Re: [PATCH net 1/2] vsock/virtio: discard packets if the transport changes

From: Stefano Garzarella
Date: Thu Jan 09 2025 - 06:00:07 EST

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On Thu, Jan 09, 2025 at 04:13:44AM -0500, Hyunwoo Kim wrote:

On Thu, Jan 09, 2025 at 10:01:31AM +0100, Stefano Garzarella wrote:

On Wed, Jan 08, 2025 at 02:31:19PM -0500, Hyunwoo Kim wrote:
> On Wed, Jan 08, 2025 at 07:06:16PM +0100, Stefano Garzarella wrote:
> > If the socket has been de-assigned or assigned to another transport,
> > we must discard any packets received because they are not expected
> > and would cause issues when we access vsk->transport.
> >
> > A possible scenario is described by Hyunwoo Kim in the attached link,
> > where after a first connect() interrupted by a signal, and a second
> > connect() failed, we can find `vsk->transport` at NULL, leading to a
> > NULL pointer dereference.
> >
> > Fixes: c0cfa2d8a788 ("vsock: add multi-transports support")
> > Reported-by: Hyunwoo Kim <v4bel@xxxxxxxxx>
> > Reported-by: Wongi Lee <qwerty@xxxxxxxxx>
> > Closes: https://lore.kernel.org/netdev/Z2LvdTTQR7dBmPb5@v4bel-B760M-AORUS-ELITE-AX/
> > Signed-off-by: Stefano Garzarella <sgarzare@xxxxxxxxxx>
> > ---
> > net/vmw_vsock/virtio_transport_common.c | 7 +++++--
> > 1 file changed, 5 insertions(+), 2 deletions(-)
> >
> > diff --git a/net/vmw_vsock/virtio_transport_common.c b/net/vmw_vsock/virtio_transport_common.c
> > index 9acc13ab3f82..51a494b69be8 100644
> > --- a/net/vmw_vsock/virtio_transport_common.c
> > +++ b/net/vmw_vsock/virtio_transport_common.c
> > @@ -1628,8 +1628,11 @@ void virtio_transport_recv_pkt(struct virtio_transport *t,
> >
> > lock_sock(sk);
> >
> > - /* Check if sk has been closed before lock_sock */
> > - if (sock_flag(sk, SOCK_DONE)) {
> > + /* Check if sk has been closed or assigned to another transport before
> > + * lock_sock (note: listener sockets are not assigned to any transport)
> > + */
> > + if (sock_flag(sk, SOCK_DONE) ||
> > + (sk->sk_state != TCP_LISTEN && vsk->transport != &t->transport)) {
>
> If a race scenario with vsock_listen() is added to the existing
> race scenario, the patch can be bypassed.
>
> In addition to the existing scenario:
> ```
> cpu0 cpu1
>
> socket(A)
>
> bind(A, {cid: VMADDR_CID_LOCAL, port: 1024})
> vsock_bind()
>
> listen(A)
> vsock_listen()
> socket(B)
>
> connect(B, {cid: VMADDR_CID_LOCAL, port: 1024})
> vsock_connect()
> lock_sock(sk);
> virtio_transport_connect()
> virtio_transport_connect()
> virtio_transport_send_pkt_info()
> vsock_loopback_send_pkt(VIRTIO_VSOCK_OP_REQUEST)
> queue_work(vsock_loopback_work)
> sk->sk_state = TCP_SYN_SENT;
> release_sock(sk);
> vsock_loopback_work()
> virtio_transport_recv_pkt(VIRTIO_VSOCK_OP_REQUEST)
> sk = vsock_find_bound_socket(&dst);
> virtio_transport_recv_listen(sk, skb)
> child = vsock_create_connected(sk);
> vsock_assign_transport()
> vvs = kzalloc(sizeof(*vvs), GFP_KERNEL);
> vsock_insert_connected(vchild);
> list_add(&vsk->connected_table, list);
> virtio_transport_send_response(vchild, skb);
> virtio_transport_send_pkt_info()
> vsock_loopback_send_pkt(VIRTIO_VSOCK_OP_RESPONSE)
> queue_work(vsock_loopback_work)
>
> vsock_loopback_work()
> virtio_transport_recv_pkt(VIRTIO_VSOCK_OP_RESPONSE)
> sk = vsock_find_bound_socket(&dst);
> lock_sock(sk);
> case TCP_SYN_SENT:
> virtio_transport_recv_connecting()
> sk->sk_state = TCP_ESTABLISHED;
> release_sock(sk);
>
> kill(connect(B));
> lock_sock(sk);
> if (signal_pending(current)) {
> sk->sk_state = sk->sk_state == TCP_ESTABLISHED ? TCP_CLOSING : TCP_CLOSE;
> sock->state = SS_UNCONNECTED; // [1]
> release_sock(sk);
>
