[PATCH] KEYS: Fix race between updating and finding a negative key

* [PATCH] KEYS: Fix race between updating and finding a negative key
@ 2017-10-24 20:06 Eric Biggers
  2017-10-25  9:41 ` Greg KH
  0 siblings, 1 reply; 3+ messages in thread
From: Eric Biggers @ 2017-10-24 20:06 UTC (permalink / raw)
  To: stable; +Cc: David Howells, Eric Biggers

From: David Howells <dhowells@redhat.com>

commit 363b02dab09b3226f3bd1420dad9c72b79a42a76 upstream.  Please apply
to 4.4-stable.

Consolidate KEY_FLAG_INSTANTIATED, KEY_FLAG_NEGATIVE and the rejection
error into one field such that:

 (1) The instantiation state can be modified/read atomically.

 (2) The error can be accessed atomically with the state.

 (3) The error isn't stored unioned with the payload pointers.

This deals with the problem that the state is spread over three different
objects (two bits and a separate variable) and reading or updating them
atomically isn't practical, given that not only can uninstantiated keys
change into instantiated or rejected keys, but rejected keys can also turn
into instantiated keys - and someone accessing the key might not be using
any locking.

The main side effect of this problem is that what was held in the payload
may change, depending on the state.  For instance, you might observe the
key to be in the rejected state.  You then read the cached error, but if
the key semaphore wasn't locked, the key might've become instantiated
between the two reads - and you might now have something in hand that isn't
actually an error code.

The state is now KEY_IS_UNINSTANTIATED, KEY_IS_POSITIVE or a negative error
code if the key is negatively instantiated.  The key_is_instantiated()
function is replaced with key_is_positive() to avoid confusion as negative
keys are also 'instantiated'.

Additionally, barriering is included:

 (1) Order payload-set before state-set during instantiation.

 (2) Order state-read before payload-read when using the key.

Further separate barriering is necessary if RCU is being used to access the
payload content after reading the payload pointers.

Fixes: 146aa8b1453b ("KEYS: Merge the type-specific data with the payload data")
Cc: stable@vger.kernel.org # v4.4+
Reported-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: David Howells <dhowells@redhat.com>
Reviewed-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Eric Biggers <ebiggers@google.com>
---
 include/linux/key.h                      | 49 ++++++++++++++++++++------------
 net/dns_resolver/dns_key.c               |  2 +-
 security/keys/big_key.c                  |  4 +--
 security/keys/encrypted-keys/encrypted.c |  2 +-
 security/keys/gc.c                       |  8 +++---
 security/keys/key.c                      | 31 +++++++++++++-------
 security/keys/keyctl.c                   |  9 +++---
 security/keys/keyring.c                  | 10 +++----
 security/keys/proc.c                     |  7 +++--
 security/keys/process_keys.c             |  2 +-
 security/keys/request_key.c              |  7 ++---
 security/keys/request_key_auth.c         |  2 +-
 security/keys/trusted.c                  |  2 +-
 security/keys/user_defined.c             |  4 +--
 14 files changed, 81 insertions(+), 58 deletions(-)

diff --git a/include/linux/key.h b/include/linux/key.h
index dcc115e8dd03..af071ca73079 100644
--- a/include/linux/key.h
+++ b/include/linux/key.h
@@ -126,6 +126,11 @@ static inline bool is_key_possessed(const key_ref_t key_ref)
 	return (unsigned long) key_ref & 1UL;
 }
 
+enum key_state {
+	KEY_IS_UNINSTANTIATED,
+	KEY_IS_POSITIVE,		/* Positively instantiated */
+};
+
 /*****************************************************************************/
 /*
  * authentication token / access credential / keyring
@@ -157,6 +162,7 @@ struct key {
 						 * - may not match RCU dereferenced payload
 						 * - payload should contain own length
 						 */
+	short			state;		/* Key state (+) or rejection error (-) */
 
