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

[Jason Wang <jasowang@redhat.com>]

On Mon, Sep 13, 2021 at 2:36 PM Michael S. Tsirkin <mst@redhat.com> 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@redhat.com>
>
> 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.

> 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

>
> > ---
> >  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
>

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