ -1574,19 +1601,20 @@ int security_context_to_sid_force(struct selinux_state *state,
 
 static int compute_sid_handle_invalid_context(
 	struct selinux_state *state,
-	struct context *scontext,
-	struct context *tcontext,
+	struct sidtab_entry *sentry,
+	struct sidtab_entry *tentry,
 	u16 tclass,
 	struct context *newcontext)
 {
 	struct policydb *policydb = &state->ss->policydb;
+	struct sidtab *sidtab = state->ss->sidtab;
 	char *s = NULL, *t = NULL, *n = NULL;
 	u32 slen, tlen, nlen;
 	struct audit_buffer *ab;
 
-	if (context_struct_to_string(policydb, scontext, &s, &slen))
+	if (sidtab_entry_to_string(policydb, sidtab, sentry, &s, &slen))
 		goto out;
-	if (context_struct_to_string(policydb, tcontext, &t, &tlen))
+	if (sidtab_entry_to_string(policydb, sidtab, tentry, &t, &tlen))
 		goto out;
 	if (context_struct_to_string(policydb, newcontext, &n, &nlen))
 		goto out;
@@ -1645,7 +1673,8 @@ static int security_compute_sid(struct selinux_state *state,
 	struct policydb *policydb;
 	struct sidtab *sidtab;
 	struct class_datum *cladatum = NULL;
-	struct context *scontext = NULL, *tcontext = NULL, newcontext;
+	struct context *scontext, *tcontext, newcontext;
+	struct sidtab_entry *sentry, *tentry;
 	struct role_trans *roletr = NULL;
 	struct avtab_key avkey;
 	struct avtab_datum *avdatum;
@@ -1682,21 +1711,24 @@ static int security_compute_sid(struct selinux_state *state,
 	policydb = &state->ss->policydb;
 	sidtab = state->ss->sidtab;
 
-	scontext = sidtab_search(sidtab, ssid);
-	if (!scontext) {
+	sentry = sidtab_search_entry(sidtab, ssid);
+	if (!sentry) {
 		pr_err("SELinux: %s:  unrecognized SID %d\n",
 		       __func__, ssid);
 		rc = -EINVAL;
 		goto out_unlock;
 	}
-	tcontext = sidtab_search(sidtab, tsid);
-	if (!tcontext) {
+	tentry = sidtab_search_entry(sidtab, tsid);
+	if (!tentry) {
 		pr_err("SELinux: %s:  unrecognized SID %d\n",
 		       __func__, tsid);
 		rc = -EINVAL;
 		goto out_unlock;
 	}
 
+	scontext = &sentry->context;
+	tcontext = &tentry->context;
+
 	if (tclass && tclass <= policydb->p_classes.nprim)
 		cladatum = policydb->class_val_to_struct[tclass - 1];
 
@@ -1797,10 +1829,8 @@ static int security_compute_sid(struct selinux_state *state,
 
 	/* Check the validity of the context. */
 	if (!policydb_context_isvalid(policydb, &newcontext)) {
-		rc = compute_sid_handle_invalid_context(state, scontext,
-							tcontext,
-							tclass,
-							&newcontext);
+		rc = compute_sid_handle_invalid_context(state, sentry, tentry,
+							tclass, &newcontext);
 		if (rc)
 			goto out_unlock;
 	}
diff --git a/security/selinux/ss/sidtab.c b/security/selinux/ss/sidtab.c
index 7d49994e8d5f..568069be922c 100644
--- a/security/selinux/ss/sidtab.c
+++ b/security/selinux/ss/sidtab.c
@@ -9,6 +9,8 @@
  */
 #include <linux/errno.h>
 #include <linux/kernel.h>
+#include <linux/list.h>
+#include <linux/rcupdate.h>
 #include <linux/slab.h>
 #include <linux/sched.h>
 #include <linux/spinlock.h>
@@ -17,6 +19,14 @@
 #include "security.h"
 #include "sidtab.h"
 
+struct sidtab_str_cache {
+	struct rcu_head rcu_member;
+	struct list_head lru_member;
+	struct sidtab_entry *parent;
+	u32 len;
+	char str[];
+};
+
 int sidtab_init(struct sidtab *s)
 {
 	u32 i;
@@ -34,24 +44,33 @@ int sidtab_init(struct sidtab *s)
 	s->convert = NULL;
 
 	spin_lock_init(&s->lock);
+
+#if CONFIG_SECURITY_SELINUX_SID2STR_CACHE_SIZE > 0
+	s->cache_free_slots = CONFIG_SECURITY_SELINUX_SID2STR_CACHE_SIZE;
+	INIT_LIST_HEAD(&s->cache_lru_list);
+	spin_lock_init(&s->cache_lock);
+#endif
 	return 0;
 }
 
 int sidtab_set_initial(struct sidtab *s, u32 sid, struct context *context)
 {
-	struct sidtab_isid_entry *entry;
+	struct sidtab_isid_entry *isid;
 	int rc;
 
 	if (sid == 0 || sid > SECINITSID_NUM)
 		return -EINVAL;
 
-	entry = &s->isids[sid - 1];
+	isid = &s->isids[sid - 1];
 
-	rc = context_cpy(&entry->context, context);
+	rc = context_cpy(&isid->entry.context, context);
 	if (rc)
 		return rc;
 
-	entry->set = 1;
+#if CONFIG_SECURITY_SELINUX_SID2STR_CACHE_SIZE > 0
+	isid->entry.cache = NULL;
+#endif
+	isid->set = 1;
 	return 0;
 }
 
@@ -88,7 +107,8 @@ static int sidtab_alloc_roots(struct sidtab *s, u32 level)
 	return 0;
 }
 
-static struct context *sidtab_do_lookup(struct sidtab *s, u32 index, int alloc)
+static struct sidtab_entry *sidtab_do_lookup(struct sidtab *s, u32 index,
+					     int alloc)
 {
 	union sidtab_entry_inner *entry;
 	u32 level, capacity_shift, leaf_index = index / SIDTAB_LEAF_ENTRIES;
@@ -125,10 +145,16 @@ static struct context *sidtab_do_lookup(struct sidtab *s, u32 index, int alloc)
 		if (!entry->ptr_leaf)
 			return NULL;
 	}
-	return &entry->ptr_leaf->entries[index % SIDTAB_LEAF_ENTRIES].context;
+	return &entry->ptr_leaf->entries[index % SIDTAB_LEAF_ENTRIES];
+}
+
+/* use when you know that there is enough entries */
+static struct context *sidtab_lookup_unsafe(struct sidtab *s, u32 index)
+{
+	return &sidtab_do_lookup(s, index, 0)->context;
 }
 
-static struct context *sidtab_lookup(struct sidtab *s, u32 index)
+static struct sidtab_entry *sidtab_lookup(struct sidtab *s, u32 index)
 {
 	/* read entries only after reading count */
 	u32 count = smp_load_acquire(&s->count);
@@ -139,33 +165,34 @@ static struct context *sidtab_lookup(struct sidtab *s, u32 index)
 	return sidtab_do_lookup(s, index, 0);
 }
 
-static struct context *sidtab_lookup_initial(struct sidtab *s, u32 sid)
+static struct sidtab_entry *sidtab_lookup_initial(struct sidtab *s, u32 sid)
 {
-	return s->isids[sid - 1].set ? &s->isids[sid - 1].context : NULL;
+	return s->isids[sid - 1].set ? &s->isids[sid - 1].entry : NULL;
 }
 
-static struct context *sidtab_search_core(struct sidtab *s, u32 sid, int force)
+static struct sidtab_entry *sidtab_search_core(struct sidtab *s, u32 sid,
+					       int force)
 {
-	struct context *context;
-
 	if (sid != 0) {
+		struct sidtab_entry *entry;
+
 		if (sid > SECINITSID_NUM)
-			context = sidtab_lookup(s, sid - (SECINITSID_NUM + 1));
+			entry = sidtab_lookup(s, sid - (SECINITSID_NUM + 1));
 		else
-			context = sidtab_lookup_initial(s, sid);
-		if (context && (!context->len || force))
-			return context;
+			entry = sidtab_lookup_initial(s, sid);
+		if (entry && (!entry->context.len || force))
+			return entry;
 	}
 
 	return sidtab_lookup_initial(s, SECINITSID_UNLABELED);
 }
 
-struct context *sidtab_search(struct sidtab *s, u32 sid)
+struct sidtab_entry *sidtab_search_entry(struct sidtab *s, u32 sid)
 {
 	return sidtab_search_core(s, sid, 0);
 }
 
-struct context *sidtab_search_force(struct sidtab *s, u32 sid)
+struct sidtab_entry *sidtab_search_entry_force(struct sidtab *s, u32 sid)
 {
 	return sidtab_search_core(s, sid, 1);
 }
@@ -230,7 +257,7 @@ static int sidtab_rcache_search(struct sidtab *s, struct context *context,
 		if (v >= SIDTAB_MAX)
 			continue;
 
-		if (context_cmp(sidtab_do_lookup(s, v, 0), context)) {
+		if (context_cmp(sidtab_lookup_unsafe(s, v), context)) {
 			sidtab_rcache_update(s, v, i);
 			*index = v;
 			return 0;
@@ -245,7 +272,7 @@ static int sidtab_reverse_lookup(struct sidtab *s, struct context *context,
 	unsigned long flags;
 	u32 count, count_locked, level, pos;
 	struct sidtab_convert_params *convert;
-	struct context *dst, *dst_convert;
+	struct sidtab_entry *dst, *dst_convert;
 	int rc;
 
 	rc = sidtab_rcache_search(s, context, index);
@@ -273,7 +300,7 @@ static int sidtab_reverse_lookup(struct sidtab *s, struct context *context,
 
 	/* if count has changed before we acquired the lock, then catch up */
 	while (count < count_locked) {
-		if (context_cmp(sidtab_do_lookup(s, count, 0), context)) {
+		if (context_cmp(sidtab_lookup_unsafe(s, count), context)) {
 			sidtab_rcache_push(s, count);
 			*index = count;
 			rc = 0;
@@ -293,7 +320,7 @@ static int sidtab_reverse_lookup(struct sidtab *s, struct context *context,
 	if (!dst)
 		goto out_unlock;
 
-	rc = context_cpy(dst, context);
+	rc = context_cpy(&dst->context, context);
 	if (rc)
 		goto out_unlock;
 
@@ -305,13 +332,14 @@ static int sidtab_reverse_lookup(struct sidtab *s, struct context *context,
 		rc = -ENOMEM;
 		dst_convert = sidtab_do_lookup(convert->target, count, 1);
 		if (!dst_convert) {
-			context_destroy(dst);
+			context_destroy(&dst->context);
 			goto out_unlock;
 		}
 
-		rc = convert->func(context, dst_convert, convert->args);
+		rc = convert->func(context, &dst_convert->context,
+				   convert->args);
 		if (rc) {
-			context_destroy(dst);
+			context_destroy(&dst->context);
 			goto out_unlock;
 		}
 
@@ -341,9 +369,9 @@ int sidtab_context_to_sid(struct sidtab *s, struct context *context, u32 *sid)
 	u32 i;
 
 	for (i = 0; i < SECINITSID_NUM; i++) {
-		struct sidtab_isid_entry *entry = &s->isids[i];
+		struct sidtab_isid_entry *isid = &s->isids[i];
 
-		if (entry->set && context_cmp(context, &entry->context)) {
+		if (isid->set && context_cmp(context, &isid->entry.context)) {
 			*sid = i + 1;
 			return 0;
 		}
@@ -453,6 +481,14 @@ int sidtab_convert(struct sidtab *s, struct sidtab_convert_params *params)
 	return rc;
 }
 
