KEYS: close race between key lookup and freeing

commit a3a87844 upstream.

When a key is being garbage collected, it's key->user would get put before
the ->destroy() callback is called, where the key is removed from it's
respective tracking structures.

This leaves a key hanging in a semi-invalid state which leaves a window open
for a different task to try an access key->user. An example is
find_keyring_by_name() which would dereference key->user for a key that is
in the process of being garbage collected (where key->user was freed but
->destroy() wasn't called yet - so it's still present in the linked list).

This would cause either a panic, or corrupt memory.

Fixes CVE-2014-9529.
Signed-off-by: Sasha Levin <sasha.levin@oracle.com>
Signed-off-by: David Howells <dhowells@redhat.com>
[bwh: Backported to 3.2: adjust indentation]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

security/keys/gc.c

@@ -186,12 +186,12 @@ static noinline void key_gc_unused_key(struct key *key)
 	if (test_bit(KEY_FLAG_INSTANTIATED, &key->flags))
 		atomic_dec(&key->user->nikeys);
 
-	key_user_put(key->user);
-
 	/* now throw away the key memory */
 	if (key->type->destroy)
 		key->type->destroy(key);
 
+	key_user_put(key->user);
+
 	kfree(key->description);
 
 #ifdef KEY_DEBUGGING