[PATCH 3.2 59/60] KEYS: Fix race between key destruction and finding a keyring by name

[Ben Hutchings]

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

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

From: David Howells <dhowells@xxxxxxxxxx>

commit 94c4554ba07adbdde396748ee7ae01e86cf2d8d7 upstream.

There appears to be a race between:

 (1) key_gc_unused_keys() which frees key->security and then calls
     keyring_destroy() to unlink the name from the name list

 (2) find_keyring_by_name() which calls key_permission(), thus accessing
     key->security, on a key before checking to see whether the key usage is 0
     (ie. the key is dead and might be cleaned up).

Fix this by calling ->destroy() before cleaning up the core key data -
including key->security.

Reported-by: Petr Matousek <pmatouse@xxxxxxxxxx>
Signed-off-by: David Howells <dhowells@xxxxxxxxxx>
[carnil: Backported to 3.2: adjust context]
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
---
 security/keys/gc.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

--- a/security/keys/gc.c
+++ b/security/keys/gc.c
@@ -172,6 +172,10 @@ static noinline void key_gc_unused_key(s
 {
 	key_check(key);
 
+	/* Throw away the key data */
+	if (key->type->destroy)
+		key->type->destroy(key);
+
 	security_key_free(key);
 
 	/* deal with the user's key tracking and quota */
@@ -186,10 +190,6 @@ static noinline void key_gc_unused_key(s
 	if (test_bit(KEY_FLAG_INSTANTIATED, &key->flags))
 		atomic_dec(&key->user->nikeys);
 
-	/* now throw away the key memory */
-	if (key->type->destroy)
-		key->type->destroy(key);
-
 	key_user_put(key->user);
 
 	kfree(key->description);

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