tipc: fix an use-after-free issue in tipc_recvmsg

Bugzilla: https://bugzilla.redhat.com/show_bug.cgi?id=2160540
Tested: compile only

commit cc19862
Author: Xin Long <lucien.xin@gmail.com>
Date:   Fri Jul 23 13:25:36 2021 -0400

    tipc: fix an use-after-free issue in tipc_recvmsg

    syzbot reported an use-after-free crash:

      BUG: KASAN: use-after-free in tipc_recvmsg+0xf77/0xf90 net/tipc/socket.c:1979
      Call Trace:
       tipc_recvmsg+0xf77/0xf90 net/tipc/socket.c:1979
       sock_recvmsg_nosec net/socket.c:943 [inline]
       sock_recvmsg net/socket.c:961 [inline]
       sock_recvmsg+0xca/0x110 net/socket.c:957
       tipc_conn_rcv_from_sock+0x162/0x2f0 net/tipc/topsrv.c:398
       tipc_conn_recv_work+0xeb/0x190 net/tipc/topsrv.c:421
       process_one_work+0x98d/0x1630 kernel/workqueue.c:2276
       worker_thread+0x658/0x11f0 kernel/workqueue.c:2422

    As Hoang pointed out, it was caused by skb_cb->bytes_read still accessed
    after calling tsk_advance_rx_queue() to free the skb in tipc_recvmsg().

    This patch is to fix it by accessing skb_cb->bytes_read earlier than
    calling tsk_advance_rx_queue().

    Fixes: f4919ff ("tipc: keep the skb in rcv queue until the whole data is read")
    Reported-by: syzbot+e6741b97d5552f97c24d@syzkaller.appspotmail.com
    Signed-off-by: Xin Long <lucien.xin@gmail.com>
    Acked-by: Jon Maloy <jmaloy@redhat.com>
    Signed-off-by: David S. Miller <davem@davemloft.net>

Signed-off-by: Xin Long <lxin@redhat.com>

Author: Xin Long

net/tipc/socket.c

@@ -1982,10 +1982,12 @@ static int tipc_recvmsg(struct socket *sock, struct msghdr *m,
 		tipc_node_distr_xmit(sock_net(sk), &xmitq);
 	}
 
-	if (!skb_cb->bytes_read)
-		tsk_advance_rx_queue(sk);
+	if (skb_cb->bytes_read)
+		goto exit;
+
+	tsk_advance_rx_queue(sk);
 
-	if (likely(!connected) || skb_cb->bytes_read)
+	if (likely(!connected))
 		goto exit;
 
 	/* Send connection flow control advertisement when applicable */