+static void sidtab_destroy_entry(struct sidtab_entry *entry)
+{
+	context_destroy(&entry->context);
+#if CONFIG_SECURITY_SELINUX_SID2STR_CACHE_SIZE > 0
+	kfree(rcu_dereference_raw(entry->cache));
+#endif
+}
+
 static void sidtab_destroy_tree(union sidtab_entry_inner entry, u32 level)
 {
 	u32 i;
@@ -473,7 +509,7 @@ static void sidtab_destroy_tree(union sidtab_entry_inner entry, u32 level)
 			return;
 
 		for (i = 0; i < SIDTAB_LEAF_ENTRIES; i++)
-			context_destroy(&node->entries[i].context);
+			sidtab_destroy_entry(&node->entries[i]);
 		kfree(node);
 	}
 }
@@ -484,7 +520,7 @@ void sidtab_destroy(struct sidtab *s)
 
 	for (i = 0; i < SECINITSID_NUM; i++)
 		if (s->isids[i].set)
-			context_destroy(&s->isids[i].context);
+			sidtab_destroy_entry(&s->isids[i].entry);
 
 	level = SIDTAB_MAX_LEVEL;
 	while (level && !s->roots[level].ptr_inner)
@@ -492,3 +528,88 @@ void sidtab_destroy(struct sidtab *s)
 
 	sidtab_destroy_tree(s->roots[level], level);
 }
+
+#if CONFIG_SECURITY_SELINUX_SID2STR_CACHE_SIZE > 0
+
+void sidtab_sid2str_put(struct sidtab *s, struct sidtab_entry *entry,
+			const char *str, u32 str_len)
+{
+	struct sidtab_str_cache *cache, *victim;
+
+	/* do not cache invalid contexts */
+	if (entry->context.len)
+		return;
+
+	/*
+	 * Skip the put operation when in non-task context to avoid the need
+	 * to disable interrupts while holding s->cache_lock.
+	 */
+	if (!in_task())
+		return;
+
+	spin_lock(&s->cache_lock);
+
+	cache = rcu_dereference_raw(entry->cache);
+	if (cache) {
+		/* entry in cache - just bump to he head of LRU list */
+		list_move(&cache->lru_member, &s->cache_lru_list);
+		goto out_unlock;
+	}
+
+	cache = kmalloc(sizeof(struct sidtab_str_cache) + str_len, GFP_ATOMIC);
+	if (!cache)
+		goto out_unlock;
+
+	if (s->cache_free_slots == 0) {
+		/* pop a cache entry from the tail and free it */
+		victim = container_of(s->cache_lru_list.prev,
+				      struct sidtab_str_cache, lru_member);
+		list_del(&victim->lru_member);
+		kfree_rcu(victim, rcu_member);
+		rcu_assign_pointer(victim->parent->cache, NULL);
+	} else {
+		s->cache_free_slots--;
+	}
+	cache->parent = entry;
+	cache->len = str_len;
+	memcpy(cache->str, str, str_len);
+	rcu_head_init(&cache->rcu_member);
+	list_add(&cache->lru_member, &s->cache_lru_list);
+
+	rcu_assign_pointer(entry->cache, cache);
+
+out_unlock:
+	spin_unlock(&s->cache_lock);
+}
+
+int sidtab_sid2str_get(struct sidtab *s, struct sidtab_entry *entry,
+		       char **out, u32 *out_len)
+{
+	struct sidtab_str_cache *cache;
+	int rc = 0;
+
+	if (entry->context.len)
+		return -ENOENT; /* do not cache invalid contexts */
+
+	rcu_read_lock();
+
+	cache = rcu_dereference(entry->cache);
+	if (!cache) {
+		rc = -ENOENT;
+	} else {
+		*out_len = cache->len;
+		if (out) {
+			*out = kmemdup(cache->str, cache->len, GFP_ATOMIC);
+			if (!*out)
+				rc = -ENOMEM;
+		}
+	}
+
+	rcu_read_unlock();
+
+	if (!rc && out)
+		sidtab_sid2str_put(s, entry, *out, *out_len);
+	return rc;
+}
+
+#endif /* CONFIG_SECURITY_SELINUX_SID2STR_CACHE_SIZE > 0 */
diff --git a/security/selinux/ss/sidtab.h b/security/selinux/ss/sidtab.h
index 1f4763141aa1..5fe67a0c307b 100644
--- a/security/selinux/ss/sidtab.h
+++ b/security/selinux/ss/sidtab.h
@@ -16,13 +16,13 @@
 
 #include "context.h"
 
-struct sidtab_entry_leaf {
+struct sidtab_entry {
 	struct context context;
+#if CONFIG_SECURITY_SELINUX_SID2STR_CACHE_SIZE > 0
+	struct sidtab_str_cache __rcu *cache;
+#endif
 };
 
-struct sidtab_node_inner;
-struct sidtab_node_leaf;
-
 union sidtab_entry_inner {
 	struct sidtab_node_inner *ptr_inner;
 	struct sidtab_node_leaf  *ptr_leaf;
@@ -38,7 +38,7 @@ union sidtab_entry_inner {
 	(SIDTAB_NODE_ALLOC_SHIFT - size_to_shift(sizeof(union sidtab_entry_inner)))
 #define SIDTAB_INNER_ENTRIES ((size_t)1 << SIDTAB_INNER_SHIFT)
 #define SIDTAB_LEAF_ENTRIES \
-	(SIDTAB_NODE_ALLOC_SIZE / sizeof(struct sidtab_entry_leaf))
+	(SIDTAB_NODE_ALLOC_SIZE / sizeof(struct sidtab_entry))
 
 #define SIDTAB_MAX_BITS 32
 #define SIDTAB_MAX U32_MAX
@@ -48,7 +48,7 @@ union sidtab_entry_inner {
 		     SIDTAB_INNER_SHIFT)
 
 struct sidtab_node_leaf {
-	struct sidtab_entry_leaf entries[SIDTAB_LEAF_ENTRIES];
+	struct sidtab_entry entries[SIDTAB_LEAF_ENTRIES];
 };
 
 struct sidtab_node_inner {
@@ -57,7 +57,7 @@ struct sidtab_node_inner {
 
 struct sidtab_isid_entry {
 	int set;
-	struct context context;
+	struct sidtab_entry entry;
 };
 
 struct sidtab_convert_params {
@@ -83,6 +83,13 @@ struct sidtab {
 	struct sidtab_convert_params *convert;
 	spinlock_t lock;
 
+#if CONFIG_SECURITY_SELINUX_SID2STR_CACHE_SIZE > 0
+	/* SID -> context string cache */
+	u32 cache_free_slots;
+	struct list_head cache_lru_list;
+	spinlock_t cache_lock;
+#endif
+
 	/* reverse lookup cache - access atomically via {READ|WRITE}_ONCE() */
 	u32 rcache[SIDTAB_RCACHE_SIZE];
 
@@ -92,8 +99,22 @@ struct sidtab {
 
 int sidtab_init(struct sidtab *s);
 int sidtab_set_initial(struct sidtab *s, u32 sid, struct context *context);
-struct context *sidtab_search(struct sidtab *s, u32 sid);
-struct context *sidtab_search_force(struct sidtab *s, u32 sid);
+struct sidtab_entry *sidtab_search_entry(struct sidtab *s, u32 sid);
+struct sidtab_entry *sidtab_search_entry_force(struct sidtab *s, u32 sid);
+
+static inline struct context *sidtab_search(struct sidtab *s, u32 sid)
+{
+	struct sidtab_entry *entry = sidtab_search_entry(s, sid);
+
+	return entry ? &entry->context : NULL;
+}
+
+static inline struct context *sidtab_search_force(struct sidtab *s, u32 sid)
+{
+	struct sidtab_entry *entry = sidtab_search_entry_force(s, sid);
+
+	return entry ? &entry->context : NULL;
+}
 
 int sidtab_convert(struct sidtab *s, struct sidtab_convert_params *params);
 
@@ -101,6 +122,25 @@ int sidtab_context_to_sid(struct sidtab *s, struct context *context, u32 *sid);
 
 void sidtab_destroy(struct sidtab *s);
 
+#if CONFIG_SECURITY_SELINUX_SID2STR_CACHE_SIZE > 0
+void sidtab_sid2str_put(struct sidtab *s, struct sidtab_entry *entry,
+			const char *str, u32 str_len);
+int sidtab_sid2str_get(struct sidtab *s, struct sidtab_entry *entry,
+		       char **out, u32 *out_len);
+#else
+static inline void sidtab_sid2str_put(struct sidtab *s,
+				      struct sidtab_entry *entry,
+				      const char *str, u32 str_len)
+{
+}
+static inline int sidtab_sid2str_get(struct sidtab *s,
+				     struct sidtab_entry *entry,
+				     char **out, u32 *out_len)
+{
+	return -ENOENT;
+}
+#endif /* CONFIG_SECURITY_SELINUX_SID2STR_CACHE_SIZE > 0 */
+
 #endif	/* _SS_SIDTAB_H_ */
 
 
-- 
2.21.0


^ permalink raw reply	[flat|nested] 4+ messages in thread

* Re: [PATCH v3] selinux: cache the SID -> context string translation
  2019-11-08  8:39 [PATCH v3] selinux: cache the SID -> context string translation Ondrej Mosnacek
@ 2019-11-08 14:09 ` Stephen Smalley
  2019-11-10 21:03 ` Paul E. McKenney
  1 sibling, 0 replies; 4+ messages in thread
From: Stephen Smalley @ 2019-11-08 14:09 UTC (permalink / raw)
  To: Ondrej Mosnacek, selinux, Paul Moore
  Cc: rcu, Paul E. McKenney, Michal Sekletar

On 11/8/19 3:39 AM, Ondrej Mosnacek wrote:
> Translating a context struct to string can be quite slow, especially if
> the context has a lot of category bits set. This can cause quite
> noticeable performance impact in situations where the translation needs
> to be done repeatedly. A common example is a UNIX datagram socket with
> the SO_PASSSEC option enabled, which is used e.g. by systemd-journald
> when receiving log messages via datagram socket. This scenario can be
> reproduced with:
> 
>      cat /dev/urandom | base64 | logger &
>      timeout 30s perf record -p $(pidof systemd-journald) -a -g
>      kill %1
>      perf report -g none --pretty raw | grep security_secid_to_secctx
> 
> Before the caching introduced by this patch, computing the context
> string (security_secid_to_secctx() function) takes up ~65% of
> systemd-journald's CPU time (assuming a context with 1024 categories
> set and Fedora x86_64 release kernel configs). After this patch
> (assuming near-perfect cache hit ratio) this overhead is reduced to just
> ~2%.
> 
> This patch addresses the issue by caching a certain number (compile-time
> configurable) of recently used context strings to speed up repeated
> translations of the same context, while using only a small amount of
> memory.
> 
> The cache is integrated into the existing sidtab table by adding a field
> to each entry, which when not NULL contains an RCU-protected pointer to
> a cache entry containing the cached string. The cache entries are kept
> in a linked list sorted according to how recently they were used. On a
> cache miss when the cache is full, the least recently used entry is
> removed to make space for the new entry.
> 
> The patch migrates security_sid_to_context_core() to use the cache (also
> a few other functions where it was possible without too much fuss, but
> these mostly use the translation for logging in case of error, which is
> rare).
> 
> Link: https://bugzilla.redhat.com/show_bug.cgi?id=1733259
> Cc: Michal Sekletar <msekleta@redhat.com>
> Signed-off-by: Ondrej Mosnacek <omosnace@redhat.com>

Reviewed-by: Stephen Smalley <sds@tycho.nsa.gov>
Tested-by: Stephen Smalley <sds@tycho.nsa.gov>

