From 363b02dab09b3226f3bd1420dad9c72b79a42a76 Mon Sep 17 00:00:00 2001
From: David Howells <dhowells@redhat.com>
Date: Wed, 4 Oct 2017 16:43:25 +0100
Subject: [PATCH] KEYS: Fix race between updating and finding a negative key

References: CVE-2017-15951, bsc#1065615, bsc#1071927
Patch-mainline: v4.14-rc6
Git-commit: 363b02dab09b3226f3bd1420dad9c72b79a42a76

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>
Acked-by: Michal Suchanek <msuchanek@suse.de>
---
 include/linux/key.h                      | 45 ++++++++++++++++++++------------
 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, 79 insertions(+), 56 deletions(-)

--- a/include/linux/key.h
+++ b/include/linux/key.h
@@ -138,6 +138,11 @@
 	struct key_type *keytype;
 };
 
+enum key_state {
+	KEY_IS_UNINSTANTIATED,
+	KEY_IS_POSITIVE,		/* Positively instantiated */
+};
+
 /*****************************************************************************/
 /*
  * authentication token / access credential / keyring
@@ -169,6 +174,7 @@
 						 * - may not match RCU dereferenced payload
 						 * - payload should contain own length
 						 */
+	short			state;		/* Key state (+) or rejection error (-) */
 
 #ifdef KEY_DEBUGGING
 	unsigned		magic;
@@ -176,18 +182,16 @@
 #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_BUILTIN	8	/* set if key is built in to the kernel */
-#define KEY_FLAG_ROOT_CAN_INVAL	9	/* set if key can be invalidated by root without permission */
-#define KEY_FLAG_KEEP		10	/* set if key should not be removed */
-#define KEY_FLAG_UID_KEYRING	11	/* 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_BUILTIN	6	/* set if key is built in to the kernel */
+#define KEY_FLAG_ROOT_CAN_INVAL	7	/* set if key can be invalidated by root without permission */
+#define KEY_FLAG_KEEP		8	/* set if key should not be removed */
+#define KEY_FLAG_UID_KEYRING	9	/* 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
@@ -213,7 +217,6 @@
 			struct list_head name_link;
 			struct assoc_array keys;
 		};
-		int reject_error;
 	};
 
 	/* This is set on a keyring to restrict the addition of a link to a key
@@ -353,17 +356,27 @@
 #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 dereference_key_rcu(KEY)					\
--- a/net/dns_resolver/dns_key.c
+++ b/net/dns_resolver/dns_key.c
@@ -224,7 +224,7 @@
 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)
--- a/security/keys/big_key.c
+++ b/security/keys/big_key.c
@@ -240,7 +240,7 @@
 
 	/* 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);
 }
@@ -272,7 +272,7 @@
 
 	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");
--- a/security/keys/encrypted-keys/encrypted.c
+++ b/security/keys/encrypted-keys/encrypted.c
@@ -854,7 +854,7 @@
 	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;
--- a/security/keys/gc.c
+++ b/security/keys/gc.c
@@ -129,15 +129,15 @@
 	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 @@
 		}
 
 		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);
--- a/security/keys/key.c
+++ b/security/keys/key.c
@@ -402,6 +402,18 @@
 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
  * semaphore need not be locked as instantiation is serialised by
@@ -424,14 +436,14 @@
 	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;
@@ -577,13 +589,10 @@
 	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);
@@ -752,8 +761,8 @@
 
 	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);
 
@@ -986,8 +995,8 @@
 
 	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);
 
--- a/security/keys/keyctl.c
+++ b/security/keys/keyctl.c
@@ -766,10 +766,9 @@
 
 	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);
@@ -901,7 +900,7 @@
 		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);
 		}
--- a/security/keys/keyring.c
+++ b/security/keys/keyring.c
@@ -414,7 +414,7 @@
 	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
@@ -553,7 +553,8 @@
 {
 	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);
 
@@ -597,9 +598,8 @@
 
 	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;
 		}
--- a/security/keys/proc.c
+++ b/security/keys/proc.c
@@ -182,6 +182,7 @@
 	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 @@
 			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),
 		   refcount_read(&key->usage),
 		   xbuf,
--- a/security/keys/process_keys.c
+++ b/security/keys/process_keys.c
@@ -730,7 +730,7 @@
 
 	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 */
--- a/security/keys/request_key.c
+++ b/security/keys/request_key.c
@@ -595,10 +595,9 @@
 			  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);
--- a/security/keys/request_key_auth.c
+++ b/security/keys/request_key_auth.c
@@ -73,7 +73,7 @@
 
 	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);
 }
 
--- a/security/keys/trusted.c
+++ b/security/keys/trusted.c
@@ -1066,7 +1066,7 @@
 	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)
--- a/security/keys/user_defined.c
+++ b/security/keys/user_defined.c
@@ -114,7 +114,7 @@
 
 	/* attach the new data, displacing the old */
 	key->expiry = prep->expiry;
-	if (!test_bit(KEY_FLAG_NEGATIVE, &key->flags))
+	if (key_is_positive(key))
 		zap = dereference_key_locked(key);
 	rcu_assign_keypointer(key, prep->payload.data[0]);
 	prep->payload.data[0] = NULL;
@@ -162,7 +162,7 @@
 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);
 }
 
