Re: [PATCH resend] vsock/virtio: avoid potential deadlock when vsock device remove

[Stefano Garzarella]

On Thu, Aug 12, 2021 at 01:30:56PM +0800, Longpeng(Mike) wrote:
> There's a potential deadlock case when remove the vsock device or
> process the RESET event:
>
>  vsock_for_each_connected_socket:
>      spin_lock_bh(&vsock_table_lock) ----------- (1)
>      ...
>          virtio_vsock_reset_sock:
>              lock_sock(sk) --------------------- (2)
>      ...
>      spin_unlock_bh(&vsock_table_lock)
>
> lock_sock() may do initiative schedule when the 'sk' is owned by
> other thread at the same time, we would receivce a warning message
> that "scheduling while atomic".
>
> Even worse, if the next task (selected by the scheduler) try to
> release a 'sk', it need to request vsock_table_lock and the deadlock
> occur, cause the system into softlockup state.
>  Call trace:
>   queued_spin_lock_slowpath
>   vsock_remove_bound
>   vsock_remove_sock
>   virtio_transport_release
>   __vsock_release
>   vsock_release
>   __sock_release
>   sock_close
>   __fput
>   ____fput
>
> So we should not require sk_lock in this case, just like the behavior
> in vhost_vsock or vmci.

The difference with vhost_vsock is that here we call it also when we 
receive an event in the event queue (for example because we are 
migrating the VM).

I think the idea of this lock was to prevent concurrency with RX loop, 
but actually if a socket is connected, it can only change state to 
TCP_CLOSING/TCP_CLOSE.

I don't think there is any problem not to take the lock, at most we 
could take the rx_lock in virtio_vsock_event_handle(), but I'm not sure 
it's necessary.

> Cc: Stefan Hajnoczi <stefanha@xxxxxxxxxx>
> Cc: Stefano Garzarella <sgarzare@xxxxxxxxxx>
> Cc: "David S. Miller" <davem@xxxxxxxxxxxxx>
> Cc: Jakub Kicinski <kuba@xxxxxxxxxx>

We should add:
Fixes: 0ea9e1d3a9e3 ("VSOCK: Introduce virtio_transport.ko")
> Signed-off-by: Longpeng(Mike) <longpeng2@xxxxxxxxxx>
> ---
> net/vmw_vsock/virtio_transport.c | 7 +++++--
> 1 file changed, 5 insertions(+), 2 deletions(-)
>
> diff --git a/net/vmw_vsock/virtio_transport.c b/net/vmw_vsock/virtio_transport.c
> index e0c2c99..4f7c99d 100644
> --- a/net/vmw_vsock/virtio_transport.c
> +++ b/net/vmw_vsock/virtio_transport.c
> @@ -357,11 +357,14 @@ static void virtio_vsock_event_fill(struct virtio_vsock *vsock)
>
> static void virtio_vsock_reset_sock(struct sock *sk)
> {
> -	lock_sock(sk);
> +	/* vmci_transport.c doesn't take sk_lock here either.  At least we're
> +	 * under vsock_table_lock so the sock cannot disappear while 
> we're
> +	 * executing.
> +	 */
> +
> 	sk->sk_state = TCP_CLOSE;
> 	sk->sk_err = ECONNRESET;
> 	sk_error_report(sk);
> -	release_sock(sk);
> }
>
> static void virtio_vsock_update_guest_cid(struct virtio_vsock *vsock)
> --
> 1.8.3.1

With the Fixes tag added:

Reviewed-by: Stefano Garzarella <sgarzare@xxxxxxxxxx>