Re: upstream kernel crashes

[Xuan Zhuo]

On Wed, 17 Aug 2022 08:13:59 +0200, Dmitry Vyukov <dvyukov@xxxxxxxxxx> 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.

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