[PATCH] KVM: nSVM: check for EFER.SVME=1 before entering guest

[PATCH] KVM: nSVM: check for EFER.SVME=1 before entering guest

[Paolo Bonzini]

EFER is set for L2 using svm_set_efer, which hardcodes EFER_SVME to 1 and hides
an incorrect value for EFER.SVME in the L1 VMCB.  Perform the check manually
to detect invalid guest state.

Reported-by: Krish Sadhukhan <krish.sadhukhan@oracle.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
---
 arch/x86/kvm/svm.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
index 08568ae9f7a1..2125c6ae5951 100644
--- a/arch/x86/kvm/svm.c
+++ b/arch/x86/kvm/svm.c
@@ -3558,6 +3558,9 @@ static bool nested_svm_vmrun_msrpm(struct vcpu_svm *svm)
 
 static bool nested_vmcb_checks(struct vmcb *vmcb)
 {
+	if ((vmcb->save.efer & EFER_SVME) == 0)
+		return false;
+
 	if ((vmcb->control.intercept & (1ULL << INTERCEPT_VMRUN)) == 0)
 		return false;
 
-- 
1.8.3.1

Re: [PATCH] KVM: nSVM: check for EFER.SVME=1 before entering guest

[Vitaly Kuznetsov]

Paolo Bonzini <pbonzini@redhat.com> writes:

> EFER is set for L2 using svm_set_efer, which hardcodes EFER_SVME to 1 and hides
> an incorrect value for EFER.SVME in the L1 VMCB.  Perform the check manually
> to detect invalid guest state.
>
> Reported-by: Krish Sadhukhan <krish.sadhukhan@oracle.com>
> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
> ---
>  arch/x86/kvm/svm.c | 3 +++
>  1 file changed, 3 insertions(+)
>
> diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
> index 08568ae9f7a1..2125c6ae5951 100644
> --- a/arch/x86/kvm/svm.c
> +++ b/arch/x86/kvm/svm.c
> @@ -3558,6 +3558,9 @@ static bool nested_svm_vmrun_msrpm(struct vcpu_svm *svm)
>  
>  static bool nested_vmcb_checks(struct vmcb *vmcb)
>  {
> +	if ((vmcb->save.efer & EFER_SVME) == 0)
> +		return false;
> +
>  	if ((vmcb->control.intercept & (1ULL << INTERCEPT_VMRUN)) == 0)
>  		return false;

Reviewed-by: Vitaly Kuznetsov <vkuznets@redhat.com>

-- 
Vitaly

Re: [PATCH] KVM: nSVM: check for EFER.SVME=1 before entering guest

[Krish Sadhukhan]

On 3/18/20 5:41 AM, Paolo Bonzini wrote:
> EFER is set for L2 using svm_set_efer, which hardcodes EFER_SVME to 1 and hides
> an incorrect value for EFER.SVME in the L1 VMCB.  Perform the check manually
> to detect invalid guest state.
>
> Reported-by: Krish Sadhukhan <krish.sadhukhan@oracle.com>
> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
> ---
>   arch/x86/kvm/svm.c | 3 +++
>   1 file changed, 3 insertions(+)
>
> diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
> index 08568ae9f7a1..2125c6ae5951 100644
> --- a/arch/x86/kvm/svm.c
> +++ b/arch/x86/kvm/svm.c
> @@ -3558,6 +3558,9 @@ static bool nested_svm_vmrun_msrpm(struct vcpu_svm *svm)
>   
>   static bool nested_vmcb_checks(struct vmcb *vmcb)
>   {
> +	if ((vmcb->save.efer & EFER_SVME) == 0)
> +		return false;
> +
>   	if ((vmcb->control.intercept & (1ULL << INTERCEPT_VMRUN)) == 0)
>   		return false;
>   

Ah! This now tells me that I forgot the KVM fix that was supposed to 
accompany my patchset.

