linux-kernel - Re: [PATCH 3/4] KVM: x86/mmu: Make sure pfn is not changed for spurious fault

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Message-ID: <Z6p8aukJgpKqg3Rn@google.com>
Date: Mon, 10 Feb 2025 14:23:38 -0800
From: Sean Christopherson <seanjc@...gle.com>
To: Yan Zhao <yan.y.zhao@...el.com>
Cc: pbonzini@...hat.com, rick.p.edgecombe@...el.com, 
	linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Subject: Re: [PATCH 3/4] KVM: x86/mmu: Make sure pfn is not changed for
 spurious fault

On Sat, Feb 08, 2025, Yan Zhao wrote:
> On Fri, Feb 07, 2025 at 07:07:06AM -0800, Sean Christopherson wrote:
> > On Fri, Feb 07, 2025, Yan Zhao wrote:
> > > Make sure pfn is not changed for a spurious fault by warning in the TDP
> > > MMU. For shadow path, only treat a prefetch fault as spurious when pfn is
> > > not changed, since the rmap removal and add are required when pfn is
> > > changed.
> > 
> > I like sanity checks, but I don't like special casing "prefetch" faults like this.
> > KVM should _never_ change the PFN of a shadow-present SPTE.  The TDP MMU already
> > BUG()s on this, and mmu_spte_update() WARNs on the transition.
> However, both TDP MMU and mmu_set_spte() return RET_PF_SPURIOUS directly before
> the BUG() in TDP MMU or mmu_spte_update() could be hit.

Ah, that's very different than treating a prefetch fault as !spurious though.  I
would be a-ok with this:

	if (is_shadow_present_pte(iter->old_spte) &&
	    (fault->prefetch || is_access_allowed(fault, iter->old_spte)) &&
	    is_last_spte(iter->old_spte, iter->level)) {
		WARN_ON_ONCE(fault->pfn != spte_to_pfn(iter->old_spte));
		return RET_PF_SPURIOUS;
	}