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

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

On Tue, Oct 27, 2020 at 01:02:59PM +0000, Felipe Franciosi wrote:
> 
> 
> > On Oct 27, 2020, at 12:56 PM, Michael S. Tsirkin <mst@redhat.com> wrote:
> > 
> > On Tue, Oct 27, 2020 at 12:53:29PM +0000, Felipe Franciosi wrote:
> >> 
> >> 
> >>> On Oct 27, 2020, at 12:25 PM, Michael S. Tsirkin <mst@redhat.com> wrote:
> >>> 
> >>> 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 ?
> >> 
> >> I thought about it. Happy to send a v2 which keeps the check and logs
> >> without throwing an error.
> >> 
> >> Separately, I'm thinking of hooking up QEMU with VRING_ERR so datapath
> >> handlers can notify QEMU that something went broken and NEEDS_RESET
> >> can be flipped on the status register, possibly along a configuration
> >> interrupt. I can see libvhost-user supports that, but are there any
> >> reasons QEMU doesn't do this already?
> > 
> > Mostly because guest support isn't there. That in turn isn't easy,
> > lots of synchronization is needed within guests.
> 
> Do you mean guest support to reset when seeing that bit in status
> following the configuration interrupt?
> 
> It should be safe, though. I can have a stab to see if it breaks
> Windows/Linux at least, and share an RFC if I get anywhere.
> 
> Unless you think it's a waste of time. Ideally guests shouldn't find
> themselves in this situation to begin with, and if they did, resetting
> would arguably just lead them into corruption again (for example). But
> it does provide a mechanism for QEMU to find out that the vhost
> backend stopped. That would help in the context of this patch.
> 
> F.


It's not a waste of time, it's just a lot of work
within guests.

> > 
> > 
> >>> 
> >>>> 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?
> >>> 
> >> 
> >> I did it briefly. I also wrote a mock userspace driver that creates
> >> this condition in a very controlled way (so I can step half-way
> >> through setting up VQs and trigger a migration, for example). But you
> >> know how manual tests are... I may have missed something.
> >> Your expert eyes are most welcome. :)
> >> 
> >> F.
> >