Do we need this check in software ? I wasn't checking the bit in KVM and 
instead I was just making sure that L0 sets that bit based on the 
setting in nested vmcb:


+static void nested_svm_set_efer(struct kvm_vcpu *vcpu, u64 
nested_vmcb_efer)
+{
+       svm_set_efer(vcpu, nested_vmcb_efer);
+
+       if (!(nested_vmcb_efer & EFER_SVME))
+               to_svm(vcpu)->vmcb->save.efer &= ~EFER_SVME;
+}
+
  static int is_external_interrupt(u32 info)
  {
         info &= SVM_EVTINJ_TYPE_MASK | SVM_EVTINJ_VALID;
@@ -3554,7 +3562,7 @@ static void enter_svm_guest_mode(struct vcpu_svm 
*svm, u64
         svm->vmcb->save.gdtr = nested_vmcb->save.gdtr;
         svm->vmcb->save.idtr = nested_vmcb->save.idtr;
         kvm_set_rflags(&svm->vcpu, nested_vmcb->save.rflags);
-       svm_set_efer(&svm->vcpu, nested_vmcb->save.efer);
+       nested_svm_set_efer(&svm->vcpu, nested_vmcb->save.efer);
         svm_set_cr0(&svm->vcpu, nested_vmcb->save.cr0);
         svm_set_cr4(&svm->vcpu, nested_vmcb->save.cr4);
         if (npt_enabled) {

Re: [PATCH] KVM: nSVM: check for EFER.SVME=1 before entering guest

[Paolo Bonzini]

On 18/03/20 19:40, Krish Sadhukhan wrote:
> 
> On 3/18/20 5:41 AM, Paolo Bonzini wrote:
>> EFER is set for L2 using svm_set_efer, which hardcodes EFER_SVME to 1
>> and hides
>> an incorrect value for EFER.SVME in the L1 VMCB.  Perform the check
>> manually
>> to detect invalid guest state.
>>
>> Reported-by: Krish Sadhukhan <krish.sadhukhan@oracle.com>
>> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
>> ---
>>   arch/x86/kvm/svm.c | 3 +++
>>   1 file changed, 3 insertions(+)
>>
>> diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
>> index 08568ae9f7a1..2125c6ae5951 100644
>> --- a/arch/x86/kvm/svm.c
>> +++ b/arch/x86/kvm/svm.c
>> @@ -3558,6 +3558,9 @@ static bool nested_svm_vmrun_msrpm(struct
>> vcpu_svm *svm)
>>     static bool nested_vmcb_checks(struct vmcb *vmcb)
>>   {
>> +    if ((vmcb->save.efer & EFER_SVME) == 0)
>> +        return false;
>> +
>>       if ((vmcb->control.intercept & (1ULL << INTERCEPT_VMRUN)) == 0)
>>           return false;
>>   
> 
> Ah! This now tells me that I forgot the KVM fix that was supposed to
> accompany my patchset.

Heh, indeed.  I was puzzled for a second after applying it, then decided
I would just fix it myself. :)

> Do we need this check in software ? I wasn't checking the bit in KVM and
> instead I was just making sure that L0 sets that bit based on the
> setting in nested vmcb:

The only effect of the function below over svm_set_efer is to guarantee
a vmrun error to happen.  Doing the test in nested_vmcb_checks is more
consistent with other must-be-one bits such as the VMRUN intercept, and
it's also a smaller patch.

Paolo

> 
> +static void nested_svm_set_efer(struct kvm_vcpu *vcpu, u64
> nested_vmcb_efer)
> +{
> +       svm_set_efer(vcpu, nested_vmcb_efer);
> +
> +       if (!(nested_vmcb_efer & EFER_SVME))
> +               to_svm(vcpu)->vmcb->save.efer &= ~EFER_SVME;
> +}
> +
>  static int is_external_interrupt(u32 info)
>  {
>         info &= SVM_EVTINJ_TYPE_MASK | SVM_EVTINJ_VALID;
> @@ -3554,7 +3562,7 @@ static void enter_svm_guest_mode(struct vcpu_svm
> *svm, u64
>         svm->vmcb->save.gdtr = nested_vmcb->save.gdtr;
>         svm->vmcb->save.idtr = nested_vmcb->save.idtr;
>         kvm_set_rflags(&svm->vcpu, nested_vmcb->save.rflags);
> -       svm_set_efer(&svm->vcpu, nested_vmcb->save.efer);
> +       nested_svm_set_efer(&svm->vcpu, nested_vmcb->save.efer);
>         svm_set_cr0(&svm->vcpu, nested_vmcb->save.cr0);
>         svm_set_cr4(&svm->vcpu, nested_vmcb->save.cr4);
>         if (npt_enabled) {
> 

