[PATCH v4] selinux: cache the SID -> context string translation

[Ondrej Mosnacek <omosnace@redhat.com>]

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 v4:
 - use rcu_dereference_protected() instead of rcu_dereference_raw() in
   sidtab_sid2str_put()
 - fix typo in comment
 - remove unnecessary rcu_head_init() call

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..6d6ce1c43b49 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_protected(entry->cache,
+					  lockdep_is_held(&s->cache_lock));
+	if (cache) {
+		/* entry in cache - just bump to the 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);
+	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