kcm: fix 0-length case for kcm_sendmsg()

Dmitry reported a kernel warning:

 WARNING: CPU: 3 PID: 2936 at net/kcm/kcmsock.c:627
 kcm_write_msgs+0x12e3/0x1b90 net/kcm/kcmsock.c:627
 CPU: 3 PID: 2936 Comm: a.out Not tainted 4.10.0-rc6+ #209
 Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Bochs 01/01/2011
 Call Trace:
  __dump_stack lib/dump_stack.c:15 [inline]
  dump_stack+0x2ee/0x3ef lib/dump_stack.c:51
  panic+0x1fb/0x412 kernel/panic.c:179
  __warn+0x1c4/0x1e0 kernel/panic.c:539
  warn_slowpath_null+0x2c/0x40 kernel/panic.c:582
  kcm_write_msgs+0x12e3/0x1b90 net/kcm/kcmsock.c:627
  kcm_sendmsg+0x163a/0x2200 net/kcm/kcmsock.c:1029
  sock_sendmsg_nosec net/socket.c:635 [inline]
  sock_sendmsg+0xca/0x110 net/socket.c:645
  sock_write_iter+0x326/0x600 net/socket.c:848
  new_sync_write fs/read_write.c:499 [inline]
  __vfs_write+0x483/0x740 fs/read_write.c:512
  vfs_write+0x187/0x530 fs/read_write.c:560
  SYSC_write fs/read_write.c:607 [inline]
  SyS_write+0xfb/0x230 fs/read_write.c:599
  entry_SYSCALL_64_fastpath+0x1f/0xc2

when calling syscall(__NR_write, sock2, 0x208aaf27ul, 0x0ul) on a KCM
seqpacket socket. It appears that kcm_sendmsg() does not handle len==0
case correctly, which causes an empty skb is allocated and queued.
Fix this by skipping the skb allocation for len==0 case.
Reported-by: Dmitry Vyukov <dvyukov@google.com>
Cc: Tom Herbert <tom@herbertland.com>
Signed-off-by: Cong Wang <xiyou.wangcong@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/kcm/kcmsock.c

@@ -929,6 +929,7 @@ static int kcm_sendmsg(struct socket *sock, struct msghdr *msg, size_t len)
 			goto out_error;
 	}
 
+	if (msg_data_left(msg)) {
 		/* New message, alloc head skb */
 		head = alloc_skb(0, sk->sk_allocation);
 		while (!head) {
@@ -946,6 +947,7 @@ static int kcm_sendmsg(struct socket *sock, struct msghdr *msg, size_t len)
 		 * csum_and_copy_from_iter from skb_do_copy_data_nocache.
 		 */
 		skb->ip_summed = CHECKSUM_UNNECESSARY;
+	}
 
 start:
 	while (msg_data_left(msg)) {
@@ -1018,10 +1020,12 @@ static int kcm_sendmsg(struct socket *sock, struct msghdr *msg, size_t len)
 	if (eor) {
 		bool not_busy = skb_queue_empty(&sk->sk_write_queue);
 
+		if (head) {
 			/* Message complete, queue it on send buffer */
 			__skb_queue_tail(&sk->sk_write_queue, head);
 			kcm->seq_skb = NULL;
 			KCM_STATS_INCR(kcm->stats.tx_msgs);
+		}
 
 		if (msg->msg_flags & MSG_BATCH) {
 			kcm->tx_wait_more = true;