Re: [PATCH 9/9] virtio_ring: validate used buffer length

[Jason Wang]

On Mon, Sep 13, 2021 at 2:57 PM Michael S. Tsirkin <mst@xxxxxxxxxx> wrote:
>
> On Mon, Sep 13, 2021 at 02:40:09PM +0800, Jason Wang wrote:
> > On Mon, Sep 13, 2021 at 2:36 PM Michael S. Tsirkin <mst@xxxxxxxxxx> wrote:
> > >
> > > On Mon, Sep 13, 2021 at 01:53:53PM +0800, Jason Wang wrote:
> > > > This patch validate the used buffer length provided by the device
> > > > before trying to use it. This is done by record the in buffer length
> > > > in a new field in desc_state structure during virtqueue_add(), then we
> > > > can fail the virtqueue_get_buf() when we find the device is trying to
> > > > give us a used buffer length which is greater than the in buffer
> > > > length.
> > > >
> > > > Signed-off-by: Jason Wang <jasowang@xxxxxxxxxx>
> > >
> > > Hmm this was proposed in the past. The overhead here is
> > > not negligeable, so I'd like to know more -
> > > when is it a problem if the used len is too big?
> >
> > One example is: https://github.com/fuzzsa/fuzzsa-bugs/blob/master/virtio_rng.md
> >
> > And there would be more I guess.
>
> That seems to suggest hwrng validation is better, and
> I think it makes sense: will fix all rng drivers in one go.
>
> > > Don't the affected drivers already track the length somewhere
> > > and so can validated it without the extra cost in
> > > virtio core?
> >
> > Probably, but this requires the changes in each device driver. And it
> > would be easily forgotten if new drivers are introduced?
> >
> > Thanks
>
> My thinking is one just has to be aware that before enabling
> any drivers they have to be audited. We can validate
> used length but e.g. for virtio net the length is inside
> the buffer anyway.

Yes, maybe we can introduce a boolean to tell the virtio core that the
driver can do the validation by itself.

>
> If we really have to, maybe use extra->len?

The extra->len has already been used for per descriptor length.

> And maybe have a mod param so the check can be turned off e.g.
> for benchmarking purposes.

Right. And I will benchmark the differences.

