Re: [PATCH] KVM: x86: Replace BUG_ON() with WARN_ON_ONCE() on "bad" nested GPA translation

[David Laight <david.laight.linux@gmail.com>]

On Thu, 18 Jun 2026 11:57:45 -0700
Sean Christopherson <seanjc@google.com> wrote:

> If KVM attempts to translate what it thinks is an L2 GPA with a non-nested
> MMU, simply WARN and return the GPA, i.e. trust the MMU more than the
> caller, as there is zero reason to potentially panic the host kernel just
> because KVM misused an API.

Except that PANIC_ON_WARN stands a reasonable chance of being set.
So it makes little difference.

	David

> 
> Signed-off-by: Sean Christopherson <seanjc@google.com>
> ---
>  arch/x86/kvm/svm/nested.c | 3 ++-
>  arch/x86/kvm/vmx/nested.c | 3 ++-
>  2 files changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/kvm/svm/nested.c b/arch/x86/kvm/svm/nested.c
> index 9aedb88c832d..3e6c671a8dc2 100644
> --- a/arch/x86/kvm/svm/nested.c
> +++ b/arch/x86/kvm/svm/nested.c
> @@ -2152,7 +2152,8 @@ static gpa_t svm_translate_nested_gpa(struct kvm_vcpu *vcpu, gpa_t gpa,
>  	struct vcpu_svm *svm = to_svm(vcpu);
>  	struct kvm_mmu *mmu = vcpu->arch.mmu;
>  
> -	BUG_ON(!mmu_is_nested(vcpu));
> +	if (WARN_ON_ONCE(!mmu_is_nested(vcpu)))
> +		return gpa;
>  
>  	/* Non-GMET walks are always user-walks */
>  	if (!(svm->nested.ctl.misc_ctl & SVM_MISC_ENABLE_GMET))
> diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
> index 3a293640d58c..6957bb6f5cf7 100644
> --- a/arch/x86/kvm/vmx/nested.c
> +++ b/arch/x86/kvm/vmx/nested.c
> @@ -7470,7 +7470,8 @@ static gpa_t vmx_translate_nested_gpa(struct kvm_vcpu *vcpu, gpa_t gpa,
>  {
>  	struct kvm_mmu *mmu = vcpu->arch.mmu;
>  
> -	BUG_ON(!mmu_is_nested(vcpu));
> +	if (WARN_ON_ONCE(!mmu_is_nested(vcpu)))
> +		return gpa;
>  
>  	/*
>  	 * MBEC differentiates based on the effective U/S bit of
> 
> base-commit: 9d4853b044beefa21c4ee3e18c40653601a64ced