linux-kernel - Re: [PATCH 09/16] KVM: x86/mmu: Move private vs. shared check above slot validity checks

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Message-ID: <ZeufCK2Yj_8Bx7EV@google.com>
Date: Fri, 8 Mar 2024 15:28:08 -0800
From: Sean Christopherson <seanjc@...gle.com>
To: Xu Yilun <yilun.xu@...ux.intel.com>
Cc: Paolo Bonzini <pbonzini@...hat.com>, kvm@...r.kernel.org, linux-kernel@...r.kernel.org, 
	Yan Zhao <yan.y.zhao@...el.com>, Isaku Yamahata <isaku.yamahata@...el.com>, 
	Michael Roth <michael.roth@....com>, Yu Zhang <yu.c.zhang@...ux.intel.com>, 
	Chao Peng <chao.p.peng@...ux.intel.com>, Fuad Tabba <tabba@...gle.com>, 
	David Matlack <dmatlack@...gle.com>
Subject: Re: [PATCH 09/16] KVM: x86/mmu: Move private vs. shared check above
 slot validity checks

On Fri, Mar 08, 2024, Xu Yilun wrote:
> On Tue, Feb 27, 2024 at 06:41:40PM -0800, Sean Christopherson wrote:
> > Prioritize private vs. shared gfn attribute checks above slot validity
> > checks to ensure a consistent userspace ABI.  E.g. as is, KVM will exit to
> > userspace if there is no memslot, but emulate accesses to the APIC access
> > page even if the attributes mismatch.
> > 
> > Fixes: 8dd2eee9d526 ("KVM: x86/mmu: Handle page fault for private memory")
> > Cc: Yu Zhang <yu.c.zhang@...ux.intel.com>
> > Cc: Chao Peng <chao.p.peng@...ux.intel.com>
> > Cc: Fuad Tabba <tabba@...gle.com>
> > Cc: Michael Roth <michael.roth@....com>
> > Cc: Isaku Yamahata <isaku.yamahata@...el.com>
> > Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> > ---
> >  arch/x86/kvm/mmu/mmu.c | 15 ++++++++++-----
> >  1 file changed, 10 insertions(+), 5 deletions(-)
> > 
> > diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> > index 9206cfa58feb..58c5ae8be66c 100644
> > --- a/arch/x86/kvm/mmu/mmu.c
> > +++ b/arch/x86/kvm/mmu/mmu.c
> > @@ -4365,11 +4365,6 @@ static int __kvm_faultin_pfn(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault
> >  			return RET_PF_EMULATE;
> >  	}
> >  
> > -	if (fault->is_private != kvm_mem_is_private(vcpu->kvm, fault->gfn)) {
> > -		kvm_mmu_prepare_memory_fault_exit(vcpu, fault);
> > -		return -EFAULT;
> > -	}
> > -
> >  	if (fault->is_private)
> >  		return kvm_faultin_pfn_private(vcpu, fault);
> >  
> > @@ -4410,6 +4405,16 @@ static int kvm_faultin_pfn(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault,
> >  	fault->mmu_seq = vcpu->kvm->mmu_invalidate_seq;
> >  	smp_rmb();
> >  
> > +	/*
> > +	 * Check for a private vs. shared mismatch *after* taking a snapshot of
> > +	 * mmu_invalidate_seq, as changes to gfn attributes are guarded by the
> > +	 * invalidation notifier.
> 
> I didn't see how mmu_invalidate_seq influences gfn attribute judgement.
> And there is no synchronization between the below check and
> kvm_vm_set_mem_attributes(), the gfn attribute could still be changing
> after the snapshot.

There is synchronization.  If kvm_vm_set_mem_attributes() changes the attributes,
and thus bumps mmu_invalidate_seq, after kvm_faultin_pfn() takes its snapshot,
then is_page_fault_stale() will detect that an invalidation related to the gfn
occured and resume the guest *without* installing a mapping in KVM's page tables.

I.e. KVM may read the old, stale gfn attributes, but it will never actually
expose the stale attirubtes to the guest.