Re: [Qemu-devel] [PATCH v1] virtio: add checks for the size of the indirect table

[Cornelia Huck <cohuck@redhat.com>]

On Tue, 15 Jan 2019 13:08:47 +0300
Dima Stepanov <dimastep@yandex-team.ru> wrote:

> The virtqueue_pop() and virtqueue_get_avail_bytes() routines can use the
> INDIRECT table to get the data. It is possible to create a packet which
> will lead to the assert message like:
>   include/exec/memory.h:1995: void
>   address_space_read_cached(MemoryRegionCache *, hwaddr, void *, int):
>   Assertion `addr < cache->len && len <= cache->len - addr' failed.
>   Aborted
> To do it the first descriptor should have a link to the INDIRECT table
> and set the size of it to 0. It doesn't look good that the guest should
> be able to trigger the assert in qemu. Add additional check for the size
> of the INDIRECT table, which should not be 0.

Ouch, being able to crash QEMU by a specially crafted descriptor is bad.

Looking at the virtio spec, we don't seem to explicitly disallow
indirect descriptors with a zero-length table. So, as an alternative to
marking the device broken, we could also skip over such a descriptor.
Not sure whether that makes sense, though.

> 
> Signed-off-by: Dima Stepanov <dimastep@yandex-team.ru>
> ---
>  hw/virtio/virtio.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/hw/virtio/virtio.c b/hw/virtio/virtio.c
> index 22bd1ac..a1ff647 100644
> --- a/hw/virtio/virtio.c
> +++ b/hw/virtio/virtio.c
> @@ -646,7 +646,7 @@ void virtqueue_get_avail_bytes(VirtQueue *vq, unsigned int *in_bytes,
>          vring_desc_read(vdev, &desc, desc_cache, i);
>  
>          if (desc.flags & VRING_DESC_F_INDIRECT) {
> -            if (desc.len % sizeof(VRingDesc)) {
> +            if (!desc.len || (desc.len % sizeof(VRingDesc))) {
>                  virtio_error(vdev, "Invalid size for indirect buffer table");
>                  goto err;
>              }
> @@ -902,7 +902,7 @@ void *virtqueue_pop(VirtQueue *vq, size_t sz)
>      desc_cache = &caches->desc;
>      vring_desc_read(vdev, &desc, desc_cache, i);
>      if (desc.flags & VRING_DESC_F_INDIRECT) {
> -        if (desc.len % sizeof(VRingDesc)) {
> +        if (!desc.len || (desc.len % sizeof(VRingDesc))) {
>              virtio_error(vdev, "Invalid size for indirect buffer table");
>              goto done;
>          }