 #ifdef KEY_DEBUGGING
 	unsigned		magic;
@@ -165,19 +171,17 @@ struct key {
 #endif
 
 	unsigned long		flags;		/* status flags (change with bitops) */
-#define KEY_FLAG_INSTANTIATED	0	/* set if key has been instantiated */
-#define KEY_FLAG_DEAD		1	/* set if key type has been deleted */
-#define KEY_FLAG_REVOKED	2	/* set if key had been revoked */
-#define KEY_FLAG_IN_QUOTA	3	/* set if key consumes quota */
-#define KEY_FLAG_USER_CONSTRUCT	4	/* set if key is being constructed in userspace */
-#define KEY_FLAG_NEGATIVE	5	/* set if key is negative */
-#define KEY_FLAG_ROOT_CAN_CLEAR	6	/* set if key can be cleared by root without permission */
-#define KEY_FLAG_INVALIDATED	7	/* set if key has been invalidated */
-#define KEY_FLAG_TRUSTED	8	/* set if key is trusted */
-#define KEY_FLAG_TRUSTED_ONLY	9	/* set if keyring only accepts links to trusted keys */
-#define KEY_FLAG_BUILTIN	10	/* set if key is builtin */
-#define KEY_FLAG_ROOT_CAN_INVAL	11	/* set if key can be invalidated by root without permission */
-#define KEY_FLAG_UID_KEYRING	12	/* set if key is a user or user session keyring */
+#define KEY_FLAG_DEAD		0	/* set if key type has been deleted */
+#define KEY_FLAG_REVOKED	1	/* set if key had been revoked */
+#define KEY_FLAG_IN_QUOTA	2	/* set if key consumes quota */
+#define KEY_FLAG_USER_CONSTRUCT	3	/* set if key is being constructed in userspace */
+#define KEY_FLAG_ROOT_CAN_CLEAR	4	/* set if key can be cleared by root without permission */
+#define KEY_FLAG_INVALIDATED	5	/* set if key has been invalidated */
+#define KEY_FLAG_TRUSTED	6	/* set if key is trusted */
+#define KEY_FLAG_TRUSTED_ONLY	7	/* set if keyring only accepts links to trusted keys */
+#define KEY_FLAG_BUILTIN	8	/* set if key is builtin */
+#define KEY_FLAG_ROOT_CAN_INVAL	9	/* set if key can be invalidated by root without permission */
+#define KEY_FLAG_UID_KEYRING	10	/* set if key is a user or user session keyring */
 
 	/* the key type and key description string
 	 * - the desc is used to match a key against search criteria
@@ -203,7 +207,6 @@ struct key {
 			struct list_head name_link;
 			struct assoc_array keys;
 		};
-		int reject_error;
 	};
 };
 
@@ -319,17 +322,27 @@ extern void key_set_timeout(struct key *, unsigned);
 #define	KEY_NEED_SETATTR 0x20	/* Require permission to change attributes */
 #define	KEY_NEED_ALL	0x3f	/* All the above permissions */
 
+static inline short key_read_state(const struct key *key)
+{
+	/* Barrier versus mark_key_instantiated(). */
+	return smp_load_acquire(&key->state);
+}
+
 /**
- * key_is_instantiated - Determine if a key has been positively instantiated
+ * key_is_positive - Determine if a key has been positively instantiated
  * @key: The key to check.
  *
  * Return true if the specified key has been positively instantiated, false
  * otherwise.
  */
-static inline bool key_is_instantiated(const struct key *key)
+static inline bool key_is_positive(const struct key *key)
+{
+	return key_read_state(key) == KEY_IS_POSITIVE;
+}
+
+static inline bool key_is_negative(const struct key *key)
 {
-	return test_bit(KEY_FLAG_INSTANTIATED, &key->flags) &&
-		!test_bit(KEY_FLAG_NEGATIVE, &key->flags);
+	return key_read_state(key) < 0;
 }
 
