[Trusty SRU PATCH 1/2] sctp: avoid BUG_ON on sctp_wait_for_sndbuf

[Thadeu Lima de Souza Cascardo]

From: Marcelo Ricardo Leitner <marcelo.leitner at gmail.com>

Alexander Popov reported that an application may trigger a BUG_ON in
sctp_wait_for_sndbuf if the socket tx buffer is full, a thread is
waiting on it to queue more data and meanwhile another thread peels off
the association being used by the first thread.

This patch replaces the BUG_ON call with a proper error handling. It
will return -EPIPE to the original sendmsg call, similarly to what would
have been done if the association wasn't found in the first place.

Acked-by: Alexander Popov <alex.popov at linux.com>
Signed-off-by: Marcelo Ricardo Leitner <marcelo.leitner at gmail.com>
Reviewed-by: Xin Long <lucien.xin at gmail.com>
Signed-off-by: David S. Miller <davem at davemloft.net>
(backported from commit 2dcab598484185dea7ec22219c76dcdd59e3cb90)
[cascardo: keep sctp_lock_sock instead of lock_sock]
CVE-2017-5986
Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo at canonical.com>
---
 net/sctp/socket.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/net/sctp/socket.c b/net/sctp/socket.c
index 2aac2d8..da2ff8f 100644
--- a/net/sctp/socket.c
+++ b/net/sctp/socket.c
@@ -6722,7 +6722,8 @@ static int sctp_wait_for_sndbuf(struct sctp_association *asoc, long *timeo_p,
 		 */
 		sctp_release_sock(sk);
 		current_timeo = schedule_timeout(current_timeo);
-		BUG_ON(sk != asoc->base.sk);
+		if (sk != asoc->base.sk)
+			goto do_error;
 		sctp_lock_sock(sk);
 
 		*timeo_p = current_timeo;
-- 
2.9.3