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

BugLink: http://bugs.launchpad.net/bugs/1705238
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: Stefan Bader <stefan.bader@canonical.com>
Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@canonical.com>

diff --git a/ipc/mqueue.c b/ipc/mqueue.c
index f490ffb07b6dd6df3a0138915c2b5de13adb256b..5cad589d3df5c8eda8aa67b051af0583d8779943 100644 (file)
--- a/ipc/mqueue.c
+++ b/ipc/mqueue.c
@@ -1249,8 +1249,10 @@ 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;