 #define rcu_dereference_key(KEY)					\
diff --git a/net/dns_resolver/dns_key.c b/net/dns_resolver/dns_key.c
index c79b85eb4d4c..6abc5012200b 100644
--- a/net/dns_resolver/dns_key.c
+++ b/net/dns_resolver/dns_key.c
@@ -224,7 +224,7 @@ static int dns_resolver_match_preparse(struct key_match_data *match_data)
 static void dns_resolver_describe(const struct key *key, struct seq_file *m)
 {
 	seq_puts(m, key->description);
-	if (key_is_instantiated(key)) {
+	if (key_is_positive(key)) {
 		int err = PTR_ERR(key->payload.data[dns_key_error]);
 
 		if (err)
diff --git a/security/keys/big_key.c b/security/keys/big_key.c
index 907c1522ee46..08c4cc5c2973 100644
--- a/security/keys/big_key.c
+++ b/security/keys/big_key.c
@@ -138,7 +138,7 @@ void big_key_revoke(struct key *key)
 
 	/* clear the quota */
 	key_payload_reserve(key, 0);
-	if (key_is_instantiated(key) &&
+	if (key_is_positive(key) &&
 	    (size_t)key->payload.data[big_key_len] > BIG_KEY_FILE_THRESHOLD)
 		vfs_truncate(path, 0);
 }
@@ -170,7 +170,7 @@ void big_key_describe(const struct key *key, struct seq_file *m)
 
 	seq_puts(m, key->description);
 
-	if (key_is_instantiated(key))
+	if (key_is_positive(key))
 		seq_printf(m, ": %zu [%s]",
 			   datalen,
 			   datalen > BIG_KEY_FILE_THRESHOLD ? "file" : "buff");
diff --git a/security/keys/encrypted-keys/encrypted.c b/security/keys/encrypted-keys/encrypted.c
index dbd75de136d7..ce295c0c1da0 100644
--- a/security/keys/encrypted-keys/encrypted.c
+++ b/security/keys/encrypted-keys/encrypted.c
@@ -852,7 +852,7 @@ static int encrypted_update(struct key *key, struct key_preparsed_payload *prep)
 	size_t datalen = prep->datalen;
 	int ret = 0;
 
-	if (test_bit(KEY_FLAG_NEGATIVE, &key->flags))
+	if (key_is_negative(key))
 		return -ENOKEY;
 	if (datalen <= 0 || datalen > 32767 || !prep->data)
 		return -EINVAL;
diff --git a/security/keys/gc.c b/security/keys/gc.c
index 9cb4fe4478a1..1659094d684d 100644
--- a/security/keys/gc.c
+++ b/security/keys/gc.c
@@ -129,15 +129,15 @@ static noinline void key_gc_unused_keys(struct list_head *keys)
 	while (!list_empty(keys)) {
 		struct key *key =
 			list_entry(keys->next, struct key, graveyard_link);
+		short state = key->state;
+
 		list_del(&key->graveyard_link);
 
 		kdebug("- %u", key->serial);
 		key_check(key);
 
 		/* Throw away the key data if the key is instantiated */
-		if (test_bit(KEY_FLAG_INSTANTIATED, &key->flags) &&
-		    !test_bit(KEY_FLAG_NEGATIVE, &key->flags) &&
-		    key->type->destroy)
+		if (state == KEY_IS_POSITIVE && key->type->destroy)
 			key->type->destroy(key);
 
 		security_key_free(key);
@@ -151,7 +151,7 @@ static noinline void key_gc_unused_keys(struct list_head *keys)
 		}
 
 		atomic_dec(&key->user->nkeys);
-		if (test_bit(KEY_FLAG_INSTANTIATED, &key->flags))
+		if (state != KEY_IS_UNINSTANTIATED)
 			atomic_dec(&key->user->nikeys);
 
 		key_user_put(key->user);
diff --git a/security/keys/key.c b/security/keys/key.c
index 2751ab4a7946..4d971bf88ac3 100644
--- a/security/keys/key.c
+++ b/security/keys/key.c
@@ -395,6 +395,18 @@ int key_payload_reserve(struct key *key, size_t datalen)
 }
 EXPORT_SYMBOL(key_payload_reserve);
 