Thanks

>
>
>
> > >
> > > > ---
> > > >  drivers/virtio/virtio_ring.c | 23 +++++++++++++++++++++++
> > > >  1 file changed, 23 insertions(+)
> > > >
> > > > diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
> > > > index d2ca0a7365f8..b8374a6144f3 100644
> > > > --- a/drivers/virtio/virtio_ring.c
> > > > +++ b/drivers/virtio/virtio_ring.c
> > > > @@ -69,6 +69,7 @@
> > > >  struct vring_desc_state_split {
> > > >       void *data;                     /* Data for callback. */
> > > >       struct vring_desc *indir_desc;  /* Indirect descriptor, if any. */
> > > > +     u64 buflen;                     /* In buffer length */
> > > >  };
> > > >
> > > >  struct vring_desc_state_packed {
> > > > @@ -76,6 +77,7 @@ struct vring_desc_state_packed {
> > > >       struct vring_packed_desc *indir_desc; /* Indirect descriptor, if any. */
> > > >       u16 num;                        /* Descriptor list length. */
> > > >       u16 last;                       /* The last desc state in a list. */
> > > > +     u64 buflen;                     /* In buffer length */
> > > >  };
> > > >
> > > >  struct vring_desc_extra {
> > > > @@ -490,6 +492,7 @@ static inline int virtqueue_add_split(struct virtqueue *_vq,
> > > >       unsigned int i, n, avail, descs_used, prev, err_idx;
> > > >       int head;
> > > >       bool indirect;
> > > > +     u64 buflen = 0;
> > > >
> > > >       START_USE(vq);
> > > >
> > > > @@ -571,6 +574,7 @@ static inline int virtqueue_add_split(struct virtqueue *_vq,
> > > >                                                    VRING_DESC_F_NEXT |
> > > >                                                    VRING_DESC_F_WRITE,
> > > >                                                    indirect);
> > > > +                     buflen += sg->length;
> > > >               }
> > > >       }
> > > >       /* Last one doesn't continue. */
> > > > @@ -605,6 +609,7 @@ static inline int virtqueue_add_split(struct virtqueue *_vq,
> > > >
> > > >       /* Store token and indirect buffer state. */
> > > >       vq->split.desc_state[head].data = data;
> > > > +     vq->split.desc_state[head].buflen = buflen;
> > > >       if (indirect)
> > > >               vq->split.desc_state[head].indir_desc = desc;
> > > >       else
> > > > @@ -784,6 +789,11 @@ static void *virtqueue_get_buf_ctx_split(struct virtqueue *_vq,
> > > >               BAD_RING(vq, "id %u is not a head!\n", i);
> > > >               return NULL;
> > > >       }
> > > > +     if (unlikely(*len > vq->split.desc_state[i].buflen)) {
> > > > +             BAD_RING(vq, "used len %d is larger than in buflen %lld\n",
> > > > +                     *len, vq->split.desc_state[i].buflen);
> > > > +             return NULL;
> > > > +     }
> > > >
> > > >       /* detach_buf_split clears data, so grab it now. */
> > > >       ret = vq->split.desc_state[i].data;
> > > > @@ -1062,6 +1072,7 @@ static int virtqueue_add_indirect_packed(struct vring_virtqueue *vq,
> > > >       unsigned int i, n, err_idx;
> > > >       u16 head, id;
> > > >       dma_addr_t addr;
> > > > +     u64 buflen = 0;
> > > >
> > > >       head = vq->packed.next_avail_idx;
> > > >       desc = alloc_indirect_packed(total_sg, gfp);
> > > > @@ -1089,6 +1100,8 @@ static int virtqueue_add_indirect_packed(struct vring_virtqueue *vq,
> > > >                       desc[i].addr = cpu_to_le64(addr);
> > > >                       desc[i].len = cpu_to_le32(sg->length);
> > > >                       i++;
> > > > +                     if (n >= out_sgs)
> > > > +                             buflen += sg->length;
> > > >               }
> > > >       }
> > > >
> > > > @@ -1141,6 +1154,7 @@ static int virtqueue_add_indirect_packed(struct vring_virtqueue *vq,
> > > >       vq->packed.desc_state[id].data = data;
> > > >       vq->packed.desc_state[id].indir_desc = desc;
> > > >       vq->packed.desc_state[id].last = id;
> > > > +     vq->packed.desc_state[id].buflen = buflen;
> > > >
> > > >       vq->num_added += 1;
> > > >
> > > > @@ -1176,6 +1190,7 @@ static inline int virtqueue_add_packed(struct virtqueue *_vq,
> > > >       unsigned int i, n, c, descs_used, err_idx;
> > > >       __le16 head_flags, flags;
> > > >       u16 head, id, prev, curr, avail_used_flags;
> > > > +     u64 buflen = 0;
> > > >
> > > >       START_USE(vq);
> > > >
> > > > @@ -1250,6 +1265,8 @@ static inline int virtqueue_add_packed(struct virtqueue *_vq,
> > > >                                       1 << VRING_PACKED_DESC_F_AVAIL |
> > > >                                       1 << VRING_PACKED_DESC_F_USED;
> > > >                       }
> > > > +                     if (n >= out_sgs)
> > > > +                             buflen += sg->length;
> > > >               }
> > > >       }
> > > >
> > > > @@ -1268,6 +1285,7 @@ static inline int virtqueue_add_packed(struct virtqueue *_vq,
> > > >       vq->packed.desc_state[id].data = data;
> > > >       vq->packed.desc_state[id].indir_desc = ctx;
> > > >       vq->packed.desc_state[id].last = prev;
> > > > +     vq->packed.desc_state[id].buflen = buflen;
> > > >
> > > >       /*
> > > >        * A driver MUST NOT make the first descriptor in the list
> > > > @@ -1455,6 +1473,11 @@ static void *virtqueue_get_buf_ctx_packed(struct virtqueue *_vq,
> > > >               BAD_RING(vq, "id %u is not a head!\n", id);
> > > >               return NULL;
> > > >       }
> > > > +     if (unlikely(*len > vq->packed.desc_state[id].buflen)) {
> > > > +             BAD_RING(vq, "used len %d is larger than in buflen %lld\n",
> > > > +                     *len, vq->packed.desc_state[id].buflen);
> > > > +             return NULL;
> > > > +     }
> > > >
> > > >       /* detach_buf_packed clears data, so grab it now. */
> > > >       ret = vq->packed.desc_state[id].data;
> > > > --
> > > > 2.25.1
> > >
>