linux-kernel - Re: [PATCH v15 21/23] KVM: MMU: Disable fast path for private memslots

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Date: Fri, 10 May 2024 10:47:00 -0700
From: Isaku Yamahata <isaku.yamahata@...el.com>
To: Sean Christopherson <seanjc@...gle.com>
Cc: Michael Roth <michael.roth@....com>,
	Paolo Bonzini <pbonzini@...hat.com>, kvm@...r.kernel.org,
	linux-coco@...ts.linux.dev, linux-mm@...ck.org,
	linux-crypto@...r.kernel.org, x86@...nel.org,
	linux-kernel@...r.kernel.org, tglx@...utronix.de, mingo@...hat.com,
	jroedel@...e.de, thomas.lendacky@....com, hpa@...or.com,
	ardb@...nel.org, vkuznets@...hat.com, jmattson@...gle.com,
	luto@...nel.org, dave.hansen@...ux.intel.com, slp@...hat.com,
	pgonda@...gle.com, peterz@...radead.org,
	srinivas.pandruvada@...ux.intel.com, rientjes@...gle.com,
	dovmurik@...ux.ibm.com, tobin@....com, bp@...en8.de, vbabka@...e.cz,
	kirill@...temov.name, ak@...ux.intel.com, tony.luck@...el.com,
	sathyanarayanan.kuppuswamy@...ux.intel.com, alpergun@...gle.com,
	jarkko@...nel.org, ashish.kalra@....com, nikunj.dadhania@....com,
	pankaj.gupta@....com, liam.merwick@...cle.com, papaluri@....com,
	Isaku Yamahata <isaku.yamahata@...el.com>,
	isaku.yamahata@...ux.intel.com, rick.p.edgecombe@...el.com
Subject: Re: [PATCH v15 21/23] KVM: MMU: Disable fast path for private
 memslots

On Fri, May 10, 2024 at 08:59:09AM -0700,
Sean Christopherson <seanjc@...gle.com> wrote:

> On Fri, May 10, 2024, Michael Roth wrote:
> > On Fri, May 10, 2024 at 03:50:26PM +0200, Paolo Bonzini wrote:
> > > On Fri, May 10, 2024 at 3:47 PM Sean Christopherson <seanjc@...gle.com> wrote:
> > > >
> > > > > +      * Since software-protected VMs don't have a notion of a shared vs.
> > > > > +      * private that's separate from what KVM is tracking, the above
> > > > > +      * KVM_EXIT_MEMORY_FAULT condition wouldn't occur, so avoid the
> > > > > +      * special handling for that case for now.
> > > >
> > > > Very technically, it can occur if userspace _just_ modified the attributes.  And
> > > > as I've said multiple times, at least for now, I want to avoid special casing
> > > > SW-protected VMs unless it is *absolutely* necessary, because their sole purpose
> > > > is to allow testing flows that are impossible to excercise without SNP/TDX hardware.
> > > 
> > > Yep, it is not like they have to be optimized.
> > 
> > Ok, I thought there were maybe some future plans to use sw-protected VMs
> > to get some added protections from userspace. But even then there'd
> > probably still be extra considerations for how to handle access tracking
> > so white-listing them probably isn't right anyway.
> > 
> > I was also partly tempted to take this route because it would cover this
> > TDX patch as well:
> > 
> >   https://lore.kernel.org/lkml/91c797997b57056224571e22362321a23947172f.1705965635.git.isaku.yamahata@intel.com/
> 
> Hmm, I'm pretty sure that patch is trying to fix the exact same issue you are
> fixing, just in a less precise way.  S-EPT entries only support RWX=0 and RWX=111b,
> i.e. it should be impossible to have a write-fault to a present S-EPT entry.
> 
> And if TDX is running afoul of this code:
> 
> 	if (!fault->present)
> 		return !kvm_ad_enabled();
> 
> then KVM should do the sane thing and require A/D support be enabled for TDX.
> 
> And if it's something else entirely, that changelog has some explaining to do.

Yes, it's for KVM_EXIT_MEMORY_FAULT case.  Because Secure-EPT has non-present or
all RWX allowed, fast page fault always returns RET_PF_INVALID by
is_shadow_present_pte() check.

I lightly tested the patch at [1] and it works for TDX KVM.

[1] https://github.com/mdroth/linux/commit/39643f9f6da6265d39d633a703c53997985c1208

Just in case for that patch,
Reviewed-by: Isaku Yamahata <isaku.yamahata@...el.com>
-- 
Isaku Yamahata <isaku.yamahata@...el.com>