+/*
+ * Change the key state to being instantiated.
+ */
+static void mark_key_instantiated(struct key *key, int reject_error)
+{
+	/* Commit the payload before setting the state; barrier versus
+	 * key_read_state().
+	 */
+	smp_store_release(&key->state,
+			  (reject_error < 0) ? reject_error : KEY_IS_POSITIVE);
+}
+
 /*
  * Instantiate a key and link it into the target keyring atomically.  Must be
  * called with the target keyring's semaphore writelocked.  The target key's
@@ -418,14 +430,14 @@ static int __key_instantiate_and_link(struct key *key,
 	mutex_lock(&key_construction_mutex);
 
 	/* can't instantiate twice */
-	if (!test_bit(KEY_FLAG_INSTANTIATED, &key->flags)) {
+	if (key->state == KEY_IS_UNINSTANTIATED) {
 		/* instantiate the key */
 		ret = key->type->instantiate(key, prep);
 
 		if (ret == 0) {
 			/* mark the key as being instantiated */
 			atomic_inc(&key->user->nikeys);
-			set_bit(KEY_FLAG_INSTANTIATED, &key->flags);
+			mark_key_instantiated(key, 0);
 
 			if (test_and_clear_bit(KEY_FLAG_USER_CONSTRUCT, &key->flags))
 				awaken = 1;
@@ -553,13 +565,10 @@ int key_reject_and_link(struct key *key,
 	mutex_lock(&key_construction_mutex);
 
 	/* can't instantiate twice */
-	if (!test_bit(KEY_FLAG_INSTANTIATED, &key->flags)) {
+	if (key->state == KEY_IS_UNINSTANTIATED) {
 		/* mark the key as being negatively instantiated */
 		atomic_inc(&key->user->nikeys);
-		key->reject_error = -error;
-		smp_wmb();
-		set_bit(KEY_FLAG_NEGATIVE, &key->flags);
-		set_bit(KEY_FLAG_INSTANTIATED, &key->flags);
+		mark_key_instantiated(key, -error);
 		now = current_kernel_time();
 		key->expiry = now.tv_sec + timeout;
 		key_schedule_gc(key->expiry + key_gc_delay);
@@ -731,8 +740,8 @@ static inline key_ref_t __key_update(key_ref_t key_ref,
 
 	ret = key->type->update(key, prep);
 	if (ret == 0)
-		/* updating a negative key instantiates it */
-		clear_bit(KEY_FLAG_NEGATIVE, &key->flags);
+		/* Updating a negative key positively instantiates it */
+		mark_key_instantiated(key, 0);
 
 	up_write(&key->sem);
 
@@ -967,8 +976,8 @@ int key_update(key_ref_t key_ref, const void *payload, size_t plen)
 
 	ret = key->type->update(key, &prep);
 	if (ret == 0)
-		/* updating a negative key instantiates it */
-		clear_bit(KEY_FLAG_NEGATIVE, &key->flags);
+		/* Updating a negative key positively instantiates it */
+		mark_key_instantiated(key, 0);
 
 	up_write(&key->sem);
 
diff --git a/security/keys/keyctl.c b/security/keys/keyctl.c
index a009dc66eb8f..2e741e1a8712 100644
--- a/security/keys/keyctl.c
+++ b/security/keys/keyctl.c
@@ -738,10 +738,9 @@ long keyctl_read_key(key_serial_t keyid, char __user *buffer, size_t buflen)
 
 	key = key_ref_to_ptr(key_ref);
 