> ---
> 
> Changes in v3:
>   - add rcu@vger.kernel.org and Paul McKenney to Cc for review of the RCU
>     logic
>   - add __rcu annotation to the cache entry pointer (sidtab.c now passes
>     sparse checks with C=1)
> 
> Changes in v2:
>   - skip sidtab_sid2str_put() when in non-task context to prevent
>     deadlock while avoiding the need to lock the spinlock with
>     irqsave/-restore (which is slower)
> 
>   security/selinux/Kconfig       |  11 ++
>   security/selinux/ss/services.c | 138 +++++++++++++++----------
>   security/selinux/ss/sidtab.c   | 179 +++++++++++++++++++++++++++------
>   security/selinux/ss/sidtab.h   |  58 +++++++++--
>   4 files changed, 294 insertions(+), 92 deletions(-)
> 
> diff --git a/security/selinux/Kconfig b/security/selinux/Kconfig
> index 5711689deb6a..35fe8878cf1c 100644
> --- a/security/selinux/Kconfig
> +++ b/security/selinux/Kconfig
> @@ -85,3 +85,14 @@ config SECURITY_SELINUX_CHECKREQPROT_VALUE
>   	  via /selinux/checkreqprot if authorized by policy.
>   
>   	  If you are unsure how to answer this question, answer 0.
> +
> +config SECURITY_SELINUX_SID2STR_CACHE_SIZE
> +	int "NSA SELinux SID to context string translation cache size"
> +	depends on SECURITY_SELINUX
> +	default 256
> +	help
> +	  This option defines the size of the internal SID -> context string
> +	  cache, which improves the performance of context to string
> +	  conversion.  Setting this option to 0 disables the cache completely.
> +
> +	  If unsure, keep the default value.
> diff --git a/security/selinux/ss/services.c b/security/selinux/ss/services.c
> index 3a29e7c24ba9..b6dda5261166 100644
> --- a/security/selinux/ss/services.c
> +++ b/security/selinux/ss/services.c
> @@ -91,6 +91,12 @@ static int context_struct_to_string(struct policydb *policydb,
>   				    char **scontext,
>   				    u32 *scontext_len);
>   
> +static int sidtab_entry_to_string(struct policydb *policydb,
> +				  struct sidtab *sidtab,
> +				  struct sidtab_entry *entry,
> +				  char **scontext,
> +				  u32 *scontext_len);
> +
>   static void context_struct_compute_av(struct policydb *policydb,
>   				      struct context *scontext,
>   				      struct context *tcontext,
> @@ -716,20 +722,21 @@ static void context_struct_compute_av(struct policydb *policydb,
>   }
>   
>   static int security_validtrans_handle_fail(struct selinux_state *state,
> -					   struct context *ocontext,
> -					   struct context *ncontext,
> -					   struct context *tcontext,
> +					   struct sidtab_entry *oentry,
> +					   struct sidtab_entry *nentry,
> +					   struct sidtab_entry *tentry,
>   					   u16 tclass)
>   {
>   	struct policydb *p = &state->ss->policydb;
> +	struct sidtab *sidtab = state->ss->sidtab;
>   	char *o = NULL, *n = NULL, *t = NULL;
>   	u32 olen, nlen, tlen;
>   
> -	if (context_struct_to_string(p, ocontext, &o, &olen))
> +	if (sidtab_entry_to_string(p, sidtab, oentry, &o, &olen))
>   		goto out;
> -	if (context_struct_to_string(p, ncontext, &n, &nlen))
> +	if (sidtab_entry_to_string(p, sidtab, nentry, &n, &nlen))
>   		goto out;
> -	if (context_struct_to_string(p, tcontext, &t, &tlen))
> +	if (sidtab_entry_to_string(p, sidtab, tentry, &t, &tlen))
>   		goto out;
>   	audit_log(audit_context(), GFP_ATOMIC, AUDIT_SELINUX_ERR,
>   		  "op=security_validate_transition seresult=denied"
> @@ -751,9 +758,9 @@ static int security_compute_validatetrans(struct selinux_state *state,
>   {
>   	struct policydb *policydb;
>   	struct sidtab *sidtab;
> -	struct context *ocontext;
> -	struct context *ncontext;
> -	struct context *tcontext;
> +	struct sidtab_entry *oentry;
> +	struct sidtab_entry *nentry;
> +	struct sidtab_entry *tentry;
>   	struct class_datum *tclass_datum;
>   	struct constraint_node *constraint;
>   	u16 tclass;
> @@ -779,24 +786,24 @@ static int security_compute_validatetrans(struct selinux_state *state,
>   	}
>   	tclass_datum = policydb->class_val_to_struct[tclass - 1];
>   
> -	ocontext = sidtab_search(sidtab, oldsid);
> -	if (!ocontext) {
> +	oentry = sidtab_search_entry(sidtab, oldsid);
> +	if (!oentry) {
>   		pr_err("SELinux: %s:  unrecognized SID %d\n",
>   			__func__, oldsid);
>   		rc = -EINVAL;
>   		goto out;
>   	}
>   
> -	ncontext = sidtab_search(sidtab, newsid);
> -	if (!ncontext) {
> +	nentry = sidtab_search_entry(sidtab, newsid);
> +	if (!nentry) {
>   		pr_err("SELinux: %s:  unrecognized SID %d\n",
>   			__func__, newsid);
>   		rc = -EINVAL;
>   		goto out;
>   	}
>   
> -	tcontext = sidtab_search(sidtab, tasksid);
> -	if (!tcontext) {
> +	tentry = sidtab_search_entry(sidtab, tasksid);
> +	if (!tentry) {
>   		pr_err("SELinux: %s:  unrecognized SID %d\n",
>   			__func__, tasksid);
>   		rc = -EINVAL;
> @@ -805,15 +812,16 @@ static int security_compute_validatetrans(struct selinux_state *state,
>   
>   	constraint = tclass_datum->validatetrans;
>   	while (constraint) {
> -		if (!constraint_expr_eval(policydb, ocontext, ncontext,
> -					  tcontext, constraint->expr)) {
> +		if (!constraint_expr_eval(policydb, &oentry->context,
> +					  &nentry->context, &tentry->context,
> +					  constraint->expr)) {
>   			if (user)
>   				rc = -EPERM;
>   			else
>   				rc = security_validtrans_handle_fail(state,
> -								     ocontext,
> -								     ncontext,
> -								     tcontext,
> +								     oentry,
> +								     nentry,
> +								     tentry,
>   								     tclass);
>   			goto out;
>   		}
> @@ -855,7 +863,7 @@ int security_bounded_transition(struct selinux_state *state,
>   {
>   	struct policydb *policydb;
>   	struct sidtab *sidtab;
> -	struct context *old_context, *new_context;
> +	struct sidtab_entry *old_entry, *new_entry;
>   	struct type_datum *type;
>   	int index;
>   	int rc;
> @@ -869,16 +877,16 @@ int security_bounded_transition(struct selinux_state *state,
>   	sidtab = state->ss->sidtab;
>   
>   	rc = -EINVAL;
> -	old_context = sidtab_search(sidtab, old_sid);
> -	if (!old_context) {
> +	old_entry = sidtab_search_entry(sidtab, old_sid);
> +	if (!old_entry) {
>   		pr_err("SELinux: %s: unrecognized SID %u\n",
>   		       __func__, old_sid);
>   		goto out;
>   	}
>   
>   	rc = -EINVAL;
> -	new_context = sidtab_search(sidtab, new_sid);
> -	if (!new_context) {
> +	new_entry = sidtab_search_entry(sidtab, new_sid);
> +	if (!new_entry) {
>   		pr_err("SELinux: %s: unrecognized SID %u\n",
>   		       __func__, new_sid);
>   		goto out;
> @@ -886,10 +894,10 @@ int security_bounded_transition(struct selinux_state *state,
>   
>   	rc = 0;
>   	/* type/domain unchanged */
> -	if (old_context->type == new_context->type)
> +	if (old_entry->context.type == new_entry->context.type)
>   		goto out;
>   
> -	index = new_context->type;
> +	index = new_entry->context.type;
>   	while (true) {
>   		type = policydb->type_val_to_struct[index - 1];
>   		BUG_ON(!type);
> @@ -901,7 +909,7 @@ int security_bounded_transition(struct selinux_state *state,
>   
>   		/* @newsid is bounded by @oldsid */
>   		rc = 0;
> -		if (type->bounds == old_context->type)
> +		if (type->bounds == old_entry->context.type)
>   			break;
>   
>   		index = type->bounds;
> @@ -912,10 +920,10 @@ int security_bounded_transition(struct selinux_state *state,
>   		char *new_name = NULL;
>   		u32 length;
>   
> -		if (!context_struct_to_string(policydb, old_context,
> -					      &old_name, &length) &&
> -		    !context_struct_to_string(policydb, new_context,
> -					      &new_name, &length)) {
> +		if (!sidtab_entry_to_string(policydb, sidtab, old_entry,
> +					    &old_name, &length) &&
> +		    !sidtab_entry_to_string(policydb, sidtab, new_entry,
> +					    &new_name, &length)) {
>   			audit_log(audit_context(),
>   				  GFP_ATOMIC, AUDIT_SELINUX_ERR,
>   				  "op=security_bounded_transition "
> @@ -1255,6 +1263,23 @@ static int context_struct_to_string(struct policydb *p,
>   	return 0;
>   }
>   
> +static int sidtab_entry_to_string(struct policydb *p,
> +				  struct sidtab *sidtab,
> +				  struct sidtab_entry *entry,
> +				  char **scontext, u32 *scontext_len)
> +{
> +	int rc = sidtab_sid2str_get(sidtab, entry, scontext, scontext_len);
> +
> +	if (rc != -ENOENT)
> +		return rc;
> +
> +	rc = context_struct_to_string(p, &entry->context, scontext,
> +				      scontext_len);
> +	if (!rc && scontext)
> +		sidtab_sid2str_put(sidtab, entry, *scontext, *scontext_len);
> +	return rc;
> +}
> +
>   #include "initial_sid_to_string.h"
>   
>   const char *security_get_initial_sid_context(u32 sid)
> @@ -1271,7 +1296,7 @@ static int security_sid_to_context_core(struct selinux_state *state,
>   {
>   	struct policydb *policydb;
>   	struct sidtab *sidtab;
> -	struct context *context;
> +	struct sidtab_entry *entry;
>   	int rc = 0;
>   
>   	if (scontext)
> @@ -1302,21 +1327,23 @@ static int security_sid_to_context_core(struct selinux_state *state,
>   	read_lock(&state->ss->policy_rwlock);
>   	policydb = &state->ss->policydb;
>   	sidtab = state->ss->sidtab;
> +
>   	if (force)
> -		context = sidtab_search_force(sidtab, sid);
> +		entry = sidtab_search_entry_force(sidtab, sid);
>   	else
> -		context = sidtab_search(sidtab, sid);
> -	if (!context) {
> +		entry = sidtab_search_entry(sidtab, sid);
> +	if (!entry) {
>   		pr_err("SELinux: %s:  unrecognized SID %d\n",
>   			__func__, sid);
>   		rc = -EINVAL;
>   		goto out_unlock;
>   	}
> -	if (only_invalid && !context->len)
> -		rc = 0;
> -	else
> -		rc = context_struct_to_string(policydb, context, scontext,
> -					      scontext_len);
> +	if (only_invalid && !entry->context.len)
> +		goto out_unlock;
> +
> +	rc = sidtab_entry_to_string(policydb, sidtab, entry, scontext,
> +				    scontext_len);
> +
>   out_unlock:
>   	read_unlock(&state->ss->policy_rwlock);
>   out:
> @@ -1574,19 +1601,20 @@ int security_context_to_sid_force(struct selinux_state *state,
>   
>   static int compute_sid_handle_invalid_context(
>   	struct selinux_state *state,
> -	struct context *scontext,
> -	struct context *tcontext,
> +	struct sidtab_entry *sentry,
> +	struct sidtab_entry *tentry,
>   	u16 tclass,
>   	struct context *newcontext)
>   {
>   	struct policydb *policydb = &state->ss->policydb;
> +	struct sidtab *sidtab = state->ss->sidtab;
>   	char *s = NULL, *t = NULL, *n = NULL;
>   	u32 slen, tlen, nlen;
>   	struct audit_buffer *ab;
>   
> -	if (context_struct_to_string(policydb, scontext, &s, &slen))
> +	if (sidtab_entry_to_string(policydb, sidtab, sentry, &s, &slen))
>   		goto out;
> -	if (context_struct_to_string(policydb, tcontext, &t, &tlen))
> +	if (sidtab_entry_to_string(policydb, sidtab, tentry, &t, &tlen))
>   		goto out;
>   	if (context_struct_to_string(policydb, newcontext, &n, &nlen))
>   		goto out;
> @@ -1645,7 +1673,8 @@ static int security_compute_sid(struct selinux_state *state,
>   	struct policydb *policydb;
>   	struct sidtab *sidtab;
>   	struct class_datum *cladatum = NULL;
> -	struct context *scontext = NULL, *tcontext = NULL, newcontext;
> +	struct context *scontext, *tcontext, newcontext;
> +	struct sidtab_entry *sentry, *tentry;
>   	struct role_trans *roletr = NULL;
>   	struct avtab_key avkey;
>   	struct avtab_datum *avdatum;
> @@ -1682,21 +1711,24 @@ static int security_compute_sid(struct selinux_state *state,
>   	policydb = &state->ss->policydb;
>   	sidtab = state->ss->sidtab;
>   
> -	scontext = sidtab_search(sidtab, ssid);
> -	if (!scontext) {
> +	sentry = sidtab_search_entry(sidtab, ssid);
> +	if (!sentry) {
>   		pr_err("SELinux: %s:  unrecognized SID %d\n",
>   		       __func__, ssid);
>   		rc = -EINVAL;
>   		goto out_unlock;
>   	}
> -	tcontext = sidtab_search(sidtab, tsid);
> -	if (!tcontext) {
> +	tentry = sidtab_search_entry(sidtab, tsid);
> +	if (!tentry) {
>   		pr_err("SELinux: %s:  unrecognized SID %d\n",
>   		       __func__, tsid);
>   		rc = -EINVAL;
>   		goto out_unlock;
>   	}
>   
> +	scontext = &sentry->context;
> +	tcontext = &tentry->context;
> +
>   	if (tclass && tclass <= policydb->p_classes.nprim)
>   		cladatum = policydb->class_val_to_struct[tclass - 1];
>   
> @@ -1797,10 +1829,8 @@ static int security_compute_sid(struct selinux_state *state,
>   
>   	/* Check the validity of the context. */
>   	if (!policydb_context_isvalid(policydb, &newcontext)) {
> -		rc = compute_sid_handle_invalid_context(state, scontext,
> -							tcontext,
> -							tclass,
> -							&newcontext);
> +		rc = compute_sid_handle_invalid_context(state, sentry, tentry,
> +							tclass, &newcontext);
>   		if (rc)
>   			goto out_unlock;
>   	}
> diff --git a/security/selinux/ss/sidtab.c b/security/selinux/ss/sidtab.c
> index 7d49994e8d5f..568069be922c 100644
> --- a/security/selinux/ss/sidtab.c
> +++ b/security/selinux/ss/sidtab.c
> @@ -9,6 +9,8 @@
>    */
>   #include <linux/errno.h>
>   #include <linux/kernel.h>
> +#include <linux/list.h>
> +#include <linux/rcupdate.h>
>   #include <linux/slab.h>
>   #include <linux/sched.h>
>   #include <linux/spinlock.h>
> @@ -17,6 +19,14 @@
>   #include "security.h"
>   #include "sidtab.h"
>   
> +struct sidtab_str_cache {
> +	struct rcu_head rcu_member;
> +	struct list_head lru_member;
> +	struct sidtab_entry *parent;
> +	u32 len;
> +	char str[];
> +};
> +
>   int sidtab_init(struct sidtab *s)
>   {
>   	u32 i;
> @@ -34,24 +44,33 @@ int sidtab_init(struct sidtab *s)
