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

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>
Cc: stable@kernel.org
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 3 insertions, 1 deletions

diff --git a/ipc/mqueue.c b/ipc/mqueue.c
index c9ff943f19ab..eb1391b52c6f 100644
--- a/ipc/mqueue.c
+++ b/ipc/mqueue.c
@@ -1270,8 +1270,10 @@ 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;