Re: [PATCH v7 15/26] KVM: nSVM: Add missing consistency check for nCR3 validity

[Sean Christopherson]

On Tue, Mar 03, 2026, Yosry Ahmed wrote:
> >From the APM Volume #2, 15.25.4 (24593—Rev. 3.42—March 2024):
> 
> 	When VMRUN is executed with nested paging enabled
> 	(NP_ENABLE = 1), the following conditions are considered illegal
> 	state combinations, in addition to those mentioned in
> 	“Canonicalization and Consistency Checks”:
> 	• Any MBZ bit of nCR3 is set.
> 	• Any G_PAT.PA field has an unsupported type encoding or any
> 	reserved field in G_PAT has a nonzero value.
> 
> Add the consistency check for nCR3 being a legal GPA with no MBZ bits
> set. The G_PAT.PA check was proposed separately [*].
> 
> [*]https://lore.kernel.org/kvm/20260205214326.1029278-3-jmattson@xxxxxxxxxx/
> 
> Fixes: 4b16184c1cca ("KVM: SVM: Initialize Nested Nested MMU context on VMRUN")
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Yosry Ahmed <yosry@xxxxxxxxxx>
> ---
>  arch/x86/kvm/svm/nested.c | 5 +++++
>  1 file changed, 5 insertions(+)
> 
> diff --git a/arch/x86/kvm/svm/nested.c b/arch/x86/kvm/svm/nested.c
> index 613d5e2e7c3d1..3aaa4f0bb31ab 100644
> --- a/arch/x86/kvm/svm/nested.c
> +++ b/arch/x86/kvm/svm/nested.c
> @@ -348,6 +348,11 @@ static bool nested_vmcb_check_controls(struct kvm_vcpu *vcpu,
>  	if (CC(control->asid == 0))
>  		return false;
>  
> +	if (control->nested_ctl & SVM_NESTED_CTL_NP_ENABLE) {
> +		if (CC(!kvm_vcpu_is_legal_gpa(vcpu, control->nested_cr3)))
> +			return false;

Put the full if-statement in CC(), that way the tracepoint will capture the entire
clause, i.e. will help the reader understand than nested_cr3 was checked
specifically because NPT was enabled.

	if (CC((control->nested_ctl & SVM_NESTED_CTL_NP_ENABLE) &&
	       !kvm_vcpu_is_legal_gpa(vcpu, control->nested_cr3)))
		return false;