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

[Wafer]

> -----Original Message-----
> From: Eugenio Perez Martin <eperezma@redhat.com>
> Sent: 2024年12月12日 15:33
> 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
> 
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> 
> 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?
> 

Thanks, u are right.

> > > > 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
> > > >
> >