Re: [PATCH] vhost: Fix bad return of descriptors to SVQ

[Jason Wang]

On Fri, Apr 1, 2022 at 3:31 PM Eugenio Perez Martin <eperezma@redhat.com> wrote:
>
> On Fri, Apr 1, 2022 at 4:30 AM Jason Wang <jasowang@redhat.com> wrote:
> >
> > On Fri, Apr 1, 2022 at 2:14 AM Eugenio Pérez <eperezma@redhat.com> wrote:
> > >
> > > Only the first one of them were properly enqueued back.
> > >
> > > Fixes: 100890f7ca ("vhost: Shadow virtqueue buffers forwarding")
> > > Signed-off-by: Eugenio Pérez <eperezma@redhat.com>
> > > ---
> > >  hw/virtio/vhost-shadow-virtqueue.c | 17 +++++++++++++++--
> > >  1 file changed, 15 insertions(+), 2 deletions(-)
> > >
> > > diff --git a/hw/virtio/vhost-shadow-virtqueue.c 
> > > b/hw/virtio/vhost-shadow-virtqueue.c
> > > index b232803d1b..c17506df20 100644
> > > --- a/hw/virtio/vhost-shadow-virtqueue.c
> > > +++ b/hw/virtio/vhost-shadow-virtqueue.c
> > > @@ -333,13 +333,25 @@ static void 
> > > vhost_svq_disable_notification(VhostShadowVirtqueue *svq)
> > >      svq->vring.avail->flags |= cpu_to_le16(VRING_AVAIL_F_NO_INTERRUPT);
> > >  }
> > >
> > > +static uint16_t vhost_svq_last_desc_of_chain(VhostShadowVirtqueue *svq,
> > > +                                             uint16_t i)
> > > +{
> > > +    vring_desc_t *descs = svq->vring.desc;
> > > +
> > > +    while (le16_to_cpu(descs[i].flags) & VRING_DESC_F_NEXT) {
> > > +        i = le16_to_cpu(descs[i].next);
> >
> >
> > This seems to be a guest trigger-able infinite loop?
> >
>
> This is the list of the SVQ vring. We could consider an infinite loop
> triggable by the device if it can write the vring directly.
>

Ok.

> I can add a counter in the loop, or to maintain an internal copy of
> the vring so it's completely hardened against malicious/bad devices.
> It should be done for packed vring anyway.

Yes, let's do that. It would be better if we don't trust the device.

Thanks

>
> Thanks!
>
> > Thanks
> >
> >
> > > +    }
> > > +
> > > +    return i;
> > > +}
> > > +
> > >  static VirtQueueElement *vhost_svq_get_buf(VhostShadowVirtqueue *svq,
> > >                                             uint32_t *len)
> > >  {
> > >      vring_desc_t *descs = svq->vring.desc;
> > >      const vring_used_t *used = svq->vring.used;
> > >      vring_used_elem_t used_elem;
> > > -    uint16_t last_used;
> > > +    uint16_t last_used, last_used_chain;
> > >
> > >      if (!vhost_svq_more_used(svq)) {
> > >          return NULL;
> > > @@ -365,7 +377,8 @@ static VirtQueueElement 
> > > *vhost_svq_get_buf(VhostShadowVirtqueue *svq,
> > >          return NULL;
> > >      }
> > >
> > > -    descs[used_elem.id].next = svq->free_head;
> > > +    last_used_chain = vhost_svq_last_desc_of_chain(svq, used_elem.id);
> > > +    descs[last_used_chain].next = svq->free_head;
> > >      svq->free_head = used_elem.id;
> > >
> > >      *len = used_elem.len;
> > > --
> > > 2.27.0
> > >
> >
>