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

[Xu Yilun]

On Fri, Mar 08, 2024 at 03:28:08PM -0800, Sean Christopherson wrote:
> 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@xxxxxxxxxxxxxxx>
> > > Cc: Chao Peng <chao.p.peng@xxxxxxxxxxxxxxx>
> > > Cc: Fuad Tabba <tabba@xxxxxxxxxx>
> > > Cc: Michael Roth <michael.roth@xxxxxxx>
> > > Cc: Isaku Yamahata <isaku.yamahata@xxxxxxxxx>
> > > Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
> > > ---
> > >  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.

That makes sense!  I was just thinking of the racing for below few lines,

	if (fault->is_private != kvm_mem_is_private(vcpu->kvm, fault->gfn)) {
		kvm_mmu_prepare_memory_fault_exit(vcpu, fault);
		return -EFAULT;
	}

But the guarding is actually for the whole kvm_faultin_pfn(). It is the
the same mechanism between getting old gfn attributes and getting old pfn.

I wonder if we could instead add some general comments at

   fault->mmu_seq = vcpu->kvm->mmu_invalidate_seq;

about the snapshot and is_page_fault_stale() thing.

Thanks,
Yilun
>