Re: upstream kernel crashes

["Michael S. Tsirkin" <mst@redhat.com>]

On Wed, Aug 17, 2022 at 02:36:31PM +0800, Xuan Zhuo wrote:
> On Wed, 17 Aug 2022 08:13:59 +0200, Dmitry Vyukov <dvyukov@google.com> wrote:
> > On Mon, 15 Aug 2022 17:32:06 -0400, Michael wrote:
> > > So if you pass the size parameter for a legacy device it will
> > > try to make the ring smaller and that is not legal with
> > > legacy at all. But the driver treats legacy and modern
> > > the same, it allocates a smaller queue anyway.
> > >
> > > Lo and behold, I pass disable-modern=on to qemu and it happily
> > > corrupts memory exactly the same as GCP does.
> >
> > Ouch!
> >
> > I understand that the host does the actual corruption,
> > but could you think of any additional debug checking in the guest
> > that would caught this in future? Potentially only when KASAN
> > is enabled which can verify validity of memory ranges.
> > Some kind of additional layer of sanity checking.
> >
> > This caused a bit of a havoc for syzbot with almost 100 unique
> > crash signatures, so would be useful to catch such issues more
> > reliably in future.
> 
> We can add a check to vring size before calling vp_legacy_set_queue_address().
> Checking the memory range directly is a bit cumbersome.
> 
> Thanks.

With a comment along the lines of

/* Legacy virtio pci has no way to communicate a change in vq size to
 * the hypervisor. If ring sizes don't match hypervisor will happily
 * corrupt memory.
 */


> diff --git a/drivers/virtio/virtio_pci_legacy.c b/drivers/virtio/virtio_pci_legacy.c
> index 2257f1b3d8ae..0673831f45b6 100644
> --- a/drivers/virtio/virtio_pci_legacy.c
> +++ b/drivers/virtio/virtio_pci_legacy.c
> @@ -146,6 +146,8 @@ static struct virtqueue *setup_vq(struct virtio_pci_device *vp_dev,
>                 goto out_del_vq;
>         }
> 
> +       BUG_ON(num != virtqueue_get_vring_size(vq));
> +
>         /* activate the queue */
>         vp_legacy_set_queue_address(&vp_dev->ldev, index, q_pfn);
> 
> 
> >
> > Thanks

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