>   	s->convert = NULL;
>   
>   	spin_lock_init(&s->lock);
> +
> +#if CONFIG_SECURITY_SELINUX_SID2STR_CACHE_SIZE > 0
> +	s->cache_free_slots = CONFIG_SECURITY_SELINUX_SID2STR_CACHE_SIZE;
> +	INIT_LIST_HEAD(&s->cache_lru_list);
> +	spin_lock_init(&s->cache_lock);
> +#endif
>   	return 0;
>   }
>   
>   int sidtab_set_initial(struct sidtab *s, u32 sid, struct context *context)
>   {
> -	struct sidtab_isid_entry *entry;
> +	struct sidtab_isid_entry *isid;
>   	int rc;
>   
>   	if (sid == 0 || sid > SECINITSID_NUM)
>   		return -EINVAL;
>   
> -	entry = &s->isids[sid - 1];
> +	isid = &s->isids[sid - 1];
>   
> -	rc = context_cpy(&entry->context, context);
> +	rc = context_cpy(&isid->entry.context, context);
>   	if (rc)
>   		return rc;
>   
> -	entry->set = 1;
> +#if CONFIG_SECURITY_SELINUX_SID2STR_CACHE_SIZE > 0
> +	isid->entry.cache = NULL;
> +#endif
> +	isid->set = 1;
>   	return 0;
>   }
>   
> @@ -88,7 +107,8 @@ static int sidtab_alloc_roots(struct sidtab *s, u32 level)
>   	return 0;
>   }
>   
> -static struct context *sidtab_do_lookup(struct sidtab *s, u32 index, int alloc)
> +static struct sidtab_entry *sidtab_do_lookup(struct sidtab *s, u32 index,
> +					     int alloc)
>   {
>   	union sidtab_entry_inner *entry;
>   	u32 level, capacity_shift, leaf_index = index / SIDTAB_LEAF_ENTRIES;
> @@ -125,10 +145,16 @@ static struct context *sidtab_do_lookup(struct sidtab *s, u32 index, int alloc)
>   		if (!entry->ptr_leaf)
>   			return NULL;
>   	}
> -	return &entry->ptr_leaf->entries[index % SIDTAB_LEAF_ENTRIES].context;
> +	return &entry->ptr_leaf->entries[index % SIDTAB_LEAF_ENTRIES];
> +}
> +
> +/* use when you know that there is enough entries */
> +static struct context *sidtab_lookup_unsafe(struct sidtab *s, u32 index)
> +{
> +	return &sidtab_do_lookup(s, index, 0)->context;
>   }
>   
> -static struct context *sidtab_lookup(struct sidtab *s, u32 index)
> +static struct sidtab_entry *sidtab_lookup(struct sidtab *s, u32 index)
>   {
>   	/* read entries only after reading count */
>   	u32 count = smp_load_acquire(&s->count);
> @@ -139,33 +165,34 @@ static struct context *sidtab_lookup(struct sidtab *s, u32 index)
>   	return sidtab_do_lookup(s, index, 0);
>   }
>   
> -static struct context *sidtab_lookup_initial(struct sidtab *s, u32 sid)
> +static struct sidtab_entry *sidtab_lookup_initial(struct sidtab *s, u32 sid)
>   {
> -	return s->isids[sid - 1].set ? &s->isids[sid - 1].context : NULL;
> +	return s->isids[sid - 1].set ? &s->isids[sid - 1].entry : NULL;
>   }
>   
> -static struct context *sidtab_search_core(struct sidtab *s, u32 sid, int force)
> +static struct sidtab_entry *sidtab_search_core(struct sidtab *s, u32 sid,
> +					       int force)
>   {
> -	struct context *context;
> -
>   	if (sid != 0) {
> +		struct sidtab_entry *entry;
> +
>   		if (sid > SECINITSID_NUM)
> -			context = sidtab_lookup(s, sid - (SECINITSID_NUM + 1));
> +			entry = sidtab_lookup(s, sid - (SECINITSID_NUM + 1));
>   		else
> -			context = sidtab_lookup_initial(s, sid);
> -		if (context && (!context->len || force))
> -			return context;
> +			entry = sidtab_lookup_initial(s, sid);
> +		if (entry && (!entry->context.len || force))
> +			return entry;
>   	}
>   
>   	return sidtab_lookup_initial(s, SECINITSID_UNLABELED);
>   }
>   
> -struct context *sidtab_search(struct sidtab *s, u32 sid)
> +struct sidtab_entry *sidtab_search_entry(struct sidtab *s, u32 sid)
>   {
>   	return sidtab_search_core(s, sid, 0);
>   }
>   
> -struct context *sidtab_search_force(struct sidtab *s, u32 sid)
> +struct sidtab_entry *sidtab_search_entry_force(struct sidtab *s, u32 sid)
>   {
>   	return sidtab_search_core(s, sid, 1);
>   }
> @@ -230,7 +257,7 @@ static int sidtab_rcache_search(struct sidtab *s, struct context *context,
>   		if (v >= SIDTAB_MAX)
>   			continue;
>   
> -		if (context_cmp(sidtab_do_lookup(s, v, 0), context)) {
> +		if (context_cmp(sidtab_lookup_unsafe(s, v), context)) {
>   			sidtab_rcache_update(s, v, i);
>   			*index = v;
>   			return 0;
> @@ -245,7 +272,7 @@ static int sidtab_reverse_lookup(struct sidtab *s, struct context *context,
>   	unsigned long flags;
>   	u32 count, count_locked, level, pos;
>   	struct sidtab_convert_params *convert;
> -	struct context *dst, *dst_convert;
> +	struct sidtab_entry *dst, *dst_convert;
>   	int rc;
>   
>   	rc = sidtab_rcache_search(s, context, index);
> @@ -273,7 +300,7 @@ static int sidtab_reverse_lookup(struct sidtab *s, struct context *context,
>   
>   	/* if count has changed before we acquired the lock, then catch up */
>   	while (count < count_locked) {
> -		if (context_cmp(sidtab_do_lookup(s, count, 0), context)) {
> +		if (context_cmp(sidtab_lookup_unsafe(s, count), context)) {
>   			sidtab_rcache_push(s, count);
>   			*index = count;
>   			rc = 0;
> @@ -293,7 +320,7 @@ static int sidtab_reverse_lookup(struct sidtab *s, struct context *context,
>   	if (!dst)
>   		goto out_unlock;
>   
> -	rc = context_cpy(dst, context);
> +	rc = context_cpy(&dst->context, context);
>   	if (rc)
>   		goto out_unlock;
>   
> @@ -305,13 +332,14 @@ static int sidtab_reverse_lookup(struct sidtab *s, struct context *context,
>   		rc = -ENOMEM;
>   		dst_convert = sidtab_do_lookup(convert->target, count, 1);
>   		if (!dst_convert) {
> -			context_destroy(dst);
> +			context_destroy(&dst->context);
>   			goto out_unlock;
>   		}
>   
> -		rc = convert->func(context, dst_convert, convert->args);
> +		rc = convert->func(context, &dst_convert->context,
> +				   convert->args);
>   		if (rc) {
> -			context_destroy(dst);
> +			context_destroy(&dst->context);
>   			goto out_unlock;
>   		}
>   
> @@ -341,9 +369,9 @@ int sidtab_context_to_sid(struct sidtab *s, struct context *context, u32 *sid)
>   	u32 i;
>   
>   	for (i = 0; i < SECINITSID_NUM; i++) {
> -		struct sidtab_isid_entry *entry = &s->isids[i];
> +		struct sidtab_isid_entry *isid = &s->isids[i];
>   
> -		if (entry->set && context_cmp(context, &entry->context)) {
> +		if (isid->set && context_cmp(context, &isid->entry.context)) {
>   			*sid = i + 1;
>   			return 0;
>   		}
> @@ -453,6 +481,14 @@ int sidtab_convert(struct sidtab *s, struct sidtab_convert_params *params)
>   	return rc;
>   }
>   
> +static void sidtab_destroy_entry(struct sidtab_entry *entry)
> +{
> +	context_destroy(&entry->context);
> +#if CONFIG_SECURITY_SELINUX_SID2STR_CACHE_SIZE > 0
> +	kfree(rcu_dereference_raw(entry->cache));
> +#endif
> +}
> +
>   static void sidtab_destroy_tree(union sidtab_entry_inner entry, u32 level)
>   {
>   	u32 i;
> @@ -473,7 +509,7 @@ static void sidtab_destroy_tree(union sidtab_entry_inner entry, u32 level)
>   			return;
>   
>   		for (i = 0; i < SIDTAB_LEAF_ENTRIES; i++)
> -			context_destroy(&node->entries[i].context);
> +			sidtab_destroy_entry(&node->entries[i]);
>   		kfree(node);
>   	}
>   }
> @@ -484,7 +520,7 @@ void sidtab_destroy(struct sidtab *s)
>   
>   	for (i = 0; i < SECINITSID_NUM; i++)
>   		if (s->isids[i].set)
> -			context_destroy(&s->isids[i].context);
> +			sidtab_destroy_entry(&s->isids[i].entry);
>   
>   	level = SIDTAB_MAX_LEVEL;
>   	while (level && !s->roots[level].ptr_inner)
> @@ -492,3 +528,88 @@ void sidtab_destroy(struct sidtab *s)
>   
>   	sidtab_destroy_tree(s->roots[level], level);
>   }
> +
> +#if CONFIG_SECURITY_SELINUX_SID2STR_CACHE_SIZE > 0
> +
> +void sidtab_sid2str_put(struct sidtab *s, struct sidtab_entry *entry,
> +			const char *str, u32 str_len)
> +{
> +	struct sidtab_str_cache *cache, *victim;
> +
> +	/* do not cache invalid contexts */
> +	if (entry->context.len)
> +		return;
> +
> +	/*
> +	 * Skip the put operation when in non-task context to avoid the need
> +	 * to disable interrupts while holding s->cache_lock.
> +	 */
> +	if (!in_task())
> +		return;
> +
> +	spin_lock(&s->cache_lock);
> +
> +	cache = rcu_dereference_raw(entry->cache);
> +	if (cache) {
> +		/* entry in cache - just bump to he head of LRU list */
> +		list_move(&cache->lru_member, &s->cache_lru_list);
> +		goto out_unlock;
> +	}
> +
> +	cache = kmalloc(sizeof(struct sidtab_str_cache) + str_len, GFP_ATOMIC);
> +	if (!cache)
> +		goto out_unlock;
> +
> +	if (s->cache_free_slots == 0) {
> +		/* pop a cache entry from the tail and free it */
> +		victim = container_of(s->cache_lru_list.prev,
> +				      struct sidtab_str_cache, lru_member);
> +		list_del(&victim->lru_member);
> +		kfree_rcu(victim, rcu_member);
> +		rcu_assign_pointer(victim->parent->cache, NULL);
> +	} else {
> +		s->cache_free_slots--;
> +	}
> +	cache->parent = entry;
> +	cache->len = str_len;
> +	memcpy(cache->str, str, str_len);
> +	rcu_head_init(&cache->rcu_member);
> +	list_add(&cache->lru_member, &s->cache_lru_list);
> +
> +	rcu_assign_pointer(entry->cache, cache);
> +
> +out_unlock:
> +	spin_unlock(&s->cache_lock);
> +}
> +
> +int sidtab_sid2str_get(struct sidtab *s, struct sidtab_entry *entry,
> +		       char **out, u32 *out_len)
> +{
> +	struct sidtab_str_cache *cache;
> +	int rc = 0;
> +
> +	if (entry->context.len)
> +		return -ENOENT; /* do not cache invalid contexts */
> +
> +	rcu_read_lock();
> +
> +	cache = rcu_dereference(entry->cache);
> +	if (!cache) {
> +		rc = -ENOENT;
> +	} else {
> +		*out_len = cache->len;
> +		if (out) {
> +			*out = kmemdup(cache->str, cache->len, GFP_ATOMIC);
> +			if (!*out)
> +				rc = -ENOMEM;
> +		}
> +	}
> +
> +	rcu_read_unlock();
> +
> +	if (!rc && out)
> +		sidtab_sid2str_put(s, entry, *out, *out_len);
> +	return rc;
> +}
> +
> +#endif /* CONFIG_SECURITY_SELINUX_SID2STR_CACHE_SIZE > 0 */
> diff --git a/security/selinux/ss/sidtab.h b/security/selinux/ss/sidtab.h
> index 1f4763141aa1..5fe67a0c307b 100644
> --- a/security/selinux/ss/sidtab.h
> +++ b/security/selinux/ss/sidtab.h
> @@ -16,13 +16,13 @@
>   
>   #include "context.h"
>   
> -struct sidtab_entry_leaf {
> +struct sidtab_entry {
>   	struct context context;
> +#if CONFIG_SECURITY_SELINUX_SID2STR_CACHE_SIZE > 0
> +	struct sidtab_str_cache __rcu *cache;
> +#endif
>   };
>   
> -struct sidtab_node_inner;
> -struct sidtab_node_leaf;
> -
>   union sidtab_entry_inner {
>   	struct sidtab_node_inner *ptr_inner;
>   	struct sidtab_node_leaf  *ptr_leaf;
> @@ -38,7 +38,7 @@ union sidtab_entry_inner {
>   	(SIDTAB_NODE_ALLOC_SHIFT - size_to_shift(sizeof(union sidtab_entry_inner)))
>   #define SIDTAB_INNER_ENTRIES ((size_t)1 << SIDTAB_INNER_SHIFT)
>   #define SIDTAB_LEAF_ENTRIES \
> -	(SIDTAB_NODE_ALLOC_SIZE / sizeof(struct sidtab_entry_leaf))
> +	(SIDTAB_NODE_ALLOC_SIZE / sizeof(struct sidtab_entry))
>   
>   #define SIDTAB_MAX_BITS 32
>   #define SIDTAB_MAX U32_MAX
> @@ -48,7 +48,7 @@ union sidtab_entry_inner {
>   		     SIDTAB_INNER_SHIFT)
>   
>   struct sidtab_node_leaf {
> -	struct sidtab_entry_leaf entries[SIDTAB_LEAF_ENTRIES];
> +	struct sidtab_entry entries[SIDTAB_LEAF_ENTRIES];
>   };
>   
>   struct sidtab_node_inner {
> @@ -57,7 +57,7 @@ struct sidtab_node_inner {
>   
>   struct sidtab_isid_entry {
>   	int set;
> -	struct context context;
> +	struct sidtab_entry entry;
>   };
>   
>   struct sidtab_convert_params {
> @@ -83,6 +83,13 @@ struct sidtab {
>   	struct sidtab_convert_params *convert;
>   	spinlock_t lock;
>   
> +#if CONFIG_SECURITY_SELINUX_SID2STR_CACHE_SIZE > 0
> +	/* SID -> context string cache */
> +	u32 cache_free_slots;
> +	struct list_head cache_lru_list;
> +	spinlock_t cache_lock;
> +#endif
> +
>   	/* reverse lookup cache - access atomically via {READ|WRITE}_ONCE() */
>   	u32 rcache[SIDTAB_RCACHE_SIZE];
>   
> @@ -92,8 +99,22 @@ struct sidtab {
>   
>   int sidtab_init(struct sidtab *s);
>   int sidtab_set_initial(struct sidtab *s, u32 sid, struct context *context);
> -struct context *sidtab_search(struct sidtab *s, u32 sid);
> -struct context *sidtab_search_force(struct sidtab *s, u32 sid);
> +struct sidtab_entry *sidtab_search_entry(struct sidtab *s, u32 sid);
> +struct sidtab_entry *sidtab_search_entry_force(struct sidtab *s, u32 sid);
> +
> +static inline struct context *sidtab_search(struct sidtab *s, u32 sid)
> +{
> +	struct sidtab_entry *entry = sidtab_search_entry(s, sid);
> +
> +	return entry ? &entry->context : NULL;
> +}
> +
> +static inline struct context *sidtab_search_force(struct sidtab *s, u32 sid)
> +{
> +	struct sidtab_entry *entry = sidtab_search_entry_force(s, sid);
> +
> +	return entry ? &entry->context : NULL;
> +}
>   
>   int sidtab_convert(struct sidtab *s, struct sidtab_convert_params *params);
>   
> @@ -101,6 +122,25 @@ int sidtab_context_to_sid(struct sidtab *s, struct context *context, u32 *sid);
>   
>   void sidtab_destroy(struct sidtab *s);
>   
> +#if CONFIG_SECURITY_SELINUX_SID2STR_CACHE_SIZE > 0
> +void sidtab_sid2str_put(struct sidtab *s, struct sidtab_entry *entry,
> +			const char *str, u32 str_len);
> +int sidtab_sid2str_get(struct sidtab *s, struct sidtab_entry *entry,
> +		       char **out, u32 *out_len);
> +#else
> +static inline void sidtab_sid2str_put(struct sidtab *s,
> +				      struct sidtab_entry *entry,
> +				      const char *str, u32 str_len)
> +{
> +}
> +static inline int sidtab_sid2str_get(struct sidtab *s,
> +				     struct sidtab_entry *entry,
> +				     char **out, u32 *out_len)
> +{
> +	return -ENOENT;
> +}
> +#endif /* CONFIG_SECURITY_SELINUX_SID2STR_CACHE_SIZE > 0 */
> +
>   #endif	/* _SS_SIDTAB_H_ */
>   
>   
> 


