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

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

On Fri, Jul 17, 2026 at 10:39:40AM +0200, David Hildenbrand (Arm) wrote:
> On 7/17/26 07:48, Michael S. Tsirkin wrote:
> > On Thu, Jul 16, 2026 at 05:59:05PM +0200, David Hildenbrand (Arm) wrote:
> >>> Or do we just always trust virtio mem devices explicitly?
> >>
> >> It's hard for me to understand where we draw the line, really.
> >>
> >> But maybe MST can clarify what we care about in virtio world where the
> >> hypervisor is fully in charge of the device,
> > 
> > Generally:
> > - The guest is expected to whitelist drivers (most drivers have not
> >   been audited).
> 
> But even if you audited your driver, who makes sure that we consider all ways
> where the device could mess with us?

A lot of this is up to a correct setup. For example, make sure all
filesystems are encrypted and refuse to mount unencrypted ones.

> Something feels off here.
> 
> Handling selected out-of-spec scenarios like this feels like a band-aid. Happy
> to be corrected.

Well Documentation/security/snp-tdx-threat-model.rst puts it like this:
	It is important to note
	that this doesn’t imply that the host or VMM are intentionally
	malicious, but that there exists a security value in having a small CoCo
	VM TCB.

and

	While traditionally the host has unlimited access to guest data and can
	leverage this access to attack the guest, the CoCo systems mitigate such
	attacks by adding security features like guest data confidentiality and
	integrity protection.


now, when we are talking about "mitigation" it is indeed becoming a bit
murky.


For me, a rule of thumb I came up with is that if the validation happens
to also be helful for users e.g. to work around buggy devices,
or maybe because we feel failing gracefully is nice because this
will allow to later make use of this config and old drivers will
fail but at least not panic, then it is good to include.


> -- 
> Cheers,
> 
> David