-	if (test_bit(KEY_FLAG_NEGATIVE, &key->flags)) {
-		ret = -ENOKEY;
-		goto error2;
-	}
+	ret = key_read_state(key);
+	if (ret < 0)
+		goto error2; /* Negatively instantiated */
 
 	/* see if we can read it directly */
 	ret = key_permission(key_ref, KEY_NEED_READ);
@@ -873,7 +872,7 @@ long keyctl_chown_key(key_serial_t id, uid_t user, gid_t group)
 		atomic_dec(&key->user->nkeys);
 		atomic_inc(&newowner->nkeys);
 
-		if (test_bit(KEY_FLAG_INSTANTIATED, &key->flags)) {
+		if (key->state != KEY_IS_UNINSTANTIATED) {
 			atomic_dec(&key->user->nikeys);
 			atomic_inc(&newowner->nikeys);
 		}
diff --git a/security/keys/keyring.c b/security/keys/keyring.c
index 0c8dd4fbe130..ef828238cdc0 100644
--- a/security/keys/keyring.c
+++ b/security/keys/keyring.c
@@ -407,7 +407,7 @@ static void keyring_describe(const struct key *keyring, struct seq_file *m)
 	else
 		seq_puts(m, "[anon]");
 
-	if (key_is_instantiated(keyring)) {
+	if (key_is_positive(keyring)) {
 		if (keyring->keys.nr_leaves_on_tree != 0)
 			seq_printf(m, ": %lu", keyring->keys.nr_leaves_on_tree);
 		else
@@ -522,7 +522,8 @@ static int keyring_search_iterator(const void *object, void *iterator_data)
 {
 	struct keyring_search_context *ctx = iterator_data;
 	const struct key *key = keyring_ptr_to_key(object);
-	unsigned long kflags = key->flags;
+	unsigned long kflags = READ_ONCE(key->flags);
+	short state = READ_ONCE(key->state);
 
 	kenter("{%d}", key->serial);
 
@@ -566,9 +567,8 @@ static int keyring_search_iterator(const void *object, void *iterator_data)
 
 	if (ctx->flags & KEYRING_SEARCH_DO_STATE_CHECK) {
 		/* we set a different error code if we pass a negative key */
-		if (kflags & (1 << KEY_FLAG_NEGATIVE)) {
-			smp_rmb();
-			ctx->result = ERR_PTR(key->reject_error);
+		if (state < 0) {
+			ctx->result = ERR_PTR(state);
 			kleave(" = %d [neg]", ctx->skipped_ret);
 			goto skipped;
 		}
diff --git a/security/keys/proc.c b/security/keys/proc.c
index b9f531c9e4fa..036128682463 100644
--- a/security/keys/proc.c
+++ b/security/keys/proc.c
@@ -182,6 +182,7 @@ static int proc_keys_show(struct seq_file *m, void *v)
 	unsigned long timo;
 	key_ref_t key_ref, skey_ref;
 	char xbuf[16];
+	short state;
 	int rc;
 
 	struct keyring_search_context ctx = {
@@ -240,17 +241,19 @@ static int proc_keys_show(struct seq_file *m, void *v)
 			sprintf(xbuf, "%luw", timo / (60*60*24*7));
 	}
 
+	state = key_read_state(key);
+
 #define showflag(KEY, LETTER, FLAG) \
 	(test_bit(FLAG,	&(KEY)->flags) ? LETTER : '-')
 
 	seq_printf(m, "%08x %c%c%c%c%c%c%c %5d %4s %08x %5d %5d %-9.9s ",
 		   key->serial,
-		   showflag(key, 'I', KEY_FLAG_INSTANTIATED),
+		   state != KEY_IS_UNINSTANTIATED ? 'I' : '-',
 		   showflag(key, 'R', KEY_FLAG_REVOKED),
 		   showflag(key, 'D', KEY_FLAG_DEAD),
 		   showflag(key, 'Q', KEY_FLAG_IN_QUOTA),
 		   showflag(key, 'U', KEY_FLAG_USER_CONSTRUCT),
-		   showflag(key, 'N', KEY_FLAG_NEGATIVE),
+		   state < 0 ? 'N' : '-',
 		   showflag(key, 'i', KEY_FLAG_INVALIDATED),
 		   atomic_read(&key->usage),
 		   xbuf,
diff --git a/security/keys/process_keys.c b/security/keys/process_keys.c
index 7dd050f24261..ac1d5b2b1626 100644
--- a/security/keys/process_keys.c
+++ b/security/keys/process_keys.c
@@ -727,7 +727,7 @@ try_again:
 
