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

[Halil Pasic <pasic@linux.ibm.com>]

On Mon, 22 Nov 2021 12:08:22 +0100
Stefano Garzarella <sgarzare@redhat.com> wrote:

> On Mon, Nov 22, 2021 at 08:55:24AM +0100, Stefano Garzarella wrote:
> >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@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."
> >>  
> >>>>
> >>>> 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.  
> 
> Michael sent a patch to suppress the validation, so I think we should 
> just fix vhost-vsock. I mean something like this:
> 
> diff --git a/drivers/vhost/vsock.c b/drivers/vhost/vsock.c
> index 938aefbc75ec..4e3b95af7ee4 100644
> --- a/drivers/vhost/vsock.c
> +++ b/drivers/vhost/vsock.c
> @@ -554,7 +554,7 @@ static void vhost_vsock_handle_tx_kick(struct vhost_work *work)
>                          virtio_transport_free_pkt(pkt);
> 
>                  len += sizeof(pkt->hdr);
> -               vhost_add_used(vq, head, len);
> +               vhost_add_used(vq, head, 0);
>                  total_len += len;
>                  added = true;
>          } while(likely(!vhost_exceeds_weight(vq, ++pkts, total_len)));
> 
> I checked and the problem is there from the first commit, so we should 
> add:
> 
> Fixes: 433fc58e6bf2 ("VSOCK: Introduce vhost_vsock.ko")
> 
> I tested this patch and it works even without suppressing validation in 
> the virtio core.  But for backwards compatibility we have to suppress it 
> for sure as Michael did.
> 
> Maybe we can have a patch just with this change to backport it easily 
> and one after to clean up a bit the code that was added after (len, 
> total_len).
> 
> @Halil Let me know if you want to do it, otherwise I can do it.
> 

It is fine, it was you guys who figured out the solution so I think
it should either be Jason or you who take credit for the patch. Thanks
for addressing the issue this quickly!

Regards,
Halil