linux-kernel - Re: [PATCH] KVM: x86/mmu: Reject direct bits in gpa passed to KVM_PRE_FAULT

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Message-ID: <aE97lRNOmJ3g4Xkk@yzhao56-desk.sh.intel.com>
Date: Mon, 16 Jun 2025 10:04:05 +0800
From: Yan Zhao <yan.y.zhao@...el.com>
To: Sean Christopherson <seanjc@...gle.com>
CC: Paolo Bonzini <pbonzini@...hat.com>, <linux-kernel@...r.kernel.org>,
	<kvm@...r.kernel.org>, <xiaoyao.li@...el.com>
Subject: Re: [PATCH] KVM: x86/mmu: Reject direct bits in gpa passed to
 KVM_PRE_FAULT_MEMORY

On Fri, Jun 13, 2025 at 01:16:38PM -0700, Sean Christopherson wrote:
> On Fri, Jun 13, 2025, Yan Zhao wrote:
> > On Thu, Jun 12, 2025 at 12:49:43AM -0400, Paolo Bonzini wrote:
> > > Only let userspace pass the same addresses that were used in KVM_SET_USER_MEMORY_REGION
> > > (or KVM_SET_USER_MEMORY_REGION2); gpas in the the upper half of the address space
> > > are an implementation detail of TDX and KVM.
> > > 
> > > Extracted from a patch by Sean Christopherson <seanjc@...gle.com>.
> > > 
> > > Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>
> > > ---
> > >  arch/x86/kvm/mmu/mmu.c | 3 +++
> > >  1 file changed, 3 insertions(+)
> > > 
> > > diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> > > index a4040578b537..4e06e2e89a8f 100644
> > > --- a/arch/x86/kvm/mmu/mmu.c
> > > +++ b/arch/x86/kvm/mmu/mmu.c
> > > @@ -4903,6 +4903,9 @@ long kvm_arch_vcpu_pre_fault_memory(struct kvm_vcpu *vcpu,
> > >  	if (!vcpu->kvm->arch.pre_fault_allowed)
> > >  		return -EOPNOTSUPP;
> > >  
> > > +	if (kvm_is_gfn_alias(vcpu->kvm, gpa_to_gfn(range->gpa)))
> > > +		return -EINVAL;
> > > +
> > Do we need the same check in kvm_vm_ioctl_set_mem_attributes()?
> 
> Yeah, we probably should disallow aliases there.  It should be benign?  Because
> memslots disallow aliases, and so the aliased gfn entries in the xarray will never
> actually be consumed.
Yes, it's benign after this patch.
Userspace may find that setting attribute for an aliased gfn has no effect.
Though there's  a "kvm_mem_is_private(vcpu->kvm, gpa_to_gfn(cr2_or_gpa)" in
kvm_mmu_page_fault(), it's only for KVM_X86_SW_PROTECTED_VM. So it's benign,
just odd :)