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Message-ID: <occeblxotmpsq4gqjjued62ar5ngqxehmmrj7jg3ynzsz2vfcy@4jzl7slmqkft> Date: Wed, 3 May 2023 09:38:50 +0200 From: Stefano Garzarella <sgarzare@...hat.com> To: Bobby Eshleman <bobbyeshleman@...il.com> Cc: Stefan Hajnoczi <stefanha@...hat.com>, Cong Wang <xiyou.wangcong@...il.com>, Cong Wang <cong.wang@...edance.com>, Bobby Eshleman <bobby.eshleman@...edance.com>, kvm@...r.kernel.org, netdev@...r.kernel.org, virtualization@...ts.linux-foundation.org Subject: Re: [Patch net] vsock: improve tap delivery accuracy 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@...edance.com> >> > >> > 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@...hat.com> >> > Cc: Stefano Garzarella <sgarzare@...hat.com> >> > Cc: Bobby Eshleman <bobby.eshleman@...edance.com> >> > Signed-off-by: Cong Wang <cong.wang@...edance.com> >> > --- >> > 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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