[PATCH 3.2 142/152] KEYS: close race between key lookup and freeing

[Ben Hutchings]

3.2.67-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Sasha Levin <sasha.levin@xxxxxxxxxx>

commit a3a8784454692dd72e5d5d34dcdab17b4420e74c 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@xxxxxxxxxx>
Signed-off-by: David Howells <dhowells@xxxxxxxxxx>
[bwh: Backported to 3.2: adjust indentation]
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
---
 security/keys/gc.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

--- a/security/keys/gc.c
+++ b/security/keys/gc.c
@@ -186,12 +186,12 @@ static noinline void key_gc_unused_key(s
 	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

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