^ permalink raw reply	[flat|nested] 4+ messages in thread

* Re: [PATCH v3] selinux: cache the SID -> context string translation
  2019-11-08  8:39 [PATCH v3] selinux: cache the SID -> context string translation Ondrej Mosnacek
  2019-11-08 14:09 ` Stephen Smalley
@ 2019-11-10 21:03 ` Paul E. McKenney
  2019-11-11  8:32   ` Ondrej Mosnacek
  1 sibling, 1 reply; 4+ messages in thread
From: Paul E. McKenney @ 2019-11-10 21:03 UTC (permalink / raw)
  To: Ondrej Mosnacek
  Cc: selinux, Paul Moore, Stephen Smalley, rcu, Michal Sekletar

On Fri, Nov 08, 2019 at 09:39:09AM +0100, Ondrej Mosnacek wrote:
> Translating a context struct to string can be quite slow, especially if
> the context has a lot of category bits set. This can cause quite
> noticeable performance impact in situations where the translation needs
> to be done repeatedly. A common example is a UNIX datagram socket with
> the SO_PASSSEC option enabled, which is used e.g. by systemd-journald
> when receiving log messages via datagram socket. This scenario can be
> reproduced with:
> 
>     cat /dev/urandom | base64 | logger &
>     timeout 30s perf record -p $(pidof systemd-journald) -a -g
>     kill %1
>     perf report -g none --pretty raw | grep security_secid_to_secctx
> 
> Before the caching introduced by this patch, computing the context
> string (security_secid_to_secctx() function) takes up ~65% of
> systemd-journald's CPU time (assuming a context with 1024 categories
> set and Fedora x86_64 release kernel configs). After this patch
> (assuming near-perfect cache hit ratio) this overhead is reduced to just
> ~2%.
> 
> This patch addresses the issue by caching a certain number (compile-time
> configurable) of recently used context strings to speed up repeated
> translations of the same context, while using only a small amount of
> memory.
> 
> The cache is integrated into the existing sidtab table by adding a field
> to each entry, which when not NULL contains an RCU-protected pointer to
> a cache entry containing the cached string. The cache entries are kept
> in a linked list sorted according to how recently they were used. On a
> cache miss when the cache is full, the least recently used entry is
> removed to make space for the new entry.
> 
> The patch migrates security_sid_to_context_core() to use the cache (also
> a few other functions where it was possible without too much fuss, but
> these mostly use the translation for logging in case of error, which is
> rare).
> 
> Link: https://bugzilla.redhat.com/show_bug.cgi?id=1733259
> Cc: Michal Sekletar <msekleta@redhat.com>
> Signed-off-by: Ondrej Mosnacek <omosnace@redhat.com>

Some questions and comments below.

