Re: [PATCH 2/2] KVM: SVM: Revert clearing of C-bit on GPA in #NPF handler

From: Tom Lendacky
Date: Fri Jun 25 2021 - 09:27:04 EST

On 6/24/21 9:03 PM, Sean Christopherson wrote:
> Don't clear the C-bit in the #NPF handler, as it is a legal GPA bit for
> non-SEV guests, and for SEV guests the C-bit is dropped before the GPA
> hits the NPT in hardware. Clearing the bit for non-SEV guests causes KVM
> to mishandle #NPFs with that collide with the host's C-bit.
> Although the APM doesn't explicitly state that the C-bit is not reserved
> for non-SEV, Tom Lendacky confirmed that the following snippet about the
> effective reduction due to the C-bit does indeed apply only to SEV guests.
> Note that because guest physical addresses are always translated
> through the nested page tables, the size of the guest physical address
> space is not impacted by any physical address space reduction indicated
> in CPUID 8000_001F[EBX]. If the C-bit is a physical address bit however,
> the guest physical address space is effectively reduced by 1 bit.
> And for SEV guests, the APM clearly states that the bit is dropped before
> walking the nested page tables.
> If the C-bit is an address bit, this bit is masked from the guest
> physical address when it is translated through the nested page tables.
> Consequently, the hypervisor does not need to be aware of which pages
> the guest has chosen to mark private.
> Note, the bogus C-bit clearing was removed from legacy #PF handler in
> commit 6d1b867d0456 ("KVM: SVM: Don't strip the C-bit from CR2 on #PF
> interception").
> Fixes: 0ede79e13224 ("KVM: SVM: Clear C-bit from the page fault address")
> Cc: Peter Gonda <pgonda@xxxxxxxxxx>
> Cc: Brijesh Singh <brijesh.singh@xxxxxxx>
> Cc: Tom Lendacky <thomas.lendacky@xxxxxxx>
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>

Yep, definitely wasn't correct to be using an SME based macro on a guest
address. And as the APM states, the encryption bit is stripped for SEV
guests, so looks correct to me.

Acked-by: Tom Lendacky <thomas.lendacky@xxxxxxx>

> ---
> arch/x86/kvm/svm/svm.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
> diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
> index 8834822c00cd..ca5614a48b21 100644
> --- a/arch/x86/kvm/svm/svm.c
> +++ b/arch/x86/kvm/svm/svm.c
> @@ -1923,7 +1923,7 @@ static int npf_interception(struct kvm_vcpu *vcpu)
> {
> struct vcpu_svm *svm = to_svm(vcpu);
> - u64 fault_address = __sme_clr(svm->vmcb->control.exit_info_2);
> + u64 fault_address = svm->vmcb->control.exit_info_2;
> u64 error_code = svm->vmcb->control.exit_info_1;
> trace_kvm_page_fault(fault_address, error_code);