[PATCH] KEYS: close race between key lookup and freeing

[Sasha Levin <sasha.levin@oracle.com>]

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.

Signed-off-by: Sasha Levin <sasha.levin@oracle.com>
---
 security/keys/gc.c |    4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/security/keys/gc.c b/security/keys/gc.c
index 9609a7f..c795237 100644
--- a/security/keys/gc.c
+++ b/security/keys/gc.c
@@ -148,12 +148,12 @@ static noinline void key_gc_unused_keys(struct list_head *keys)
 		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
-- 
1.7.10.4