linux-kernel - Re: [PATCH v2 3/8] KVM: x86: nSVM: Add validity check for vmcb12 g

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Message-ID: <mcxo54ct7bsu2d6xmalpsae6wq32ykjh6dd4kfgcgru474dqre@47flraln5zz5>
Date: Thu, 22 Jan 2026 01:40:49 +0000
From: Yosry Ahmed <yosry.ahmed@...ux.dev>
To: Jim Mattson <jmattson@...gle.com>
Cc: Sean Christopherson <seanjc@...gle.com>, 
	Paolo Bonzini <pbonzini@...hat.com>, Thomas Gleixner <tglx@...utronix.de>, 
	Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>, 
	Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org, "H. Peter Anvin" <hpa@...or.com>, 
	Shuah Khan <shuah@...nel.org>, kvm@...r.kernel.org, linux-kernel@...r.kernel.org, 
	linux-kselftest@...r.kernel.org
Subject: Re: [PATCH v2 3/8] KVM: x86: nSVM: Add validity check for vmcb12
 g_pat

On Thu, Jan 15, 2026 at 03:21:42PM -0800, Jim Mattson wrote:
> Add a validity check for g_pat, so that when nested paging is enabled for
> vmcb12, an invalid g_pat causes an immediate VMEXIT with exit code
> VMEXIT_INVALID, as specified in the APM, volume 2: "Nested Paging and
> VMRUN/VMEXIT."
> 
> Update the signature of __nested_vmcb_check_save() to include a pointer to
> a struct vmcb_ctrl_area_cached, since the g_pat validity check depend on
> the nested paging control bit.
> 
> Fixes: 3d6368ef580a ("KVM: SVM: Add VMRUN handler")
> Signed-off-by: Jim Mattson <jmattson@...gle.com>
> ---
>  arch/x86/kvm/svm/nested.c | 12 +++++++++---
>  1 file changed, 9 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/x86/kvm/svm/nested.c b/arch/x86/kvm/svm/nested.c
> index 07a57a43fc3b..e65291434be9 100644
> --- a/arch/x86/kvm/svm/nested.c
> +++ b/arch/x86/kvm/svm/nested.c
> @@ -369,7 +369,8 @@ static bool __nested_vmcb_check_controls(struct kvm_vcpu *vcpu,
>  
>  /* Common checks that apply to both L1 and L2 state.  */
>  static bool __nested_vmcb_check_save(struct kvm_vcpu *vcpu,
> -				     struct vmcb_save_area_cached *save)
> +				     struct vmcb_save_area_cached *save,
> +				     struct vmcb_ctrl_area_cached *control)
>  {
>  	if (CC(!(save->efer & EFER_SVME)))
>  		return false;
> @@ -400,6 +401,10 @@ static bool __nested_vmcb_check_save(struct kvm_vcpu *vcpu,
>  	if (CC(!kvm_valid_efer(vcpu, save->efer)))
>  		return false;
>  
> +	if (CC((control->nested_ctl & SVM_NESTED_CTL_NP_ENABLE) &&
> +	       npt_enabled && !kvm_pat_valid(save->g_pat)))

If this lands after "KVM: nSVM: Drop the non-architectural consistency
check for NP_ENABLE" [1], the npt_enabled check can be dropped, as
SVM_NESTED_CTL_NP_ENABLE will be cleared if the guest cannot use NPTs.

[1]https://lore.kernel.org/kvm/20260115011312.3675857-16-yosry.ahmed@linux.dev/

> +		return false;
> +
>  	return true;
>  }
>  
> @@ -407,8 +412,9 @@ static bool nested_vmcb_check_save(struct kvm_vcpu *vcpu)
>  {
>  	struct vcpu_svm *svm = to_svm(vcpu);
>  	struct vmcb_save_area_cached *save = &svm->nested.save;
> +	struct vmcb_ctrl_area_cached *ctl = &svm->nested.ctl;
>  
> -	return __nested_vmcb_check_save(vcpu, save);
> +	return __nested_vmcb_check_save(vcpu, save, ctl);
>  }
>  
>  static bool nested_vmcb_check_controls(struct kvm_vcpu *vcpu)
> @@ -1892,7 +1898,7 @@ static int svm_set_nested_state(struct kvm_vcpu *vcpu,
>  	if (!(save->cr0 & X86_CR0_PG) ||
>  	    !(save->cr0 & X86_CR0_PE) ||
>  	    (save->rflags & X86_EFLAGS_VM) ||
> -	    !__nested_vmcb_check_save(vcpu, &save_cached))
> +	    !__nested_vmcb_check_save(vcpu, &save_cached, &ctl_cached))

save_cached here is from vmcb01, but ctl_cached is from vmcb12, right?
So we're conditioning the PAT validity check on L1's gPAT on NPT being
enabled for L2 IIUC.

Perhaps we should pass 'bool guest_npt' instead of the cached control
area?  Then we can just pass npt_enabled here IIUC.

>  		goto out_free;
>  
>  
> -- 
> 2.52.0.457.g6b5491de43-goog
>