Re: [PATCH v2 1/4] virtio-mem: validate device-reported block size

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

On Thu, Jul 23, 2026 at 04:23:34PM -0700, Carlos Bilbao wrote:
> On 7/20/26 02:19, David Hildenbrand (Arm) wrote:
> 
> > > > > We've the virto-mem config struct layout and the kernel source, so for
> > > > > obvious fixes like a NULL check, static analysis is better than fuzzing.
> > > > > Claude took a few mins to find me two examples:
> > > > > 
> > > > > Patch 1: virtio-mem: reject non-power-of-two device_block_size
> > > > > This one is for virtio_mem_init() to check if
> > > > > !is_power_of_2(vm->device_block_size)
> > > > > 
> > > > > Patch 2: virto-mem: validate region_size and usable_region_size
> > > > > THis one checks region_size != 0 and vm->usable_reion_size >
> > > > > vm->region_size.
> > > > > 
> > > > > An endless factory of "silly" checks like these are low hanging fruit.
> > > > "silly" is the right word.
> > > "silly" in what way?
> > > 
> > As in producing "silly" low-hanging fruit patches that don't move the needle
> > when it comes to security.
> > 
> > > Seriously, I'm trying to figure out what you all care about here and
> > > what exactly the threat model you want this driver to work in, and I'm
> > > getting conflicting answers.
> > > 
> > > Either you all do worry about the "device" sending bad data and want to
> > > protect from that, or you don't and you trust it.  Pick one please so
> > > that we know how to deal with these bug reports we are getting.
> > > 
> > > For example, for USB we have said our threat model is:
> > > 
> > >    - we do NOT trust the device before a driver is bound to the device,
> > >      so if a malicious device can do something to the kernel, the kernel
> > >      needs to be fixed.
> > >    - During the probe() call for a USB driver, the driver does NOT trust
> > >      the device, and again, anything a malicious device can do to the
> > >      kernel, the kernel should fix.
> > >    - After probe() for a USB driver succeeds, it's up to the driver if it
> > >      wants to validate all data coming from the device or not.  Right
> > >      now, in general, the kernel trusts the device at that point in time
> > >      so additional checks are discretionary and at the whim of the
> > >      maintainer.
> > > 
> > > For that last point, I will note that some BIG users of Linux (i.e.
> > > billions of Android devices) still explicitly do NOT want to trust the
> > > USB device at this point in time, and are relying on the kernel to
> > > protect the system from bad devices.  In that case, various patches have
> > > been taken to different drivers and subsystems to play whack-a-mole on
> > > while Android gets their act together to finally come up with a solid
> > > defensive plan (like ChromeOS has had for a decade.)  It will be seen
> > > which happens first, all drivers are properly fuzzed and fixed up, or
> > > Android gets their act together and finally fixes their b0rked system
> > > trust model.  I think Android management is relying on the kernel
> > > community to do the kernel work as they keep refusing to staff the
> > > userspace work that they need to do here...
> > Right, and for virtio devices trusting the device after probe is just extremely
> > questionable.
> > 
> > What changes during probe that the device suddenly sends us good data?
> > 
> > Why would a hypervisor that tried to break us before probe not try to break us
> > after probe?
> > 
> > > And yes, I really need to write this up in a more solid document for USB
> > > and get it into the tree, but at least this email thread has forced me
> > > to write down the above :)
> > > 
> > > 
> > > So, again, for virtio drivers, what exactly do you all want to say is
> > > your threat model that the drivers need to handle?  Can you all agree on
> > > something please?  Otherwise, for new developers like Hari, this is
> > > totaly confusion as to what they should be doing.
> > Well, I am also totally confused why we end up checking against some MUST
> > clauses in the spec, but not against others.
> > 
> > I am very much in favor of making virtio-mem completely safe to use even in
> > coco, where it is currently not used at all.
> > 
> > If it's really about "don't let a device trigger any unexpected kernel code
> > execution by sanitizing all input data", fine with me. We should do exactly
> > that. Try checking all MUST clauses etc.
> > 
> > But I don't think doing the "low hanging fruit" adds any security. It should be
> > done properly or not at all.
> 
> 
> I think you're mistaken in assuming that these "silly" fixes don't move the
> needle at all when it comes to security. We can't really quantify these
> things, and one extra NULL check by itself is probably not going to make a
> meaningful difference. But, IMHO, this should be treated as the opposite of
> "death by a thousand cuts", many small hardening improvements, each
> individually insignificant, can collectively make the kernel substantially
> more robust over long periods of time.
> 
> 
> Thanks,
> 
> Carlos


Simply put - no.

DoS attacks, including NULL ptr accesses, are outside the confidential
computing security boundary.  They have to be - denying service to
whoever is not paying them is exactly how cloud vendors get paid.


My preference is to ignore such non-issues - we need to focus on
handling real ones.


Besides, adding random changes all over the code for non issues
can easily introduce new security bugs. That's the classic death by a
thousand cuts.

Thanks,
-- 
MST