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

["Roger Pau Monné" <roger.pau@citrix.com>]

On Thu, Apr 10, 2025 at 08:27:49AM +0200, Jan Beulich wrote:
> 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.

I have a draft with this approach, and it seems quite better, as it
allows to get rid of hvm_emulate_one_mmio() and the special casing
done in hvm_hap_nested_page_fault().

Thanks, Roger.