linux-kernel - [PATCH 3.2 58/59] mqueue: fix a use-after-free in sys_mq

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Message-ID: <lsq.1503061993.780790059@decadent.org.uk>
Date:   Fri, 18 Aug 2017 14:13:13 +0100
From:   Ben Hutchings <ben@...adent.org.uk>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org
CC:     akpm@...ux-foundation.org, stable@...nel.org,
        "Cong Wang" <xiyou.wangcong@...il.com>,
        "Linus Torvalds" <torvalds@...ux-foundation.org>,
        "GeneBlue" <geneblue.mail@...il.com>,
        "Manfred Spraul" <manfred@...orfullife.com>
Subject: [PATCH 3.2 58/59] mqueue: fix a use-after-free in sys_mq_notify()

3.2.92-rc1 review patch.  If anyone has any objections, please let me know.

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

From: Cong Wang <xiyou.wangcong@...il.com>

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@...il.com>
Signed-off-by: Cong Wang <xiyou.wangcong@...il.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>
Cc: Manfred Spraul <manfred@...orfullife.com>
Cc: stable@...nel.org
Signed-off-by: Linus Torvalds <torvalds@...ux-foundation.org>
Signed-off-by: Ben Hutchings <ben@...adent.org.uk>
---
 ipc/mqueue.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

--- a/ipc/mqueue.c
+++ b/ipc/mqueue.c
@@ -1095,8 +1095,10 @@ 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;