> ---
> 
> Changes in v3:
>  - add rcu@vger.kernel.org and Paul McKenney to Cc for review of the RCU
>    logic
>  - add __rcu annotation to the cache entry pointer (sidtab.c now passes
>    sparse checks with C=1)
> 
> Changes in v2:
>  - skip sidtab_sid2str_put() when in non-task context to prevent
>    deadlock while avoiding the need to lock the spinlock with
>    irqsave/-restore (which is slower)
> 
>  security/selinux/Kconfig       |  11 ++
>  security/selinux/ss/services.c | 138 +++++++++++++++----------
>  security/selinux/ss/sidtab.c   | 179 +++++++++++++++++++++++++++------
>  security/selinux/ss/sidtab.h   |  58 +++++++++--
>  4 files changed, 294 insertions(+), 92 deletions(-)
> 
> diff --git a/security/selinux/Kconfig b/security/selinux/Kconfig
> index 5711689deb6a..35fe8878cf1c 100644
> --- a/security/selinux/Kconfig
> +++ b/security/selinux/Kconfig
> @@ -85,3 +85,14 @@ config SECURITY_SELINUX_CHECKREQPROT_VALUE
>  	  via /selinux/checkreqprot if authorized by policy.
>  
>  	  If you are unsure how to answer this question, answer 0.
> +
> +config SECURITY_SELINUX_SID2STR_CACHE_SIZE
> +	int "NSA SELinux SID to context string translation cache size"
> +	depends on SECURITY_SELINUX
> +	default 256
> +	help
> +	  This option defines the size of the internal SID -> context string
> +	  cache, which improves the performance of context to string
> +	  conversion.  Setting this option to 0 disables the cache completely.
> +
> +	  If unsure, keep the default value.
> diff --git a/security/selinux/ss/services.c b/security/selinux/ss/services.c
> index 3a29e7c24ba9..b6dda5261166 100644
> --- a/security/selinux/ss/services.c
> +++ b/security/selinux/ss/services.c
> @@ -91,6 +91,12 @@ static int context_struct_to_string(struct policydb *policydb,
>  				    char **scontext,
>  				    u32 *scontext_len);
>  
> +static int sidtab_entry_to_string(struct policydb *policydb,
> +				  struct sidtab *sidtab,
> +				  struct sidtab_entry *entry,
> +				  char **scontext,
> +				  u32 *scontext_len);
> +
>  static void context_struct_compute_av(struct policydb *policydb,
>  				      struct context *scontext,
>  				      struct context *tcontext,
> @@ -716,20 +722,21 @@ static void context_struct_compute_av(struct policydb *policydb,
>  }
>  
>  static int security_validtrans_handle_fail(struct selinux_state *state,
> -					   struct context *ocontext,
> -					   struct context *ncontext,
> -					   struct context *tcontext,
> +					   struct sidtab_entry *oentry,
> +					   struct sidtab_entry *nentry,
> +					   struct sidtab_entry *tentry,
>  					   u16 tclass)
>  {
>  	struct policydb *p = &state->ss->policydb;
> +	struct sidtab *sidtab = state->ss->sidtab;
>  	char *o = NULL, *n = NULL, *t = NULL;
>  	u32 olen, nlen, tlen;
>  
> -	if (context_struct_to_string(p, ocontext, &o, &olen))
> +	if (sidtab_entry_to_string(p, sidtab, oentry, &o, &olen))
>  		goto out;
> -	if (context_struct_to_string(p, ncontext, &n, &nlen))
> +	if (sidtab_entry_to_string(p, sidtab, nentry, &n, &nlen))
>  		goto out;
> -	if (context_struct_to_string(p, tcontext, &t, &tlen))
> +	if (sidtab_entry_to_string(p, sidtab, tentry, &t, &tlen))
>  		goto out;
>  	audit_log(audit_context(), GFP_ATOMIC, AUDIT_SELINUX_ERR,
>  		  "op=security_validate_transition seresult=denied"
> @@ -751,9 +758,9 @@ static int security_compute_validatetrans(struct selinux_state *state,
>  {
>  	struct policydb *policydb;
>  	struct sidtab *sidtab;
> -	struct context *ocontext;
> -	struct context *ncontext;
> -	struct context *tcontext;
> +	struct sidtab_entry *oentry;
> +	struct sidtab_entry *nentry;
> +	struct sidtab_entry *tentry;
>  	struct class_datum *tclass_datum;
>  	struct constraint_node *constraint;
>  	u16 tclass;
> @@ -779,24 +786,24 @@ static int security_compute_validatetrans(struct selinux_state *state,
>  	}
>  	tclass_datum = policydb->class_val_to_struct[tclass - 1];
>  
> -	ocontext = sidtab_search(sidtab, oldsid);
> -	if (!ocontext) {
> +	oentry = sidtab_search_entry(sidtab, oldsid);
> +	if (!oentry) {
>  		pr_err("SELinux: %s:  unrecognized SID %d\n",
>  			__func__, oldsid);
>  		rc = -EINVAL;
>  		goto out;
>  	}
>  
> -	ncontext = sidtab_search(sidtab, newsid);
> -	if (!ncontext) {
> +	nentry = sidtab_search_entry(sidtab, newsid);
> +	if (!nentry) {
>  		pr_err("SELinux: %s:  unrecognized SID %d\n",
>  			__func__, newsid);
>  		rc = -EINVAL;
>  		goto out;
>  	}
>  
> -	tcontext = sidtab_search(sidtab, tasksid);
> -	if (!tcontext) {
> +	tentry = sidtab_search_entry(sidtab, tasksid);
> +	if (!tentry) {
>  		pr_err("SELinux: %s:  unrecognized SID %d\n",
>  			__func__, tasksid);
>  		rc = -EINVAL;
> @@ -805,15 +812,16 @@ static int security_compute_validatetrans(struct selinux_state *state,
>  
>  	constraint = tclass_datum->validatetrans;
>  	while (constraint) {
> -		if (!constraint_expr_eval(policydb, ocontext, ncontext,
> -					  tcontext, constraint->expr)) {
> +		if (!constraint_expr_eval(policydb, &oentry->context,
> +					  &nentry->context, &tentry->context,
> +					  constraint->expr)) {
>  			if (user)
>  				rc = -EPERM;
>  			else
>  				rc = security_validtrans_handle_fail(state,
> -								     ocontext,
> -								     ncontext,
> -								     tcontext,
> +								     oentry,
> +								     nentry,
> +								     tentry,
>  								     tclass);
>  			goto out;
>  		}
> @@ -855,7 +863,7 @@ int security_bounded_transition(struct selinux_state *state,
>  {
>  	struct policydb *policydb;
>  	struct sidtab *sidtab;
> -	struct context *old_context, *new_context;
> +	struct sidtab_entry *old_entry, *new_entry;
>  	struct type_datum *type;
>  	int index;
>  	int rc;
> @@ -869,16 +877,16 @@ int security_bounded_transition(struct selinux_state *state,
>  	sidtab = state->ss->sidtab;
>  
>  	rc = -EINVAL;
> -	old_context = sidtab_search(sidtab, old_sid);
> -	if (!old_context) {
> +	old_entry = sidtab_search_entry(sidtab, old_sid);
> +	if (!old_entry) {
>  		pr_err("SELinux: %s: unrecognized SID %u\n",
>  		       __func__, old_sid);
>  		goto out;
>  	}
>  
>  	rc = -EINVAL;
> -	new_context = sidtab_search(sidtab, new_sid);
> -	if (!new_context) {
> +	new_entry = sidtab_search_entry(sidtab, new_sid);
> +	if (!new_entry) {
>  		pr_err("SELinux: %s: unrecognized SID %u\n",
>  		       __func__, new_sid);
>  		goto out;
> @@ -886,10 +894,10 @@ int security_bounded_transition(struct selinux_state *state,
>  
>  	rc = 0;
>  	/* type/domain unchanged */
> -	if (old_context->type == new_context->type)
> +	if (old_entry->context.type == new_entry->context.type)
>  		goto out;
>  
> -	index = new_context->type;
> +	index = new_entry->context.type;
>  	while (true) {
>  		type = policydb->type_val_to_struct[index - 1];
>  		BUG_ON(!type);
> @@ -901,7 +909,7 @@ int security_bounded_transition(struct selinux_state *state,
>  
>  		/* @newsid is bounded by @oldsid */
>  		rc = 0;
> -		if (type->bounds == old_context->type)
> +		if (type->bounds == old_entry->context.type)
>  			break;
>  
>  		index = type->bounds;
> @@ -912,10 +920,10 @@ int security_bounded_transition(struct selinux_state *state,
>  		char *new_name = NULL;
>  		u32 length;
>  
> -		if (!context_struct_to_string(policydb, old_context,
> -					      &old_name, &length) &&
> -		    !context_struct_to_string(policydb, new_context,
> -					      &new_name, &length)) {
> +		if (!sidtab_entry_to_string(policydb, sidtab, old_entry,
> +					    &old_name, &length) &&
> +		    !sidtab_entry_to_string(policydb, sidtab, new_entry,
> +					    &new_name, &length)) {
>  			audit_log(audit_context(),
>  				  GFP_ATOMIC, AUDIT_SELINUX_ERR,
>  				  "op=security_bounded_transition "
> @@ -1255,6 +1263,23 @@ static int context_struct_to_string(struct policydb *p,
>  	return 0;
>  }
>  
> +static int sidtab_entry_to_string(struct policydb *p,
> +				  struct sidtab *sidtab,
> +				  struct sidtab_entry *entry,
> +				  char **scontext, u32 *scontext_len)
> +{
> +	int rc = sidtab_sid2str_get(sidtab, entry, scontext, scontext_len);
> +
> +	if (rc != -ENOENT)
> +		return rc;
> +
> +	rc = context_struct_to_string(p, &entry->context, scontext,
> +				      scontext_len);
> +	if (!rc && scontext)
> +		sidtab_sid2str_put(sidtab, entry, *scontext, *scontext_len);
> +	return rc;
> +}
> +
>  #include "initial_sid_to_string.h"
>  
>  const char *security_get_initial_sid_context(u32 sid)
> @@ -1271,7 +1296,7 @@ static int security_sid_to_context_core(struct selinux_state *state,
>  {
>  	struct policydb *policydb;
>  	struct sidtab *sidtab;
> -	struct context *context;
> +	struct sidtab_entry *entry;
>  	int rc = 0;
>  
>  	if (scontext)
> @@ -1302,21 +1327,23 @@ static int security_sid_to_context_core(struct selinux_state *state,
>  	read_lock(&state->ss->policy_rwlock);
>  	policydb = &state->ss->policydb;
>  	sidtab = state->ss->sidtab;
> +
>  	if (force)
> -		context = sidtab_search_force(sidtab, sid);
> +		entry = sidtab_search_entry_force(sidtab, sid);
>  	else
> -		context = sidtab_search(sidtab, sid);
> -	if (!context) {
> +		entry = sidtab_search_entry(sidtab, sid);
> +	if (!entry) {
>  		pr_err("SELinux: %s:  unrecognized SID %d\n",
>  			__func__, sid);
>  		rc = -EINVAL;
>  		goto out_unlock;
>  	}
> -	if (only_invalid && !context->len)
> -		rc = 0;
> -	else
> -		rc = context_struct_to_string(policydb, context, scontext,
> -					      scontext_len);
> +	if (only_invalid && !entry->context.len)
> +		goto out_unlock;
> +
> +	rc = sidtab_entry_to_string(policydb, sidtab, entry, scontext,
> +				    scontext_len);
> +
>  out_unlock:
>  	read_unlock(&state->ss->policy_rwlock);
>  out:
> @@ -1574,19 +1601,20 @@ int security_context_to_sid_force(struct selinux_state *state,
>  
>  static int compute_sid_handle_invalid_context(
>  	struct selinux_state *state,
> -	struct context *scontext,
> -	struct context *tcontext,
> +	struct sidtab_entry *sentry,
> +	struct sidtab_entry *tentry,
>  	u16 tclass,
>  	struct context *newcontext)
>  {
>  	struct policydb *policydb = &state->ss->policydb;
> +	struct sidtab *sidtab = state->ss->sidtab;
>  	char *s = NULL, *t = NULL, *n = NULL;
>  	u32 slen, tlen, nlen;
>  	struct audit_buffer *ab;
>  
> -	if (context_struct_to_string(policydb, scontext, &s, &slen))
> +	if (sidtab_entry_to_string(policydb, sidtab, sentry, &s, &slen))
>  		goto out;
> -	if (context_struct_to_string(policydb, tcontext, &t, &tlen))
> +	if (sidtab_entry_to_string(policydb, sidtab, tentry, &t, &tlen))
>  		goto out;
>  	if (context_struct_to_string(policydb, newcontext, &n, &nlen))
>  		goto out;
> @@ -1645,7 +1673,8 @@ static int security_compute_sid(struct selinux_state *state,
>  	struct policydb *policydb;
>  	struct sidtab *sidtab;
>  	struct class_datum *cladatum = NULL;
> -	struct context *scontext = NULL, *tcontext = NULL, newcontext;
> +	struct context *scontext, *tcontext, newcontext;
> +	struct sidtab_entry *sentry, *tentry;
>  	struct role_trans *roletr = NULL;
>  	struct avtab_key avkey;
>  	struct avtab_datum *avdatum;
> @@ -1682,21 +1711,24 @@ static int security_compute_sid(struct selinux_state *state,
>  	policydb = &state->ss->policydb;
>  	sidtab = state->ss->sidtab;
>  
> -	scontext = sidtab_search(sidtab, ssid);
> -	if (!scontext) {
> +	sentry = sidtab_search_entry(sidtab, ssid);
> +	if (!sentry) {
>  		pr_err("SELinux: %s:  unrecognized SID %d\n",
>  		       __func__, ssid);
>  		rc = -EINVAL;
>  		goto out_unlock;
>  	}
> -	tcontext = sidtab_search(sidtab, tsid);
> -	if (!tcontext) {
> +	tentry = sidtab_search_entry(sidtab, tsid);
> +	if (!tentry) {
>  		pr_err("SELinux: %s:  unrecognized SID %d\n",
>  		       __func__, tsid);
>  		rc = -EINVAL;
>  		goto out_unlock;
>  	}
>  
> +	scontext = &sentry->context;
> +	tcontext = &tentry->context;
> +
>  	if (tclass && tclass <= policydb->p_classes.nprim)
>  		cladatum = policydb->class_val_to_struct[tclass - 1];
>  
> @@ -1797,10 +1829,8 @@ static int security_compute_sid(struct selinux_state *state,
>  
>  	/* Check the validity of the context. */
>  	if (!policydb_context_isvalid(policydb, &newcontext)) {
> -		rc = compute_sid_handle_invalid_context(state, scontext,
> -							tcontext,
> -							tclass,
> -							&newcontext);
> +		rc = compute_sid_handle_invalid_context(state, sentry, tentry,
> +							tclass, &newcontext);
>  		if (rc)
>  			goto out_unlock;
>  	}
> diff --git a/security/selinux/ss/sidtab.c b/security/selinux/ss/sidtab.c
> index 7d49994e8d5f..568069be922c 100644
> --- a/security/selinux/ss/sidtab.c
> +++ b/security/selinux/ss/sidtab.c
> @@ -9,6 +9,8 @@
>   */
>  #include <linux/errno.h>
>  #include <linux/kernel.h>
> +#include <linux/list.h>
> +#include <linux/rcupdate.h>
>  #include <linux/slab.h>
>  #include <linux/sched.h>
>  #include <linux/spinlock.h>
> @@ -17,6 +19,14 @@
>  #include "security.h"
>  #include "sidtab.h"
>  
> +struct sidtab_str_cache {
> +	struct rcu_head rcu_member;
> +	struct list_head lru_member;
> +	struct sidtab_entry *parent;
> +	u32 len;
> +	char str[];
> +};
> +
>  int sidtab_init(struct sidtab *s)
>  {
>  	u32 i;
> @@ -34,24 +44,33 @@ int sidtab_init(struct sidtab *s)
>  	s->convert = NULL;
>  
>  	spin_lock_init(&s->lock);
> +
> +#if CONFIG_SECURITY_SELINUX_SID2STR_CACHE_SIZE > 0
> +	s->cache_free_slots = CONFIG_SECURITY_SELINUX_SID2STR_CACHE_SIZE;
> +	INIT_LIST_HEAD(&s->cache_lru_list);
> +	spin_lock_init(&s->cache_lock);
> +#endif
>  	return 0;
>  }
>  
>  int sidtab_set_initial(struct sidtab *s, u32 sid, struct context *context)
>  {
> -	struct sidtab_isid_entry *entry;
> +	struct sidtab_isid_entry *isid;
>  	int rc;
>  
>  	if (sid == 0 || sid > SECINITSID_NUM)
>  		return -EINVAL;
>  
> -	entry = &s->isids[sid - 1];
> +	isid = &s->isids[sid - 1];
>  
> -	rc = context_cpy(&entry->context, context);
> +	rc = context_cpy(&isid->entry.context, context);
>  	if (rc)
>  		return rc;
>  
> -	entry->set = 1;
> +#if CONFIG_SECURITY_SELINUX_SID2STR_CACHE_SIZE > 0
> +	isid->entry.cache = NULL;
> +#endif
> +	isid->set = 1;
>  	return 0;
>  }
>  
> @@ -88,7 +107,8 @@ static int sidtab_alloc_roots(struct sidtab *s, u32 level)
>  	return 0;
>  }
>  
> -static struct context *sidtab_do_lookup(struct sidtab *s, u32 index, int alloc)
> +static struct sidtab_entry *sidtab_do_lookup(struct sidtab *s, u32 index,
> +					     int alloc)
>  {
>  	union sidtab_entry_inner *entry;
>  	u32 level, capacity_shift, leaf_index = index / SIDTAB_LEAF_ENTRIES;
> @@ -125,10 +145,16 @@ static struct context *sidtab_do_lookup(struct sidtab *s, u32 index, int alloc)
>  		if (!entry->ptr_leaf)
>  			return NULL;
>  	}
> -	return &entry->ptr_leaf->entries[index % SIDTAB_LEAF_ENTRIES].context;
> +	return &entry->ptr_leaf->entries[index % SIDTAB_LEAF_ENTRIES];
> +}
> +
> +/* use when you know that there is enough entries */
> +static struct context *sidtab_lookup_unsafe(struct sidtab *s, u32 index)
> +{
> +	return &sidtab_do_lookup(s, index, 0)->context;
>  }
>  
> -static struct context *sidtab_lookup(struct sidtab *s, u32 index)
> +static struct sidtab_entry *sidtab_lookup(struct sidtab *s, u32 index)
>  {
>  	/* read entries only after reading count */
>  	u32 count = smp_load_acquire(&s->count);
> @@ -139,33 +165,34 @@ static struct context *sidtab_lookup(struct sidtab *s, u32 index)
>  	return sidtab_do_lookup(s, index, 0);
>  }
>  
> -static struct context *sidtab_lookup_initial(struct sidtab *s, u32 sid)
> +static struct sidtab_entry *sidtab_lookup_initial(struct sidtab *s, u32 sid)
>  {
> -	return s->isids[sid - 1].set ? &s->isids[sid - 1].context : NULL;
> +	return s->isids[sid - 1].set ? &s->isids[sid - 1].entry : NULL;
>  }
>  
> -static struct context *sidtab_search_core(struct sidtab *s, u32 sid, int force)
> +static struct sidtab_entry *sidtab_search_core(struct sidtab *s, u32 sid,
> +					       int force)
>  {
> -	struct context *context;
> -
>  	if (sid != 0) {
> +		struct sidtab_entry *entry;
> +
>  		if (sid > SECINITSID_NUM)
> -			context = sidtab_lookup(s, sid - (SECINITSID_NUM + 1));
> +			entry = sidtab_lookup(s, sid - (SECINITSID_NUM + 1));
>  		else
> -			context = sidtab_lookup_initial(s, sid);
> -		if (context && (!context->len || force))
> -			return context;
> +			entry = sidtab_lookup_initial(s, sid);
> +		if (entry && (!entry->context.len || force))
> +			return entry;
>  	}
>  
>  	return sidtab_lookup_initial(s, SECINITSID_UNLABELED);
>  }
>  
> -struct context *sidtab_search(struct sidtab *s, u32 sid)
> +struct sidtab_entry *sidtab_search_entry(struct sidtab *s, u32 sid)
>  {
>  	return sidtab_search_core(s, sid, 0);
>  }
>  
> -struct context *sidtab_search_force(struct sidtab *s, u32 sid)
> +struct sidtab_entry *sidtab_search_entry_force(struct sidtab *s, u32 sid)
>  {
>  	return sidtab_search_core(s, sid, 1);
>  }
> @@ -230,7 +257,7 @@ static int sidtab_rcache_search(struct sidtab *s, struct context *context,
>  		if (v >= SIDTAB_MAX)
>  			continue;
>  
> -		if (context_cmp(sidtab_do_lookup(s, v, 0), context)) {
> +		if (context_cmp(sidtab_lookup_unsafe(s, v), context)) {
>  			sidtab_rcache_update(s, v, i);
>  			*index = v;
>  			return 0;
> @@ -245,7 +272,7 @@ static int sidtab_reverse_lookup(struct sidtab *s, struct context *context,
>  	unsigned long flags;
>  	u32 count, count_locked, level, pos;
>  	struct sidtab_convert_params *convert;
> -	struct context *dst, *dst_convert;
> +	struct sidtab_entry *dst, *dst_convert;
>  	int rc;
>  
>  	rc = sidtab_rcache_search(s, context, index);
> @@ -273,7 +300,7 @@ static int sidtab_reverse_lookup(struct sidtab *s, struct context *context,
>  
>  	/* if count has changed before we acquired the lock, then catch up */
>  	while (count < count_locked) {
> -		if (context_cmp(sidtab_do_lookup(s, count, 0), context)) {
> +		if (context_cmp(sidtab_lookup_unsafe(s, count), context)) {
>  			sidtab_rcache_push(s, count);
>  			*index = count;
>  			rc = 0;
> @@ -293,7 +320,7 @@ static int sidtab_reverse_lookup(struct sidtab *s, struct context *context,
>  	if (!dst)
>  		goto out_unlock;
>  
> -	rc = context_cpy(dst, context);
> +	rc = context_cpy(&dst->context, context);
>  	if (rc)
>  		goto out_unlock;
>  
> @@ -305,13 +332,14 @@ static int sidtab_reverse_lookup(struct sidtab *s, struct context *context,
>  		rc = -ENOMEM;
>  		dst_convert = sidtab_do_lookup(convert->target, count, 1);
>  		if (!dst_convert) {
> -			context_destroy(dst);
> +			context_destroy(&dst->context);
>  			goto out_unlock;
>  		}
>  
> -		rc = convert->func(context, dst_convert, convert->args);
> +		rc = convert->func(context, &dst_convert->context,
> +				   convert->args);
>  		if (rc) {
> -			context_destroy(dst);
> +			context_destroy(&dst->context);
>  			goto out_unlock;
>  		}
>  
> @@ -341,9 +369,9 @@ int sidtab_context_to_sid(struct sidtab *s, struct context *context, u32 *sid)
>  	u32 i;
>  
>  	for (i = 0; i < SECINITSID_NUM; i++) {
> -		struct sidtab_isid_entry *entry = &s->isids[i];
> +		struct sidtab_isid_entry *isid = &s->isids[i];
>  
> -		if (entry->set && context_cmp(context, &entry->context)) {
> +		if (isid->set && context_cmp(context, &isid->entry.context)) {
>  			*sid = i + 1;
>  			return 0;
>  		}
> @@ -453,6 +481,14 @@ int sidtab_convert(struct sidtab *s, struct sidtab_convert_params *params)
>  	return rc;
>  }
>  
> +static void sidtab_destroy_entry(struct sidtab_entry *entry)
> +{
> +	context_destroy(&entry->context);
> +#if CONFIG_SECURITY_SELINUX_SID2STR_CACHE_SIZE > 0
> +	kfree(rcu_dereference_raw(entry->cache));
> +#endif
> +}

