From: Cong Wang <xiyou.wangcong@gmail.com>
Date: Sun, 9 Jul 2017 13:19:55 -0700
Subject: [PATCH] mqueue: fix a use-after-free in sys_mq_notify()
References: bnc#1060662
Patch-mainline: 4.12.2
Git-commit: f991af3daabaecff34684fd51fac80319d1baad1

commit f991af3daabaecff34684fd51fac80319d1baad1 upstream.

The retry logic for netlink_attachskb() inside sys_mq_notify()
is nasty and vulnerable:

1) The sock refcnt is already released when retry is needed
2) The fd is controllable by user-space because we already
   release the file refcnt

so we when retry but the fd has been just closed by user-space
during this small window, we end up calling netlink_detachskb()
on the error path which releases the sock again, later when
the user-space closes this socket a use-after-free could be
triggered.

Setting 'sock' to NULL here should be sufficient to fix it.

Reported-by: GeneBlue <geneblue.mail@gmail.com>
Signed-off-by: Cong Wang <xiyou.wangcong@gmail.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Manfred Spraul <manfred@colorfullife.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Jiri Slaby <jslaby@suse.cz>
---
 ipc/mqueue.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/ipc/mqueue.c b/ipc/mqueue.c
index e8d41ff57241..a6ced9e07e1c 100644
--- a/ipc/mqueue.c
+++ b/ipc/mqueue.c
@@ -1253,8 +1253,10 @@ retry:
 
 			timeo = MAX_SCHEDULE_TIMEOUT;
 			ret = netlink_attachskb(sock, nc, &timeo, NULL);
-			if (ret == 1)
+			if (ret == 1) {
+				sock = NULL;
 				goto retry;
+			}
 			if (ret) {
 				sock = NULL;
 				nc = NULL;
-- 
2.14.2