Re: [PATCH v2] hw/virtio: Fix check available index on virtio loading

[Eugenio Perez Martin]

On Thu, Dec 12, 2024 at 3:30 AM Wafer <wafer@jaguarmicro.com> wrote:
>
>
>
> > -----Original Message-----
> > From: Eugenio Perez Martin <eperezma@redhat.com>
> > Sent: 2024年12月11日 20:45
> > To: Wafer <wafer@jaguarmicro.com>
> > Cc: mst@redhat.com; jasowang@redhat.com; qemu-devel@nongnu.org;
> > Angus Chen <angus.chen@jaguarmicro.com>
> > Subject: Re: [PATCH v2] hw/virtio: Fix check available index on virtio 
> > loading
> >
> > External Mail: This email originated from OUTSIDE of the organization!
> > Do not click links, open attachments or provide ANY information unless you
> > recognize the sender and know the content is safe.
> >
> >
> > On Wed, Dec 11, 2024 at 1:34 PM Wafer <Wafer@jaguarmicro.com> wrote:
> > >
> > > From: Wafer Xie <wafer@jaguarmicro.com>
> > >
> > > The virtio-1.2 specification writes:
> > >
> > > 2.7.6 The Virtqueue Available Ring:
> > > "idx field indicates where the driver would put the next descriptor
> > > entry in the ring (modulo the queue size). This starts at 0, and 
> > > increases"
> > >
> > > The idx will increase from 0 to 0xFFFF and repeat, So idx may be less
> > > than last_avail_idx.
> > >
> >
> > I don't get this change. If that happens the driver went buggy or malicious
> > and the next check nheads > vring.num should mark the vq as buggy, isn't it?
> >
>
> During the migration process, let's assume a scenario where:
> The depth of the avail ring is 0x10000, last_avail_index is 0xFFF0, and 
> avail->idx is 0xFFFFF.
> At this point, the guest VM driver sends a virtio data packet, and avail->idx 
> is updated to 0x0.
> The migration occurs, and last_avail_index is sent to the target QEMU.
> During the loading process of the target QEMU, it will check both 
> last_avail_index and avail->idx.
> In this case, last_avail_index is greater than avail->idx.
>

But (uint16_t)0x0 - (uint16_t)0xFFF0 is well defined to 0x10. So
nheads value is correct, isn't it?

> > > Fixes: 258dc7c96b ("virtio: sanity-check available index")
> > >
> > > Signed-off-by: Wafer Xie <wafer@jaguarmicro.com>
> > >
> > > --
> > > Changes in v2:
> > >  -Modify the commit id of the fix.
> > > ---
> > >  hw/virtio/virtio.c | 8 +++++++-
> > >  1 file changed, 7 insertions(+), 1 deletion(-)
> > >
> > > diff --git a/hw/virtio/virtio.c b/hw/virtio/virtio.c index
> > > a26f18908e..ae7d407113 100644
> > > --- a/hw/virtio/virtio.c
> > > +++ b/hw/virtio/virtio.c
> > > @@ -3362,7 +3362,13 @@ virtio_load(VirtIODevice *vdev, QEMUFile *f, int
> > version_id)
> > >                  continue;
> > >              }
> > >
> > > -            nheads = vring_avail_idx(&vdev->vq[i]) - 
> > > vdev->vq[i].last_avail_idx;
> > > +            if (vring_avail_idx(&vdev->vq[i]) >= 
> > > vdev->vq[i].last_avail_idx) {
> > > +                nheads = vring_avail_idx(&vdev->vq[i]) -
> > > +                         vdev->vq[i].last_avail_idx;
> > > +            } else {
> > > +                nheads = UINT16_MAX - vdev->vq[i].last_avail_idx +
> > > +                         vring_avail_idx(&vdev->vq[i]) + 1;
> > > +            }
> > >              /* Check it isn't doing strange things with descriptor 
> > > numbers. */
> > >              if (nheads > vdev->vq[i].vring.num) {
> > >                  virtio_error(vdev, "VQ %d size 0x%x Guest index 0x%x "
> > > --
> > > 2.27.0
> > >
>