| Message ID | 20210625020354.431829-3-seanjc@google.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | KVM: SVM: Final C-bit fixes? | expand |
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@google.com> > Cc: Brijesh Singh <brijesh.singh@amd.com> > Cc: Tom Lendacky <thomas.lendacky@amd.com> > Cc: stable@vger.kernel.org > Signed-off-by: Sean Christopherson <seanjc@google.com> 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@amd.com> > --- > 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); >
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);
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@google.com> Cc: Brijesh Singh <brijesh.singh@amd.com> Cc: Tom Lendacky <thomas.lendacky@amd.com> Cc: stable@vger.kernel.org Signed-off-by: Sean Christopherson <seanjc@google.com> --- arch/x86/kvm/svm/svm.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)