Re: [PATCH V5 1/4] virtio_ring: validate used buffer length

[Stefano Garzarella]

On Mon, Nov 22, 2021 at 02:25:26PM +0800, Jason Wang wrote:
> On Mon, Nov 22, 2021 at 1:49 PM Halil Pasic <pasic@xxxxxxxxxxxxx> wrote:
> > On Mon, 22 Nov 2021 06:35:18 +0100
> > Halil Pasic <pasic@xxxxxxxxxxxxx> wrote:
> >
> > > > I think it should be a common issue, looking at
> > > > vhost_vsock_handle_tx_kick(), it did:
> > > >
> > > > len += sizeof(pkt->hdr);
> > > > vhost_add_used(vq, head, len);
> > > >
> > > > which looks like a violation of the spec since it's TX.
> > >
> > > I'm not sure the lines above look like a violation of the spec. If you
> > > examine vhost_vsock_alloc_pkt() I believe that you will agree that:
> > > len == pkt->len == pkt->hdr.len
> > > which makes sense since according to the spec both tx and rx messages
> > > are hdr+payload. And I believe hdr.len is the size of the payload,
> > > although that does not seem to be properly documented by the spec.
>
> Sorry for being unclear, what I meant is that we probably should use
> zero here. TX doesn't use in buffer actually.
>
> According to the spec, 0 should be the used length:
>
> "and len the total of bytes written into the buffer."
>
> > >
> > > On the other hand tx messages are stated to be device read-only (in the
> > > spec) so if the device writes stuff, that is certainly wrong.
> > >
>
> Yes.
>
> > > If that is what happens.
> > >
> > > Looking at virtqueue_get_buf_ctx_split() I'm not sure that is what
> > > happens. My hypothesis is that we just a last descriptor is an 'in'
> > > type descriptor (i.e. a device writable one). For tx that assumption
> > > would be wrong.
> > >
> > > I will have another look at this today and send a fix patch if my
> > > suspicion is confirmed.
> >
> > If my suspicion is right something like:
> >
> > diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
> > index 00f64f2f8b72..efb57898920b 100644
> > --- a/drivers/virtio/virtio_ring.c
> > +++ b/drivers/virtio/virtio_ring.c
> > @@ -764,6 +764,7 @@ static void *virtqueue_get_buf_ctx_split(struct virtqueue *_vq,
> >         struct vring_virtqueue *vq = to_vvq(_vq);
> >         void *ret;
> >         unsigned int i;
> > +       bool has_in;
> >         u16 last_used;
> >
> >         START_USE(vq);
> > @@ -787,6 +788,9 @@ static void *virtqueue_get_buf_ctx_split(struct virtqueue *_vq,
> >                         vq->split.vring.used->ring[last_used].id);
> >         *len = virtio32_to_cpu(_vq->vdev,
> >                         vq->split.vring.used->ring[last_used].len);
> > +       has_in = virtio16_to_cpu(_vq->vdev,
> > +                       vq->split.vring.used->ring[last_used].flags)
> > +                               & VRING_DESC_F_WRITE;
>
> Did you mean vring.desc actually? If yes, it's better not depend on
> the descriptor ring which can be modified by the device. We've stored
> the flags in desc_extra[].
>
> >         if (unlikely(i >= vq->split.vring.num)) {
> >                 BAD_RING(vq, "id %u out of range\n", i);
> > @@ -796,7 +800,7 @@ static void *virtqueue_get_buf_ctx_split(struct virtqueue *_vq,
> >                 BAD_RING(vq, "id %u is not a head!\n", i);
> >                 return NULL;
> >         }
> > -       if (vq->buflen && unlikely(*len > vq->buflen[i])) {
> > +       if (has_in && q->buflen && unlikely(*len > vq->buflen[i])) {
> >                 BAD_RING(vq, "used len %d is larger than in buflen %u\n",
> >                         *len, vq->buflen[i]);
> >                 return NULL;
> >
> > would fix the problem for split. I will try that out and let you know
> > later.
>
> I'm not sure I get this, in virtqueue_add_split, the buflen[i] only
> contains the in buffer length.
>
> I think the fixes are:
>
> 1) fixing the vhost vsock

Yep, in vhost_vsock_handle_tx_kick() we should have vhost_add_used(vq, 
head, 0) since the device doesn't write anything.

> 2) use suppress_used_validation=true to let vsock driver to validate
> the in buffer length
> 3) probably a new feature so the driver can only enable the validation
> when the feature is enabled.

I fully agree with these steps.

Thanks,
Stefano