Re: [PATCH RFC 2/4] vhost/vsock: split packets to send using multiple buffers

[Stefano Garzarella <sgarzare@redhat.com>]

On Fri, Apr 05, 2019 at 09:13:56AM +0100, Stefan Hajnoczi wrote:
> On Thu, Apr 04, 2019 at 12:58:36PM +0200, Stefano Garzarella wrote:
> > @@ -139,8 +139,18 @@ vhost_transport_do_send_pkt(struct vhost_vsock *vsock,
> >  			break;
> >  		}
> >  
> > -		len = iov_length(&vq->iov[out], in);
> > -		iov_iter_init(&iov_iter, READ, &vq->iov[out], in, len);
> > +		payload_len = pkt->len - pkt->off;
> > +		iov_len = iov_length(&vq->iov[out], in);
> > +		iov_iter_init(&iov_iter, READ, &vq->iov[out], in, iov_len);
> > +
> > +		/* If the packet is greater than the space available in the
> > +		 * buffer, we split it using multiple buffers.
> > +		 */
> > +		if (payload_len > iov_len - sizeof(pkt->hdr))
> 
> Integer underflow.  iov_len is controlled by the guest and therefore
> untrusted.  Please validate iov_len before assuming it's larger than
> sizeof(pkt->hdr).
> 

Okay, I'll do it!

> > -		vhost_add_used(vq, head, sizeof(pkt->hdr) + pkt->len);
> > +		vhost_add_used(vq, head, sizeof(pkt->hdr) + payload_len);
> >  		added = true;
> >  
> > +		pkt->off += payload_len;
> > +
> > +		/* If we didn't send all the payload we can requeue the packet
> > +		 * to send it with the next available buffer.
> > +		 */
> > +		if (pkt->off < pkt->len) {
> > +			spin_lock_bh(&vsock->send_pkt_list_lock);
> > +			list_add(&pkt->list, &vsock->send_pkt_list);
> > +			spin_unlock_bh(&vsock->send_pkt_list_lock);
> > +			continue;
> 
> The virtio_transport_deliver_tap_pkt() call is skipped.  Packet capture
> should see the exact packets that are delivered.  I think this patch
> will present one large packet instead of several smaller packets that
> were actually delivered.

I'll modify virtio_transport_build_skb() to take care of pkt->off
and reading the payload size from the virtio_vsock_hdr.
Otherwise, should I introduce another field in virtio_vsock_pkt to store
the payload size?

Thanks,
Stefano