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

[Felipe Franciosi]

> 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?

> 
>> 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.