Re: [PATCH v2 1/1] vhost: Added pad cleanup if vnet_hdr is not present.

[Andrew Melnichenko]

Thanks, I'll look into it.

On Thu, Mar 28, 2024 at 6:03 AM Jason Wang <jasowang@xxxxxxxxxx> wrote:
>
> On Thu, Mar 28, 2024 at 7:44 AM Andrew Melnychenko <andrew@daynixcom> wrote:
> >
> > When the Qemu launched with vhost but without tap vnet_hdr,
> > vhost tries to copy vnet_hdr from socket iter with size 0
> > to the page that may contain some trash.
> > That trash can be interpreted as unpredictable values for
> > vnet_hdr.
> > That leads to dropping some packets and in some cases to
> > stalling vhost routine when the vhost_net tries to process
> > packets and fails in a loop.
> >
> > Qemu options:
> >   -netdev tap,vhost=on,vnet_hdr=off,...
> >
> > From security point of view, wrong values on field used later
> > tap's tap_get_user_xdp() and will affect skb gso and options.
> > Later the header(and data in headroom) should not be used by the stack.
> > Using custom socket as a backend to vhost_net can reveal some data
> > in the vnet_hdr, although it would require kernel access to implement.
> >
> > The issue happens because the value of sock_len in virtqueue is 0.
> > That value is set at vhost_net_set_features() with
> > VHOST_NET_F_VIRTIO_NET_HDR, also it's set to zero at device open()
> > and reset() routine.
> > So, currently, to trigger the issue, we need to set up qemu with
> > vhost=on,vnet_hdr=off, or do not configure vhost in the custom program.
> >
> > Signed-off-by: Andrew Melnychenko <andrew@xxxxxxxxxx>
>
> Acked-by: Jason Wang <jasowang@xxxxxxxxxx>
>
> It seems it has been merged by Michael.
>
> Thanks
>
> > ---
> >  drivers/vhost/net.c | 3 +++
> >  1 file changed, 3 insertions(+)
> >
> > diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
> > index f2ed7167c848..57411ac2d08b 100644
> > --- a/drivers/vhost/net.c
> > +++ b/drivers/vhost/net.c
> > @@ -735,6 +735,9 @@ static int vhost_net_build_xdp(struct vhost_net_virtqueue *nvq,
> >         hdr = buf;
> >         gso = &hdr->gso;
> >
> > +       if (!sock_hlen)
> > +               memset(buf, 0, pad);
> > +
> >         if ((gso->flags & VIRTIO_NET_HDR_F_NEEDS_CSUM) &&
> >             vhost16_to_cpu(vq, gso->csum_start) +
> >             vhost16_to_cpu(vq, gso->csum_offset) + 2 >
> > --
> > 2.43.0
> >
>