On Sun, Apr 16, 2023 at 04:49:00AM +0000, Bobby Eshleman wrote:
On Tue, May 02, 2023 at 04:14:18PM -0400, Stefan Hajnoczi wrote:
On Tue, May 02, 2023 at 10:44:04AM -0700, Cong Wang wrote:
> From: Cong Wang <cong.wang@xxxxxxxxxxxxx>
>
> When virtqueue_add_sgs() fails, the skb is put back to send queue,
> we should not deliver the copy to tap device in this case. So we
> need to move virtio_transport_deliver_tap_pkt() down after all
> possible failures.
>
> Fixes: 82dfb540aeb2 ("VSOCK: Add virtio vsock vsockmon hooks")
> Cc: Stefan Hajnoczi <stefanha@xxxxxxxxxx>
> Cc: Stefano Garzarella <sgarzare@xxxxxxxxxx>
> Cc: Bobby Eshleman <bobby.eshleman@xxxxxxxxxxxxx>
> Signed-off-by: Cong Wang <cong.wang@xxxxxxxxxxxxx>
> ---
> net/vmw_vsock/virtio_transport.c | 5 ++---
> 1 file changed, 2 insertions(+), 3 deletions(-)
>
> diff --git a/net/vmw_vsock/virtio_transport.c b/net/vmw_vsock/virtio_transport.c
> index e95df847176b..055678628c07 100644
> --- a/net/vmw_vsock/virtio_transport.c
> +++ b/net/vmw_vsock/virtio_transport.c
> @@ -109,9 +109,6 @@ virtio_transport_send_pkt_work(struct work_struct *work)
> if (!skb)
> break;
>
> - virtio_transport_deliver_tap_pkt(skb);
> - reply = virtio_vsock_skb_reply(skb);
> -
> sg_init_one(&hdr, virtio_vsock_hdr(skb), sizeof(*virtio_vsock_hdr(skb)));
> sgs[out_sg++] = &hdr;
> if (skb->len > 0) {
> @@ -128,6 +125,8 @@ virtio_transport_send_pkt_work(struct work_struct *work)
> break;
> }
>
> + virtio_transport_deliver_tap_pkt(skb);
I would move only the virtio_transport_deliver_tap_pkt(),
virtio_vsock_skb_reply() is not related.
> + reply = virtio_vsock_skb_reply(skb);
I don't remember the reason for the ordering, but I'm pretty sure it was
deliberate. Probably because the payload buffers could be freed as soon
as virtqueue_add_sgs() is called.
If that's no longer true with Bobby's skbuff code, then maybe it's safe
to monitor packets after they have been sent.
Stefan
Hey Stefan,
Unfortunately, skbuff doesn't change that behavior.
If I understand correctly, the problem flow you are describing
would be something like this:
Thread 0 Thread 1
guest:virtqueue_add_sgs()[@send_pkt_work]
host:vhost_vq_get_desc()[@handle_tx_kick]
host:vhost_add_used()
host:vhost_signal()
guest:virtqueue_get_buf()[@tx_work]
guest:consume_skb()
guest:deliver_tap_pkt()[@send_pkt_work]
^ use-after-free
Which I guess is possible because the receiver can consume the new
scatterlist during the processing kicked off for a previous batch?
(doesn't have to wait for the subsequent kick)
This is true, but both `send_pkt_work` and `tx_work` hold `tx_lock`, so
can they really go in parallel?
Thanks,
Stefano
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