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

["Michael S. Tsirkin" <mst@redhat.com>]

On Mon, Nov 22, 2021 at 02:50:03PM +0100, Halil Pasic wrote:
> On Mon, 22 Nov 2021 14:25:26 +0800
> Jason Wang <jasowang@redhat.com> wrote:
> 
> > On Mon, Nov 22, 2021 at 1:49 PM Halil Pasic <pasic@linux.ibm.com> wrote:
> > >
> > > On Mon, 22 Nov 2021 06:35:18 +0100
> > > Halil Pasic <pasic@linux.ibm.com> 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."
> 
> Right, I was wrong. I somehow assumed this is the total length and not
> just the number of bytes written.
> 
> > 
> > > >
> > > > 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.
> 
> Yeah, I didn't remember the semantic of
> vq->split.vring.used->ring[last_used].len
> correctly, and in fact also how exactly the rings work. So your objection
> is correct. 
> 
> Maybe updating some stuff would make it easier to not make this mistake.
> 
> For example the spec and also the linux header says:
> 
> /* le32 is used here for ids for padding reasons. */ 
> struct virtq_used_elem { 
>         /* Index of start of used descriptor chain. */ 
>         le32 id; 
>         /* Total length of the descriptor chain which was used (written to) */ 
>         le32 len; 
> };
> 
> I think that comment isn't as clear as it could be. I would prefer:
> /* The number of bytes written into the device writable portion of the
> buffer described by the descriptor chain. */
> 
> I believe "the descriptor chain which was used" includes both the
> descriptors that map the device read only and the device write
> only portions of the buffer described by the descriptor chain. And the
> total length of that descriptor chain may be defined either as a number
> of the descriptors that form the chain, or the length of the buffer.
> 
> One has to use the descriptor chain even if the whole buffer is device
> read only. So "used" == "written to" does not make any sense to me.

The virtio spec actually says

Total length of the descriptor chain which was written to

without the "used" part.

> Also something like
> int vhost_add_used(struct vhost_virtqueue *vq, unsigned int head, int bytes_written)
> instead of
> int vhost_add_used(struct vhost_virtqueue *vq, unsigned int head, int len)
> would make it easier to read the code correctly.

I think we agree here. Patches?

> > >
> > > 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.
> 
> Sorry my diff is indeed silly.
> 
> > 
> > I think the fixes are:
> > 
> > 1) fixing the vhost vsock
> > 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.
> > 
> 
> Makes sense!
> 
> Regards,
> Halil

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