vsock: Ignore signal/timeout on connect() if already established

During connect(), acting on a signal/timeout by disconnecting an already
established socket leads to several issues:

1. connect() invoking vsock_transport_cancel_pkt() ->
   virtio_transport_purge_skbs() may race with sendmsg() invoking
   virtio_transport_get_credit(). This results in a permanently elevated
   `vvs->bytes_unsent`. Which, in turn, confuses the SOCK_LINGER handling.

2. connect() resetting a connected socket's state may race with socket
   being placed in a sockmap. A disconnected socket remaining in a sockmap
   breaks sockmap's assumptions. And gives rise to WARNs.

3. connect() transitioning SS_CONNECTED -> SS_UNCONNECTED allows for a
   transport change/drop after TCP_ESTABLISHED. Which poses a problem for
   any simultaneous sendmsg() or connect() and may result in a
   use-after-free/null-ptr-deref.

Do not disconnect socket on signal/timeout. Keep the logic for unconnected
sockets: they don't linger, can't be placed in a sockmap, are rejected by
sendmsg().

[1]: https://lore.kernel.org/netdev/[email protected]/
[2]: https://lore.kernel.org/netdev/[email protected]/
[3]: https://lore.kernel.org/netdev/[email protected]/

Fixes: d021c34405 ("VSOCK: Introduce VM Sockets")
Signed-off-by: Michal Luczaj <[email protected]>
Reviewed-by: Stefano Garzarella <[email protected]>
Link: https://patch.msgid.link/[email protected]
Signed-off-by: Jakub Kicinski <[email protected]>

net/vmw_vsock/af_vsock.c

@@ -1661,18 +1661,40 @@ static int vsock_connect(struct socket *sock, struct sockaddr *addr,
 		timeout = schedule_timeout(timeout);
 		lock_sock(sk);
 
-		if (signal_pending(current)) {
-			err = sock_intr_errno(timeout);
-			sk->sk_state = sk->sk_state == TCP_ESTABLISHED ? TCP_CLOSING : TCP_CLOSE;
-			sock->state = SS_UNCONNECTED;
-			vsock_transport_cancel_pkt(vsk);
-			vsock_remove_connected(vsk);
-			goto out_wait;
-		} else if ((sk->sk_state != TCP_ESTABLISHED) && (timeout == 0)) {
-			err = -ETIMEDOUT;
+		/* Connection established. Whatever happens to socket once we
+		 * release it, that's not connect()'s concern. No need to go
+		 * into signal and timeout handling. Call it a day.
+		 *
+		 * Note that allowing to "reset" an already established socket
+		 * here is racy and insecure.
+		 */
+		if (sk->sk_state == TCP_ESTABLISHED)
+			break;
+
+		/* If connection was _not_ established and a signal/timeout came
+		 * to be, we want the socket's state reset. User space may want
+		 * to retry.
+		 *
+		 * sk_state != TCP_ESTABLISHED implies that socket is not on
+		 * vsock_connected_table. We keep the binding and the transport
+		 * assigned.
+		 */
+		if (signal_pending(current) || timeout == 0) {
+			err = timeout == 0 ? -ETIMEDOUT : sock_intr_errno(timeout);
+
+			/* Listener might have already responded with
+			 * VIRTIO_VSOCK_OP_RESPONSE. Its handling expects our
+			 * sk_state == TCP_SYN_SENT, which hereby we break.
+			 * In such case VIRTIO_VSOCK_OP_RST will follow.
+			 */
 			sk->sk_state = TCP_CLOSE;
 			sock->state = SS_UNCONNECTED;
+
+			/* Try to cancel VIRTIO_VSOCK_OP_REQUEST skb sent out by
+			 * transport->connect().
+			 */
 			vsock_transport_cancel_pkt(vsk);
+
 			goto out_wait;
 		}