I am assuming that this is called after all possible readers are done,
for example late in shutdown.  Or is there some way that a grace period
is guaranteed to have elapsed between the time this data was made
inaccessible to readers and when sidtab_destroy_entry() is invoked?

For example, suppose that sidtab_sid2str_get() is delayed just after
it fetches entry->cache.  How does this patch avoid freeing entry->cache
out from under that call to sidtab_sid2str_get()?

> +
>  static void sidtab_destroy_tree(union sidtab_entry_inner entry, u32 level)
>  {
>  	u32 i;
> @@ -473,7 +509,7 @@ static void sidtab_destroy_tree(union sidtab_entry_inner entry, u32 level)
>  			return;
>  
>  		for (i = 0; i < SIDTAB_LEAF_ENTRIES; i++)
> -			context_destroy(&node->entries[i].context);
> +			sidtab_destroy_entry(&node->entries[i]);
>  		kfree(node);
>  	}
>  }
> @@ -484,7 +520,7 @@ void sidtab_destroy(struct sidtab *s)
>  
>  	for (i = 0; i < SECINITSID_NUM; i++)
>  		if (s->isids[i].set)
> -			context_destroy(&s->isids[i].context);
> +			sidtab_destroy_entry(&s->isids[i].entry);
>  
>  	level = SIDTAB_MAX_LEVEL;
>  	while (level && !s->roots[level].ptr_inner)
> @@ -492,3 +528,88 @@ void sidtab_destroy(struct sidtab *s)
>  
>  	sidtab_destroy_tree(s->roots[level], level);
>  }
> +
> +#if CONFIG_SECURITY_SELINUX_SID2STR_CACHE_SIZE > 0
> +
> +void sidtab_sid2str_put(struct sidtab *s, struct sidtab_entry *entry,
> +			const char *str, u32 str_len)
> +{
> +	struct sidtab_str_cache *cache, *victim;
> +
> +	/* do not cache invalid contexts */
> +	if (entry->context.len)
> +		return;
> +
> +	/*
> +	 * Skip the put operation when in non-task context to avoid the need
> +	 * to disable interrupts while holding s->cache_lock.
> +	 */
> +	if (!in_task())
> +		return;
> +
> +	spin_lock(&s->cache_lock);
> +
> +	cache = rcu_dereference_raw(entry->cache);

If entry->cache is protected by s->cache_lock, you can do this:

	cache = rcu_dereference_protected(entry->cache, lockdep_is_held(s->cache_lock);

Yes, this looks a little silly just after the lock is acquired, but
future code changes might make the connection less obvious.  And there
is no additional overhead unless CONFIG_PROVE_LOCKING is set, which
won't normally be set in production.

> +	if (cache) {
> +		/* entry in cache - just bump to he head of LRU list */
> +		list_move(&cache->lru_member, &s->cache_lru_list);
> +		goto out_unlock;
> +	}
> +
> +	cache = kmalloc(sizeof(struct sidtab_str_cache) + str_len, GFP_ATOMIC);
> +	if (!cache)
> +		goto out_unlock;
> +
> +	if (s->cache_free_slots == 0) {
> +		/* pop a cache entry from the tail and free it */
> +		victim = container_of(s->cache_lru_list.prev,
> +				      struct sidtab_str_cache, lru_member);
> +		list_del(&victim->lru_member);
> +		kfree_rcu(victim, rcu_member);
> +		rcu_assign_pointer(victim->parent->cache, NULL);
> +	} else {
> +		s->cache_free_slots--;
> +	}
> +	cache->parent = entry;
> +	cache->len = str_len;
> +	memcpy(cache->str, str, str_len);
> +	rcu_head_init(&cache->rcu_member);

You don't need rcu_head_init() unless you are also going to be using
rcu_head_after_call_rcu(), which I don't see in this patch.  There is
only one use of rcu_head_after_call_rcu() compared to something like 300
uses of call_rcu(), so you probably won't ever need the rcu_head_init().

> +	list_add(&cache->lru_member, &s->cache_lru_list);
> +
> +	rcu_assign_pointer(entry->cache, cache);
> +
> +out_unlock:
> +	spin_unlock(&s->cache_lock);
> +}
> +
> +int sidtab_sid2str_get(struct sidtab *s, struct sidtab_entry *entry,
> +		       char **out, u32 *out_len)
> +{
> +	struct sidtab_str_cache *cache;
> +	int rc = 0;
> +
> +	if (entry->context.len)
> +		return -ENOENT; /* do not cache invalid contexts */
> +
> +	rcu_read_lock();
> +
> +	cache = rcu_dereference(entry->cache);
> +	if (!cache) {
> +		rc = -ENOENT;
> +	} else {
> +		*out_len = cache->len;

Interesting.  The reason the above assignment is outside of the "if"
below is so that the caller can learn the length by passing in NULL
for "out" and non-NULL for "out_len"?

