On Wed, May 03, 2023 at 09:38:50AM +0200, Stefano Garzarella wrote: > 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? > Oh good point, the tx_lock synchronizes it: 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:mutex_lock()[@tx_work] guest:deliver_tap_pkt()[@send_pkt_work] guest:mutex_unlock() guest:virtqueue_get_buf()[@tx_work] guest:consume_skb() I'm pretty sure this should be safe. Best, Bobby