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

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Date:   Thu, 12 Aug 2021 10:03:32 +0200
From:   Stefano Garzarella <sgarzare@...hat.com>
To:     "Longpeng(Mike)" <longpeng2@...wei.com>
Cc:     kvm@...r.kernel.org, virtualization@...ts.linux-foundation.org,
        netdev@...r.kernel.org, arei.gonglei@...wei.com,
        linux-kernel@...r.kernel.org,
        Stefan Hajnoczi <stefanha@...hat.com>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>
Subject: Re: [PATCH resend] vsock/virtio: avoid potential deadlock when vsock
 device remove

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@...hat.com>
>Cc: Stefano Garzarella <sgarzare@...hat.com>
>Cc: "David S. Miller" <davem@...emloft.net>
>Cc: Jakub Kicinski <kuba@...nel.org>

We should add:
Fixes: 0ea9e1d3a9e3 ("VSOCK: Introduce virtio_transport.ko")
>Signed-off-by: Longpeng(Mike) <longpeng2@...wei.com>
>---
> 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@...hat.com>