 	ret = -EIO;
 	if (!(lflags & KEY_LOOKUP_PARTIAL) &&
-	    !test_bit(KEY_FLAG_INSTANTIATED, &key->flags))
+	    key_read_state(key) == KEY_IS_UNINSTANTIATED)
 		goto invalid_key;
 
 	/* check the permissions */
diff --git a/security/keys/request_key.c b/security/keys/request_key.c
index c7a117c9a8f3..2ce733342b5a 100644
--- a/security/keys/request_key.c
+++ b/security/keys/request_key.c
@@ -594,10 +594,9 @@ int wait_for_key_construction(struct key *key, bool intr)
 			  intr ? TASK_INTERRUPTIBLE : TASK_UNINTERRUPTIBLE);
 	if (ret)
 		return -ERESTARTSYS;
-	if (test_bit(KEY_FLAG_NEGATIVE, &key->flags)) {
-		smp_rmb();
-		return key->reject_error;
-	}
+	ret = key_read_state(key);
+	if (ret < 0)
+		return ret;
 	return key_validate(key);
 }
 EXPORT_SYMBOL(wait_for_key_construction);
diff --git a/security/keys/request_key_auth.c b/security/keys/request_key_auth.c
index 4f0f112fe276..217775fcd0f3 100644
--- a/security/keys/request_key_auth.c
+++ b/security/keys/request_key_auth.c
@@ -73,7 +73,7 @@ static void request_key_auth_describe(const struct key *key,
 
 	seq_puts(m, "key:");
 	seq_puts(m, key->description);
-	if (key_is_instantiated(key))
+	if (key_is_positive(key))
 		seq_printf(m, " pid:%d ci:%zu", rka->pid, rka->callout_len);
 }
 
diff --git a/security/keys/trusted.c b/security/keys/trusted.c
index 16dec53184b6..509aedcf8310 100644
--- a/security/keys/trusted.c
+++ b/security/keys/trusted.c
@@ -1014,7 +1014,7 @@ static int trusted_update(struct key *key, struct key_preparsed_payload *prep)
 	char *datablob;
 	int ret = 0;
 
-	if (test_bit(KEY_FLAG_NEGATIVE, &key->flags))
+	if (key_is_negative(key))
 		return -ENOKEY;
 	p = key->payload.data[0];
 	if (!p->migratable)
diff --git a/security/keys/user_defined.c b/security/keys/user_defined.c
index 8705d79b2c6f..eba8a516ee9e 100644
--- a/security/keys/user_defined.c
+++ b/security/keys/user_defined.c
@@ -120,7 +120,7 @@ int user_update(struct key *key, struct key_preparsed_payload *prep)
 
 	if (ret == 0) {
 		/* attach the new data, displacing the old */
-		if (!test_bit(KEY_FLAG_NEGATIVE, &key->flags))
+		if (key_is_positive(key))
 			zap = key->payload.data[0];
 		else
 			zap = NULL;
@@ -174,7 +174,7 @@ EXPORT_SYMBOL_GPL(user_destroy);
 void user_describe(const struct key *key, struct seq_file *m)
 {
 	seq_puts(m, key->description);
-	if (key_is_instantiated(key))
+	if (key_is_positive(key))
 		seq_printf(m, ": %u", key->datalen);
 }
 
-- 
2.15.0.rc0.271.g36b669edcc-goog

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