Re: [PATCH 2/2] x86/hvm: fix write emulation of RO ranges

[Jan Beulich <jbeulich@xxxxxxxx>]

On 09.04.2025 17:33, Roger Pau Monné wrote:
> On Wed, Apr 09, 2025 at 04:08:47PM +0200, Jan Beulich wrote:
>> On 09.04.2025 16:01, Roger Pau Monné wrote:
>>>  As I understand the checks done in
>>> mmio_ro_emulated_write() are to ensure correctness, but carrying the
>>> access even when the %cr2 check fail wouldn't cause issues to Xen
>>> itself?
>>
>> Well, "wouldn't" is too strong for my taste, "shouldn't" would fit. The
>> checking is there to avoid guests playing games. Whether that prevents
>> merely in-guest just-bugs or actual XSAs we can't know until we find a
>> case where the game playing might make us do something wrong. I expect
>> it's unlikely for Xen itself to be affected. But an in-guest privilege
>> escalation would already be bad enough.
> 
> I see.  That was kind of my understanding of the checks.  At least on
> HVM it feels a bit weird to handle r/o regions this way.  It would IMO
> be more natural to use an hvm_io_handler, but that's maybe because I'm
> more familiar with those.

I guess this would be an option; I assume it's the way it is because PVHv1
inherited it from PV, and PVHv2 inherited it from PVHv1.

> And in that regard, hvm_io_handler don't seem to do any of the extra
> checking that mmio_ro_emulated_write() does with the %cr2, but maybe
> that's done by some higher layer?  AFAICT that would ultimately get
> called by hvmemul_read(), and there are no checks there at all.

That more general framework isn't page-fault specific, and hence there's
no CR2 recorded to check against.

Jan