							Thanx, Paul

> +		if (out) {
> +			*out = kmemdup(cache->str, cache->len, GFP_ATOMIC);
> +			if (!*out)
> +				rc = -ENOMEM;
> +		}
> +	}
> +
> +	rcu_read_unlock();
> +
> +	if (!rc && out)
> +		sidtab_sid2str_put(s, entry, *out, *out_len);
> +	return rc;
> +}
> +
> +#endif /* CONFIG_SECURITY_SELINUX_SID2STR_CACHE_SIZE > 0 */
> diff --git a/security/selinux/ss/sidtab.h b/security/selinux/ss/sidtab.h
> index 1f4763141aa1..5fe67a0c307b 100644
> --- a/security/selinux/ss/sidtab.h
> +++ b/security/selinux/ss/sidtab.h
> @@ -16,13 +16,13 @@
>  
>  #include "context.h"
>  
> -struct sidtab_entry_leaf {
> +struct sidtab_entry {
>  	struct context context;
> +#if CONFIG_SECURITY_SELINUX_SID2STR_CACHE_SIZE > 0
> +	struct sidtab_str_cache __rcu *cache;
> +#endif
>  };
>  
> -struct sidtab_node_inner;
> -struct sidtab_node_leaf;
> -
>  union sidtab_entry_inner {
>  	struct sidtab_node_inner *ptr_inner;
>  	struct sidtab_node_leaf  *ptr_leaf;
> @@ -38,7 +38,7 @@ union sidtab_entry_inner {
>  	(SIDTAB_NODE_ALLOC_SHIFT - size_to_shift(sizeof(union sidtab_entry_inner)))
>  #define SIDTAB_INNER_ENTRIES ((size_t)1 << SIDTAB_INNER_SHIFT)
>  #define SIDTAB_LEAF_ENTRIES \
> -	(SIDTAB_NODE_ALLOC_SIZE / sizeof(struct sidtab_entry_leaf))
> +	(SIDTAB_NODE_ALLOC_SIZE / sizeof(struct sidtab_entry))
>  
>  #define SIDTAB_MAX_BITS 32
>  #define SIDTAB_MAX U32_MAX
> @@ -48,7 +48,7 @@ union sidtab_entry_inner {
>  		     SIDTAB_INNER_SHIFT)
>  
>  struct sidtab_node_leaf {
> -	struct sidtab_entry_leaf entries[SIDTAB_LEAF_ENTRIES];
> +	struct sidtab_entry entries[SIDTAB_LEAF_ENTRIES];
>  };
>  
>  struct sidtab_node_inner {
> @@ -57,7 +57,7 @@ struct sidtab_node_inner {
>  
>  struct sidtab_isid_entry {
>  	int set;
> -	struct context context;
> +	struct sidtab_entry entry;
>  };
>  
>  struct sidtab_convert_params {
> @@ -83,6 +83,13 @@ struct sidtab {
>  	struct sidtab_convert_params *convert;
>  	spinlock_t lock;
>  
> +#if CONFIG_SECURITY_SELINUX_SID2STR_CACHE_SIZE > 0
> +	/* SID -> context string cache */
> +	u32 cache_free_slots;
> +	struct list_head cache_lru_list;
> +	spinlock_t cache_lock;
> +#endif
> +
>  	/* reverse lookup cache - access atomically via {READ|WRITE}_ONCE() */
>  	u32 rcache[SIDTAB_RCACHE_SIZE];
>  
> @@ -92,8 +99,22 @@ struct sidtab {
>  
>  int sidtab_init(struct sidtab *s);
>  int sidtab_set_initial(struct sidtab *s, u32 sid, struct context *context);
> -struct context *sidtab_search(struct sidtab *s, u32 sid);
> -struct context *sidtab_search_force(struct sidtab *s, u32 sid);
> +struct sidtab_entry *sidtab_search_entry(struct sidtab *s, u32 sid);
> +struct sidtab_entry *sidtab_search_entry_force(struct sidtab *s, u32 sid);
> +
> +static inline struct context *sidtab_search(struct sidtab *s, u32 sid)
> +{
> +	struct sidtab_entry *entry = sidtab_search_entry(s, sid);
> +
> +	return entry ? &entry->context : NULL;
> +}
> +
> +static inline struct context *sidtab_search_force(struct sidtab *s, u32 sid)
> +{
> +	struct sidtab_entry *entry = sidtab_search_entry_force(s, sid);
> +
> +	return entry ? &entry->context : NULL;
> +}
>  
>  int sidtab_convert(struct sidtab *s, struct sidtab_convert_params *params);
>  
> @@ -101,6 +122,25 @@ int sidtab_context_to_sid(struct sidtab *s, struct context *context, u32 *sid);
>  
>  void sidtab_destroy(struct sidtab *s);
>  
> +#if CONFIG_SECURITY_SELINUX_SID2STR_CACHE_SIZE > 0
> +void sidtab_sid2str_put(struct sidtab *s, struct sidtab_entry *entry,
> +			const char *str, u32 str_len);
> +int sidtab_sid2str_get(struct sidtab *s, struct sidtab_entry *entry,
> +		       char **out, u32 *out_len);
> +#else
> +static inline void sidtab_sid2str_put(struct sidtab *s,
> +				      struct sidtab_entry *entry,
> +				      const char *str, u32 str_len)
> +{
> +}
> +static inline int sidtab_sid2str_get(struct sidtab *s,
> +				     struct sidtab_entry *entry,
> +				     char **out, u32 *out_len)
> +{
> +	return -ENOENT;
> +}
> +#endif /* CONFIG_SECURITY_SELINUX_SID2STR_CACHE_SIZE > 0 */
> +
>  #endif	/* _SS_SIDTAB_H_ */
>  
>  
> -- 
> 2.21.0
> 

^ permalink raw reply	[flat|nested] 4+ messages in thread

* Re: [PATCH v3] selinux: cache the SID -> context string translation
  2019-11-10 21:03 ` Paul E. McKenney
@ 2019-11-11  8:32   ` Ondrej Mosnacek
  0 siblings, 0 replies; 4+ messages in thread
From: Ondrej Mosnacek @ 2019-11-11  8:32 UTC (permalink / raw)
  To: Paul E. McKenney
  Cc: SElinux list, Paul Moore, Stephen Smalley, rcu, Michal Sekletar

On Sun, Nov 10, 2019 at 10:03 PM Paul E. McKenney <paulmck@kernel.org> wrote:
> On Fri, Nov 08, 2019 at 09:39:09AM +0100, Ondrej Mosnacek wrote:
> > +static void sidtab_destroy_entry(struct sidtab_entry *entry)
> > +{
> > +     context_destroy(&entry->context);
> > +#if CONFIG_SECURITY_SELINUX_SID2STR_CACHE_SIZE > 0
> > +     kfree(rcu_dereference_raw(entry->cache));
> > +#endif
> > +}
>
> I am assuming that this is called after all possible readers are done,
> for example late in shutdown.  Or is there some way that a grace period
> is guaranteed to have elapsed between the time this data was made
> inaccessible to readers and when sidtab_destroy_entry() is invoked?
>
> For example, suppose that sidtab_sid2str_get() is delayed just after
> it fetches entry->cache.  How does this patch avoid freeing entry->cache
> out from under that call to sidtab_sid2str_get()?

It is not visible in this patch, but the sidtab (along with other
policy-lifetime structures) is protected by a big fat read-write lock.
The only places where sidtab_destroy() is called are (a) error paths
when initializing a new sidtab (here the sidtab isn't shared yet, so
no race) and (b) when freeing the old sidtab during policy reload - in
this case it is happening after a policy write-locked critical
section, which had removed the old sidtab pointer from the shared
structures, so at that point all sidtab readers will already be
accessing the new sidtab and the old one is visible only by the thread
doing the destruction.

>
> > +
> >  static void sidtab_destroy_tree(union sidtab_entry_inner entry, u32 level)
> >  {
> >       u32 i;
> > @@ -473,7 +509,7 @@ static void sidtab_destroy_tree(union sidtab_entry_inner entry, u32 level)
> >                       return;
> >
> >               for (i = 0; i < SIDTAB_LEAF_ENTRIES; i++)
> > -                     context_destroy(&node->entries[i].context);
> > +                     sidtab_destroy_entry(&node->entries[i]);
> >               kfree(node);
> >       }
> >  }
> > @@ -484,7 +520,7 @@ void sidtab_destroy(struct sidtab *s)
> >
> >       for (i = 0; i < SECINITSID_NUM; i++)
> >               if (s->isids[i].set)
> > -                     context_destroy(&s->isids[i].context);
> > +                     sidtab_destroy_entry(&s->isids[i].entry);
> >
> >       level = SIDTAB_MAX_LEVEL;
> >       while (level && !s->roots[level].ptr_inner)
> > @@ -492,3 +528,88 @@ void sidtab_destroy(struct sidtab *s)
> >
> >       sidtab_destroy_tree(s->roots[level], level);
> >  }
> > +
> > +#if CONFIG_SECURITY_SELINUX_SID2STR_CACHE_SIZE > 0
> > +
> > +void sidtab_sid2str_put(struct sidtab *s, struct sidtab_entry *entry,
> > +                     const char *str, u32 str_len)
> > +{
> > +     struct sidtab_str_cache *cache, *victim;
> > +
> > +     /* do not cache invalid contexts */
> > +     if (entry->context.len)
> > +             return;
> > +
> > +     /*
> > +      * Skip the put operation when in non-task context to avoid the need
> > +      * to disable interrupts while holding s->cache_lock.
> > +      */
> > +     if (!in_task())
> > +             return;
> > +
> > +     spin_lock(&s->cache_lock);
> > +
> > +     cache = rcu_dereference_raw(entry->cache);
>
> If entry->cache is protected by s->cache_lock, you can do this:
>
>         cache = rcu_dereference_protected(entry->cache, lockdep_is_held(s->cache_lock);
>
> Yes, this looks a little silly just after the lock is acquired, but
> future code changes might make the connection less obvious.  And there
> is no additional overhead unless CONFIG_PROVE_LOCKING is set, which
> won't normally be set in production.

Yeah, I realized this shortly after sending the patch. Queued this
change for v4.

>
> > +     if (cache) {
> > +             /* entry in cache - just bump to he head of LRU list */

Aaand I just spotted a typo here (he -> the). Also queued for v4.

> > +             list_move(&cache->lru_member, &s->cache_lru_list);
> > +             goto out_unlock;
> > +     }
> > +
> > +     cache = kmalloc(sizeof(struct sidtab_str_cache) + str_len, GFP_ATOMIC);
> > +     if (!cache)
> > +             goto out_unlock;
> > +
> > +     if (s->cache_free_slots == 0) {
> > +             /* pop a cache entry from the tail and free it */
> > +             victim = container_of(s->cache_lru_list.prev,
> > +                                   struct sidtab_str_cache, lru_member);
> > +             list_del(&victim->lru_member);
> > +             kfree_rcu(victim, rcu_member);
> > +             rcu_assign_pointer(victim->parent->cache, NULL);
> > +     } else {
> > +             s->cache_free_slots--;
> > +     }
> > +     cache->parent = entry;
> > +     cache->len = str_len;
> > +     memcpy(cache->str, str, str_len);
> > +     rcu_head_init(&cache->rcu_member);
>
> You don't need rcu_head_init() unless you are also going to be using
> rcu_head_after_call_rcu(), which I don't see in this patch.  There is
> only one use of rcu_head_after_call_rcu() compared to something like 300
> uses of call_rcu(), so you probably won't ever need the rcu_head_init().

Ah, right. I just instinctively tried to keep everything initialized
and since I saw that rcu_head_init() exists I figured I should better
call it here... But it makes sense that rcu_member won't be touched
until call_rcu(), which can just initialize it itself. I'll drop the
call in v4.

>
> > +     list_add(&cache->lru_member, &s->cache_lru_list);
> > +
> > +     rcu_assign_pointer(entry->cache, cache);
> > +
> > +out_unlock:
> > +     spin_unlock(&s->cache_lock);
> > +}
> > +
> > +int sidtab_sid2str_get(struct sidtab *s, struct sidtab_entry *entry,
> > +                    char **out, u32 *out_len)
> > +{
> > +     struct sidtab_str_cache *cache;
> > +     int rc = 0;
> > +
> > +     if (entry->context.len)
> > +             return -ENOENT; /* do not cache invalid contexts */
> > +
> > +     rcu_read_lock();
> > +
> > +     cache = rcu_dereference(entry->cache);
> > +     if (!cache) {
> > +             rc = -ENOENT;
> > +     } else {
> > +             *out_len = cache->len;
>
> Interesting.  The reason the above assignment is outside of the "if"
> below is so that the caller can learn the length by passing in NULL
> for "out" and non-NULL for "out_len"?

Yes, I was mirroring the logic of context_struct_to_string() (the
target of the caching), which seems to intentionally support such
feature. I'm not sure if it is ever used, but it's safer not to break
the contract.

Thank you for the review, much appreciated!

>
>                                                         Thanx, Paul
>
> > +             if (out) {
> > +                     *out = kmemdup(cache->str, cache->len, GFP_ATOMIC);
> > +                     if (!*out)
> > +                             rc = -ENOMEM;
> > +             }
> > +     }

--
Ondrej Mosnacek <omosnace at redhat dot com>
Software Engineer, Security Technologies
Red Hat, Inc.


^ permalink raw reply	[flat|nested] 4+ messages in thread

end of thread, back to index

Thread overview: 4+ messages (download: mbox.gz / follow: Atom feed)
-- links below jump to the message on this page --
2019-11-08  8:39 [PATCH v3] selinux: cache the SID -> context string translation Ondrej Mosnacek
2019-11-08 14:09 ` Stephen Smalley
2019-11-10 21:03 ` Paul E. McKenney
2019-11-11  8:32   ` Ondrej Mosnacek

RCU Archive on lore.kernel.org

Archives are clonable:
	git clone --mirror https://lore.kernel.org/rcu/0 rcu/git/0.git

	# If you have public-inbox 1.1+ installed, you may
	# initialize and index your mirror using the following commands:
	public-inbox-init -V2 rcu rcu/ https://lore.kernel.org/rcu \
		rcu@vger.kernel.org
	public-inbox-index rcu

Example config snippet for mirrors

Newsgroup available over NNTP:
	nntp://nntp.lore.kernel.org/org.kernel.vger.rcu


AGPL code for this site: git clone https://public-inbox.org/public-inbox.git