linux-kernel - [PATCH 4.2.y-ckt 019/211] KEYS: Fix race between key destruction and finding a keyring by name

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Message-Id: <1452023119-25647-20-git-send-email-kamal@canonical.com>
Date:	Tue,  5 Jan 2016 11:42:07 -0800
From:	Kamal Mostafa <kamal@...onical.com>
To:	linux-kernel@...r.kernel.org, stable@...r.kernel.org,
	kernel-team@...ts.ubuntu.com
Cc:	David Howells <dhowells@...hat.com>,
	Kamal Mostafa <kamal@...onical.com>
Subject: [PATCH 4.2.y-ckt 019/211] KEYS: Fix race between key destruction and finding a keyring by name

4.2.8-ckt1 -stable review patch.  If anyone has any objections, please let me know.

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

From: David Howells <dhowells@...hat.com>

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@...hat.com>
Signed-off-by: David Howells <dhowells@...hat.com>
Signed-off-by: Kamal Mostafa <kamal@...onical.com>
---
 security/keys/gc.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/security/keys/gc.c b/security/keys/gc.c
index c795237..39eac1f 100644
--- a/security/keys/gc.c
+++ b/security/keys/gc.c
@@ -134,6 +134,10 @@ static noinline void key_gc_unused_keys(struct list_head *keys)
 		kdebug("- %u", key->serial);
 		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 */
@@ -148,10 +152,6 @@ static noinline void key_gc_unused_keys(struct list_head *keys)
 		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);
-- 
1.9.1

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