Re: [PATCH 3/5] x86/hvm: fix handling of accesses to partial r/o MMIO pages

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

On Tue, Apr 15, 2025 at 11:41:27AM +0200, Jan Beulich wrote:
> On 15.04.2025 10:34, Roger Pau Monné wrote:
> > On Tue, Apr 15, 2025 at 09:32:37AM +0200, Jan Beulich wrote:
> >> On 14.04.2025 18:13, Roger Pau Monné wrote:
> >>> On Mon, Apr 14, 2025 at 05:24:32PM +0200, Jan Beulich wrote:
> >>>> On 14.04.2025 15:53, Roger Pau Monné wrote:
> >>>>> On Mon, Apr 14, 2025 at 08:33:44AM +0200, Jan Beulich wrote:
> >>>>>> I'm also concerned of e.g. VT-x'es APIC access MFN, which is
> >>>>>> p2m_mmio_direct.
> >>>>>
> >>>>> But that won't go into hvm_hap_nested_page_fault() when using
> >>>>> cpu_has_vmx_virtualize_apic_accesses (and thus having an APIC page
> >>>>> mapped as p2m_mmio_direct)?
> >>>>>
> >>>>> It would instead be an EXIT_REASON_APIC_ACCESS vmexit which is handled
> >>>>> differently?
> >>>>
> >>>> All true as long as things work as expected (potentially including the guest
> >>>> also behaving as expected). Also this was explicitly only an example I could
> >>>> readily think of. I'm simply wary of handle_mmio_with_translation() now
> >>>> getting things to handle it's not meant to ever see.
> >>>
> >>> How was access to MMIO r/o regions supposed to be handled before
> >>> 33c19df9a5a0 (~2015)?  I see that setting r/o MMIO p2m entries was
> >>> added way before to p2m_type_to_flags() and ept_p2m_type_to_flags()
> >>> (~2010), yet I can't figure out how writes would be handled back then
> >>> that didn't result in a p2m fault and crashing of the domain.
> >>
> >> Was that handled at all before said change?
> > 
> > Not really AFAICT, hence me wondering how where write accesses to r/o
> > MMIO regions supposed to be handled by (non-priv) domains.  Was the
> > expectation that those writes trigger an p2m violation thus crashing
> > the domain?
> 
> I think so, yes. Devices with such special areas weren't (aren't?) supposed
> to be handed to DomU-s.

Oh, I see.  That makes stuff a bit clearer.  I think we would then
also want to add some checks to {ept_}p2m_type_to_flags()?

I wonder why handling of mmio_ro_ranges was added to the HVM p2m code
in ~2010 then.  If mmio_ro_ranges is only supposed to be relevant for
the hardware domain in ~2010 an HVM dom0 was not even in sight?

Sorry to ask so many questions, I'm a bit confused about how this
was/is supposed to work.

> >> mmio_ro_do_page_fault() was
> >> (and still is) invoked for the hardware domain only, and quite likely
> >> the need for handling (discarding) writes for PVHv1 had been overlooked
> >> until someone was hit by the lack thereof.
> > 
> > I see, I didn't realize r/o MMIO was only handled for PV hardware
> > domains only.  I could arguably do the same for HVM in
> > hvm_hap_nested_page_fault().
> > 
> > Not sure whether the subpage stuff is supposed to be functional for
> > domains different than the hardware domain?  It seems to be available
> > to the hanrdware domain only for PV guests, while for HVM is available
> > for both PV and HVM domains:
> 
> DYM Dom0 and DomU here?

Indeed, sorry.

> > is_hardware_domain(currd) || subpage_mmio_write_accept(mfn, gla)
> > 
> > In hvm_hap_nested_page_fault().
> 
> See the three XHCI_SHARE_* modes. When it's XHCI_SHARE_ANY, even DomU-s
> would require this handling. It looks like a mistake that we permit the
> path to be taken for DomU-s even when the mode is XHCI_SHARE_HWDOM.

Arguable a domU will never get the device assigned in the first place
unless the share mode is set to XHCI_SHARE_ANY.  For the other modes
the device is hidden, and hence couldn't be assigned to a domU anyway.

> It
> also looks like a mistake that the PV path has remained Dom0-only, even
> in the XHCI_SHARE_ANY case. Cc-ing Marek once again ...
> 
> >>> I'm happy to look at other ways to handling this, but given there's
> >>> current logic for handling accesses to read-only regions in
> >>> hvm_hap_nested_page_fault() I think re-using that was the best way to
> >>> also handle accesses to MMIO read-only regions.
> >>>
> >>> Arguably it would already be the case that for other reasons Xen would
> >>> need to emulate an instruction that accesses a read-only MMIO region?
> >>
> >> Aiui hvm_translate_get_page() will yield HVMTRANS_bad_gfn_to_mfn for
> >> p2m_mmio_direct (after all, "direct" means we expect no emulation is
> >> needed; while arguably wrong for the introspection case, I'm not sure
> >> that and pass-through actually go together). Hence it's down to
> >> hvmemul_linear_mmio_access() -> hvmemul_phys_mmio_access() ->
> >> hvmemul_do_mmio_buffer() -> hvmemul_do_io_buffer() -> hvmemul_do_io(),
> >> which means that if hvm_io_intercept() can't handle it, the access
> >> will be forwarded to the responsible DM, or be "processed" by the
> >> internal null handler.
> >>
> >> Given this, perhaps what you do is actually fine. At the same time
> >> note how several functions in hvm/emulate.c simply fail upon
> >> encountering p2m_mmio_direct. These are all REP handlers though, so
> >> the main emulator would then try emulating the insn the non-REP way.
> > 
> > I'm open to alternative ways of handling such accesses, just used what
> > seemed more natural in the context of hvm_hap_nested_page_fault().
> > 
> > Emulation of r/o MMIO accesses failing wouldn't be an issue from Xen's
> > perspective, that would "just" result in the guest getting a #GP
> > injected.
> 
> That's not the part I'm worried about. What worries me is that we open up
> another (or better: we're widening a) way to hit the emulator in the first
> place. (Plus, as said, the issue with the not really tidy P2M type system.)

But the hit would be limited to domains having r/o p2m_mmio_direct
entries in the p2m, as otherwise the path would be unreachable?

> > Would you like me to add some of your reasoning above to the commit
> > message?
> 
> While I'd still be a little hesitant as to ack-ing of the result, I think
> that's all I'm really asking for, yes.

As said before - I'm happy to consider suggestions here, I don't want
to fix this with yet another bodge that will cause us further issues
down the road.  What I proposed seemed like the most natural way to
handle those accesses IMO, but I'm not an expert on the emulator.

Thanks, Roger.