> connect(B, {cid: VMADDR_CID_HYPERVISOR, port: 1024})
> vsock_connect(B)
> lock_sock(sk);
> vsock_assign_transport()
> virtio_transport_release()
> virtio_transport_close()
> if (!(sk->sk_state == TCP_ESTABLISHED || sk->sk_state == TCP_CLOSING))
> virtio_transport_shutdown()
> virtio_transport_send_pkt_info()
> vsock_loopback_send_pkt(VIRTIO_VSOCK_OP_SHUTDOWN)
> queue_work(vsock_loopback_work)
> schedule_delayed_work(&vsk->close_work, VSOCK_CLOSE_TIMEOUT); // [5]
> vsock_deassign_transport()
> vsk->transport = NULL;
> return -ESOCKTNOSUPPORT;
> release_sock(sk);
> vsock_loopback_work()
> virtio_transport_recv_pkt(VIRTIO_VSOCK_OP_SHUTDOWN)
> virtio_transport_recv_connected()
> virtio_transport_reset()
> virtio_transport_send_pkt_info()
> vsock_loopback_send_pkt(VIRTIO_VSOCK_OP_RST)
> queue_work(vsock_loopback_work)
> listen(B)
> vsock_listen()
> if (sock->state != SS_UNCONNECTED) // [2]
> sk->sk_state = TCP_LISTEN; // [3]
>
> vsock_loopback_work()
> virtio_transport_recv_pkt(VIRTIO_VSOCK_OP_RST)
> if ((sk->sk_state != TCP_LISTEN && vsk->transport != &t->transport)) { // [4]
> ...
>
> virtio_transport_close_timeout()
> virtio_transport_do_close()
> vsock_stream_has_data()
> return vsk->transport->stream_has_data(vsk); // null-ptr-deref
>
> ```
> (Yes, This is quite a crazy scenario, but it can actually be induced)
>
> Since sock->state is set to SS_UNCONNECTED during the first connect()[1],
> it can pass the sock->state check[2] in vsock_listen() and set
> sk->sk_state to TCP_LISTEN[3].
> If this happens, the check in the patch with
> `sk->sk_state != TCP_LISTEN` will pass[4], and a null-ptr-deref can
> still occur.)
>
> More specifically, because the sk_state has changed to TCP_LISTEN,
> virtio_transport_recv_disconnecting() will not be called by the
> loopback worker. However, a null-ptr-deref may occur in
> virtio_transport_close_timeout(), which is scheduled by
> virtio_transport_close() called in the flow of the second connect()[5].
> (The patch no longer cancels the virtio_transport_close_timeout() worker)
>
> And even if the `sk->sk_state != TCP_LISTEN` check is removed from the
> patch, it seems that a null-ptr-deref will still occur due to
> virtio_transport_close_timeout().
> It might be necessary to add worker cancellation at the
> appropriate location.

Thanks for the analysis!

Do you have time to cook a proper patch to cover this scenario?
Or we should mix this fix together with your patch (return 0 in
vsock_stream_has_data()) while we investigate a better handling?

For now, it seems better to merge them together.

Okay, since both you and Michael agree on that, I'll include your changes in this series, but adding a warning message, since it should not happen.

Is that fine with you?

It seems that covering this scenario will require more analysis and testing.

Yeah, scheduling a task during the release is tricky, especially when we are changing the transport, so I think we should handle that better.

One idea that I have it to cancel delayed works in virtio_transport_destruct(), I'll test it a bit and add a patch for that in the next version of this series.

We also need to reset SOCK_DONE after changing the transports.

Thanks,
Stefano

Next message: Peter Zijlstra: "Re: [REGRESSION] Re: [PATCH 00/24] Complete EEVDF"
Previous message: Paolo Abeni: "Re: [PATCH net v2] mctp i3c: fix MCTP I3C driver multi-thread issue"
In reply to: Hyunwoo Kim: "Re: [PATCH net 1/2] vsock/virtio: discard packets if the transport changes"
Next in thread: Hyunwoo Kim: "Re: [PATCH net 1/2] vsock/virtio: discard packets if the transport changes"
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