Re: [PATCH] KVM: nSVM: check for EFER.SVME=1 before entering guest

[Krish Sadhukhan]

On 3/19/20 3:05 AM, Paolo Bonzini wrote:
> On 18/03/20 19:40, Krish Sadhukhan wrote:
>> On 3/18/20 5:41 AM, Paolo Bonzini wrote:
>>> EFER is set for L2 using svm_set_efer, which hardcodes EFER_SVME to 1
>>> and hides
>>> an incorrect value for EFER.SVME in the L1 VMCB.  Perform the check
>>> manually
>>> to detect invalid guest state.
>>>
>>> Reported-by: Krish Sadhukhan <krish.sadhukhan@oracle.com>
>>> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
>>> ---
>>>    arch/x86/kvm/svm.c | 3 +++
>>>    1 file changed, 3 insertions(+)
>>>
>>> diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
>>> index 08568ae9f7a1..2125c6ae5951 100644
>>> --- a/arch/x86/kvm/svm.c
>>> +++ b/arch/x86/kvm/svm.c
>>> @@ -3558,6 +3558,9 @@ static bool nested_svm_vmrun_msrpm(struct
>>> vcpu_svm *svm)
>>>      static bool nested_vmcb_checks(struct vmcb *vmcb)
>>>    {
>>> +    if ((vmcb->save.efer & EFER_SVME) == 0)
>>> +        return false;
>>> +
>>>        if ((vmcb->control.intercept & (1ULL << INTERCEPT_VMRUN)) == 0)
>>>            return false;
>>>    
>> Ah! This now tells me that I forgot the KVM fix that was supposed to
>> accompany my patchset.
> Heh, indeed.  I was puzzled for a second after applying it, then decided
> I would just fix it myself. :)
>
>> Do we need this check in software ? I wasn't checking the bit in KVM and
>> instead I was just making sure that L0 sets that bit based on the
>> setting in nested vmcb:
Thanks !
> The only effect of the function below over svm_set_efer is to guarantee
> a vmrun error to happen.  Doing the test in nested_vmcb_checks is more
> consistent with other must-be-one bits such as the VMRUN intercept, and
> it's also a smaller patch.

Reviewed-by: Krish Sadhukhan <krish.sadhukhan@oracle.com>

>
> Paolo
>
>> +static void nested_svm_set_efer(struct kvm_vcpu *vcpu, u64
>> nested_vmcb_efer)
>> +{
>> +       svm_set_efer(vcpu, nested_vmcb_efer);
>> +
>> +       if (!(nested_vmcb_efer & EFER_SVME))
>> +               to_svm(vcpu)->vmcb->save.efer &= ~EFER_SVME;
>> +}
>> +
>>   static int is_external_interrupt(u32 info)
>>   {
>>          info &= SVM_EVTINJ_TYPE_MASK | SVM_EVTINJ_VALID;
>> @@ -3554,7 +3562,7 @@ static void enter_svm_guest_mode(struct vcpu_svm
>> *svm, u64
>>          svm->vmcb->save.gdtr = nested_vmcb->save.gdtr;
>>          svm->vmcb->save.idtr = nested_vmcb->save.idtr;
>>          kvm_set_rflags(&svm->vcpu, nested_vmcb->save.rflags);
>> -       svm_set_efer(&svm->vcpu, nested_vmcb->save.efer);
>> +       nested_svm_set_efer(&svm->vcpu, nested_vmcb->save.efer);
>>          svm_set_cr0(&svm->vcpu, nested_vmcb->save.cr0);
>>          svm_set_cr4(&svm->vcpu, nested_vmcb->save.cr4);
>>          if (npt_enabled) {
>>