Re: [PATCH] virtio: skip guest index check on device load

[Michael S. Tsirkin]

On Tue, Oct 27, 2020 at 11:30:49AM +0000, Stefan Hajnoczi wrote:
> On Mon, Oct 26, 2020 at 03:13:32PM +0000, Felipe Franciosi wrote:
> > QEMU must be careful when loading device state off migration streams to
> > prevent a malicious source from exploiting the emulator. Overdoing these
> > checks has the side effect of allowing a guest to "pin itself" in cloud
> > environments by messing with state which is entirely in its control.
> > 
> > Similarly to what f3081539 achieved in usb_device_post_load(), this
> > commit removes such a check from virtio_load(). Worth noting, the result
> > of a load without this check is the same as if a guest enables a VQ with
> > invalid indexes to begin with. That is, the virtual device is set in a
> > broken state (by the datapath handler) and must be reset.
> > 
> > Signed-off-by: Felipe Franciosi <felipe@nutanix.com>
> > ---
> >  hw/virtio/virtio.c | 12 ------------
> >  1 file changed, 12 deletions(-)
> > 
> > diff --git a/hw/virtio/virtio.c b/hw/virtio/virtio.c
> > index 6f8f865aff..0561bdb857 100644
> > --- a/hw/virtio/virtio.c
> > +++ b/hw/virtio/virtio.c
> > @@ -3136,8 +3136,6 @@ int virtio_load(VirtIODevice *vdev, QEMUFile *f, int 
> > version_id)
> >      RCU_READ_LOCK_GUARD();
> >      for (i = 0; i < num; i++) {
> >          if (vdev->vq[i].vring.desc) {
> > -            uint16_t nheads;
> > -
> >              /*
> >               * VIRTIO-1 devices migrate desc, used, and avail ring 
> > addresses so
> >               * only the region cache needs to be set up.  Legacy devices 
> > need
> > @@ -3157,16 +3155,6 @@ int virtio_load(VirtIODevice *vdev, QEMUFile *f, int 
> > version_id)
> >                  continue;
> >              }
> >  
> > -            nheads = vring_avail_idx(&vdev->vq[i]) - 
> > vdev->vq[i].last_avail_idx;
> > -            /* Check it isn't doing strange things with descriptor 
> > numbers. */
> > -            if (nheads > vdev->vq[i].vring.num) {
> > -                error_report("VQ %d size 0x%x Guest index 0x%x "
> > -                             "inconsistent with Host index 0x%x: delta 
> > 0x%x",
> > -                             i, vdev->vq[i].vring.num,
> > -                             vring_avail_idx(&vdev->vq[i]),
> > -                             vdev->vq[i].last_avail_idx, nheads);
> > -                return -1;
> > -            }
> 
> Michael, the commit that introduced this check seems to have been for
> debugging rather than to prevent a QEMU crash, so this removing the
> check may be safe:
> 
>   commit 258dc7c96bb4b7ca71d5bee811e73933310e168c
>   Author: Michael S. Tsirkin <mst@redhat.com>
>   Date:   Sun Oct 17 20:23:48 2010 +0200
> 
>       virtio: sanity-check available index
> 
>       Checking available index upon load instead of
>       only when vm is running makes is easier to
>       debug failures.

Agreed. Given this, let's keep the message around, just with
LOG_GUEST_ERROR ?


> Felipe: Did you audit the code to make sure the invalid avail_idx value
> and the fields it is propagated to (e.g. shadow_avail_idx) are always
> used in a safe way?