Patch "mqueue: fix a use-after-free in sys_mq_notify()" has been added to the 4.4-stable tree

This is a note to let you know that I've just added the patch titled

mqueue: fix a use-after-free in sys_mq_notify()

to the 4.4-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
mqueue-fix-a-use-after-free-in-sys_mq_notify.patch
and it can be found in the queue-4.4 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Cong Wang <[email protected]>
Date: Sun, 9 Jul 2017 13:19:55 -0700
Subject: mqueue: fix a use-after-free in sys_mq_notify()

From: Cong Wang <[email protected]>

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.

---
 ipc/mqueue.c |    4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

@@ -1251,8 +1251,10 @@ 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;

Patches currently in stable-queue which might be from [email protected] are

queue-4.4/mqueue-fix-a-use-after-free-in-sys_mq_notify.patch