sctp: do not free asoc when it is already dead in sctp_sendmsg

Now in sctp_sendmsg sctp_wait_for_sndbuf could schedule out without
holding sock sk. It means the current asoc can be freed elsewhere,
like when receiving an abort packet.

If the asoc is just created in sctp_sendmsg and sctp_wait_for_sndbuf
returns err, the asoc will be freed again due to new_asoc is not nil.
An use-after-free issue would be triggered by this.

This patch is to fix it by setting new_asoc with nil if the asoc is
already dead when cpu schedules back, so that it will not be freed
again in sctp_sendmsg.

v1->v2:
  set new_asoc as nil in sctp_sendmsg instead of sctp_wait_for_sndbuf.
Suggested-by: Neil Horman <nhorman@tuxdriver.com>
Reported-by: Dmitry Vyukov <dvyukov@google.com>
Signed-off-by: Xin Long <lucien.xin@gmail.com>
Acked-by: Neil Horman <nhorman@tuxdriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/sctp/socket.c

@@ -1971,8 +1971,14 @@ static int sctp_sendmsg(struct sock *sk, struct msghdr *msg, size_t msg_len)
 	timeo = sock_sndtimeo(sk, msg->msg_flags & MSG_DONTWAIT);
 	if (!sctp_wspace(asoc)) {
 		err = sctp_wait_for_sndbuf(asoc, &timeo, msg_len);
-		if (err)
+		if (err) {
+			if (err == -ESRCH) {
+				/* asoc is already dead. */
+				new_asoc = NULL;
+				err = -EPIPE;
+			}
 			goto out_free;
+		}
 	}
 
 	/* If an address is passed with the sendto/sendmsg call, it is used
@@ -8006,10 +8012,11 @@ static int sctp_wait_for_sndbuf(struct sctp_association *asoc, long *timeo_p,
 	for (;;) {
 		prepare_to_wait_exclusive(&asoc->wait, &wait,
 					  TASK_INTERRUPTIBLE);
+		if (asoc->base.dead)
+			goto do_dead;
 		if (!*timeo_p)
 			goto do_nonblock;
-		if (sk->sk_err || asoc->state >= SCTP_STATE_SHUTDOWN_PENDING ||
-		    asoc->base.dead)
+		if (sk->sk_err || asoc->state >= SCTP_STATE_SHUTDOWN_PENDING)
 			goto do_error;
 		if (signal_pending(current))
 			goto do_interrupted;
@@ -8034,6 +8041,10 @@ static int sctp_wait_for_sndbuf(struct sctp_association *asoc, long *timeo_p,
 
 	return err;
 
+do_dead:
+	err = -ESRCH;
+	goto out;
+
 do_error:
 	err = -EPIPE;
 	goto out;