kcm: Fix memory leak in error path of kcm_sendmsg()

syzbot reported a memory leak like below:

BUG: memory leak
unreferenced object 0xffff88810b088c00 (size 240):
  comm "syz-executor186", pid 5012, jiffies 4294943306 (age 13.680s)
  hex dump (first 32 bytes):
    00 89 08 0b 81 88 ff ff 00 00 00 00 00 00 00 00  ................
    00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
  backtrace:
    [<ffffffff83e5d5ff>] __alloc_skb+0x1ef/0x230 net/core/skbuff.c:634
    [<ffffffff84606e59>] alloc_skb include/linux/skbuff.h:1289 [inline]
    [<ffffffff84606e59>] kcm_sendmsg+0x269/0x1050 net/kcm/kcmsock.c:815
    [<ffffffff83e479c6>] sock_sendmsg_nosec net/socket.c:725 [inline]
    [<ffffffff83e479c6>] sock_sendmsg+0x56/0xb0 net/socket.c:748
    [<ffffffff83e47f55>] ____sys_sendmsg+0x365/0x470 net/socket.c:2494
    [<ffffffff83e4c389>] ___sys_sendmsg+0xc9/0x130 net/socket.c:2548
    [<ffffffff83e4c536>] __sys_sendmsg+0xa6/0x120 net/socket.c:2577
    [<ffffffff84ad7bb8>] do_syscall_x64 arch/x86/entry/common.c:50 [inline]
    [<ffffffff84ad7bb8>] do_syscall_64+0x38/0xb0 arch/x86/entry/common.c:80
    [<ffffffff84c0008b>] entry_SYSCALL_64_after_hwframe+0x63/0xcd

In kcm_sendmsg(), kcm_tx_msg(head)->last_skb is used as a cursor to append
newly allocated skbs to 'head'. If some bytes are copied, an error occurred,
and jumped to out_error label, 'last_skb' is left unmodified. A later
kcm_sendmsg() will use an obsoleted 'last_skb' reference, corrupting the
'head' frag_list and causing the leak.

This patch fixes this issue by properly updating the last allocated skb in
'last_skb'.

Fixes: ab7ac4eb ("kcm: Kernel Connection Multiplexor module")
Reported-and-tested-by: syzbot+6f98de741f7dbbfc4ccb@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=6f98de741f7dbbfc4ccbSigned-off-by: Shigeru Yoshida <syoshida@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/kcm/kcmsock.c

@@ -939,6 +939,8 @@ static int kcm_sendmsg(struct socket *sock, struct msghdr *msg, size_t len)
 
 	if (head != kcm->seq_skb)
 		kfree_skb(head);
+	else if (copied)
+		kcm_tx_msg(head)->last_skb = skb;
 
 	err = sk_stream_error(sk, msg->msg_flags, err);