[PATCH net-next] tipc: Fix NULL pointer dereference in __tipc_sendstream()

[YueHaibing]

tipc_sendstream() may send zero length packet, then tipc_msg_append()
do not alloc skb, skb_peek_tail() will get NULL, msg_set_ack_required
will trigger NULL pointer dereference.

Reported-by: syzbot+8eac6d030e7807c21...@syzkaller.appspotmail.com
Fixes: 0a3e060f340d ("tipc: add test for Nagle algorithm effectiveness")
Signed-off-by: YueHaibing <yuehaib...@huawei.com>
---
 net/tipc/socket.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/net/tipc/socket.c b/net/tipc/socket.c
index d6b67d07d22e..2943561399f1 100644
--- a/net/tipc/socket.c
+++ b/net/tipc/socket.c
@@ -1588,8 +1588,12 @@ static int __tipc_sendstream(struct socket *sock, struct 
msghdr *m, size_t dlen)
                                tsk->pkt_cnt += skb_queue_len(txq);
                        } else {
                                skb = skb_peek_tail(txq);
-                               msg_set_ack_required(buf_msg(skb));
-                               tsk->expect_ack = true;
+                               if (skb) {
+                                       msg_set_ack_required(buf_msg(skb));
+                                       tsk->expect_ack = true;
+                               } else {
+                                       tsk->expect_ack = false;
+                               }
                                tsk->msg_acc = 0;
                                tsk->pkt_cnt = 0;
                        }
-- 
2.17.1