Revert KVM's made-up consistency check on SVM's TLB control.  The APM says
that unsupported encodings are reserved, but the APM doesn't state that
VMRUN checks for a supported encoding.  Unless something is called out
in "Canonicalization and Consistency Checks" or listed as MBZ (Must Be
Zero), AMD behavior is typically to let software shoot itself in the foot.

This reverts commit 174a921b6975ef959dd82ee9e8844067a62e3ec1.

Fixes: 174a921b6975 ("nSVM: Check for reserved encodings of TLB_CONTROL in nested VMCB")
Reported-by: Stefan Sterz <s.sterz@proxmox.com>
Closes: https://lkml.kernel.org/r/b9915c9c-4cf6-051a-2d91-44cc6380f455%40proxmox.com
Cc: stable@vger.kernel.org
Signed-off-by: Sean Christopherson <seanjc@google.com>
---
 arch/x86/kvm/svm/nested.c | 15 ---------------
 1 file changed, 15 deletions(-)

diff --git a/arch/x86/kvm/svm/nested.c b/arch/x86/kvm/svm/nested.c
index 3fea8c47679e..60891b9ce25f 100644
--- a/arch/x86/kvm/svm/nested.c
+++ b/arch/x86/kvm/svm/nested.c
@@ -247,18 +247,6 @@ static bool nested_svm_check_bitmap_pa(struct kvm_vcpu *vcpu, u64 pa, u32 size)
 	    kvm_vcpu_is_legal_gpa(vcpu, addr + size - 1);
 }
 
-static bool nested_svm_check_tlb_ctl(struct kvm_vcpu *vcpu, u8 tlb_ctl)
-{
-	/* Nested FLUSHBYASID is not supported yet.  */
-	switch(tlb_ctl) {
-		case TLB_CONTROL_DO_NOTHING:
-		case TLB_CONTROL_FLUSH_ALL_ASID:
-			return true;
-		default:
-			return false;
-	}
-}
-
 static bool __nested_vmcb_check_controls(struct kvm_vcpu *vcpu,
 					 struct vmcb_ctrl_area_cached *control)
 {
@@ -278,9 +266,6 @@ static bool __nested_vmcb_check_controls(struct kvm_vcpu *vcpu,
 					   IOPM_SIZE)))
 		return false;
 
-	if (CC(!nested_svm_check_tlb_ctl(vcpu, control->tlb_ctl)))
-		return false;
-
 	if (CC((control->int_ctl & V_NMI_ENABLE_MASK) &&
 	       !vmcb12_is_intercept(control, INTERCEPT_NMI))) {
 		return false;
-- 
2.42.0.655.g421f12c284-goog

On Wed, 2023-10-18 at 12:41 -0700, Sean Christopherson wrote:
> Revert KVM's made-up consistency check on SVM's TLB control.  The APM says
> that unsupported encodings are reserved, but the APM doesn't state that
> VMRUN checks for a supported encoding.  Unless something is called out
> in "Canonicalization and Consistency Checks" or listed as MBZ (Must Be
> Zero), AMD behavior is typically to let software shoot itself in the foot.
> 
> This reverts commit 174a921b6975ef959dd82ee9e8844067a62e3ec1.
> 
> Fixes: 174a921b6975 ("nSVM: Check for reserved encodings of TLB_CONTROL in nested VMCB")
> Reported-by: Stefan Sterz <s.sterz@proxmox.com>
> Closes: https://lkml.kernel.org/r/b9915c9c-4cf6-051a-2d91-44cc6380f455%40proxmox.com
> Cc: stable@vger.kernel.org
> Signed-off-by: Sean Christopherson <seanjc@google.com>
> ---
>  arch/x86/kvm/svm/nested.c | 15 ---------------
>  1 file changed, 15 deletions(-)
> 
> diff --git a/arch/x86/kvm/svm/nested.c b/arch/x86/kvm/svm/nested.c
> index 3fea8c47679e..60891b9ce25f 100644
> --- a/arch/x86/kvm/svm/nested.c
> +++ b/arch/x86/kvm/svm/nested.c
> @@ -247,18 +247,6 @@ static bool nested_svm_check_bitmap_pa(struct kvm_vcpu *vcpu, u64 pa, u32 size)
>  	    kvm_vcpu_is_legal_gpa(vcpu, addr + size - 1);
>  }
>  
> -static bool nested_svm_check_tlb_ctl(struct kvm_vcpu *vcpu, u8 tlb_ctl)
> -{
> -	/* Nested FLUSHBYASID is not supported yet.  */
> -	switch(tlb_ctl) {
> -		case TLB_CONTROL_DO_NOTHING:
> -		case TLB_CONTROL_FLUSH_ALL_ASID:
> -			return true;
> -		default:
> -			return false;
> -	}
> -}
> -
>  static bool __nested_vmcb_check_controls(struct kvm_vcpu *vcpu,
>  					 struct vmcb_ctrl_area_cached *control)
>  {
> @@ -278,9 +266,6 @@ static bool __nested_vmcb_check_controls(struct kvm_vcpu *vcpu,
>  					   IOPM_SIZE)))
>  		return false;
>  
> -	if (CC(!nested_svm_check_tlb_ctl(vcpu, control->tlb_ctl)))
> -		return false;
> -
>  	if (CC((control->int_ctl & V_NMI_ENABLE_MASK) &&
>  	       !vmcb12_is_intercept(control, INTERCEPT_NMI))) {
>  		return false;


Yes, after checking Jim's comment (*) on this I still agree that revert is OK.
KVM never passes through the tlb_ctl field (but does copy it to the cache),
thus there is no need to sanitize it.

https://www.spinics.net/lists/kvm/msg316072.html


Reviewed-by: Maxim Levitsky <mlevitsk@redhat.com>

Best regards,
	Maxim Levitsky