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

[Halil Pasic]

On Mon, 22 Nov 2021 14:25:26 +0800
Jason Wang <jasowang@xxxxxxxxxx> 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."

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.

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.

> >
> > 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