[PATCH v2] KVM: x86: accept userspace interrupt only if no event is injected

[PATCH v2] KVM: x86: accept userspace interrupt only if no event is injected

[Paolo Bonzini]

Once an exception has been injected, any side effects related to
the exception (such as setting CR2 or DR6) have been taked place.
Therefore, once KVM sets the VM-entry interruption information
field or the AMD EVENTINJ field, the next VM-entry must deliver that
exception.

Pending interrupts are processed after injected exceptions, so
in theory it would not be a problem to use KVM_INTERRUPT when
an injected exception is present.  However, DOSEMU is using
run->ready_for_interrupt_injection to detect interrupt windows
and then using KVM_SET_SREGS/KVM_SET_REGS to inject the
interrupt manually.  For this to work, the interrupt window
must be delayed after the completion of the previous event
injection.

Cc: stable@vger.kernel.org
Reported-by: Stas Sergeev <stsp2@yandex.ru>
Tested-by: Stas Sergeev <stsp2@yandex.ru>
Fixes: 71cc849b7093 ("KVM: x86: Fix split-irqchip vs interrupt injection window request")
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
(cherry picked from commit 043264d97e9ab74cc9661c8b1f9c00c8ce24cad9)
---
 arch/x86/kvm/x86.c | 12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 4116567f3d44..5e921f1e00db 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -4358,8 +4358,18 @@ static int kvm_cpu_accept_dm_intr(struct kvm_vcpu *vcpu)
 
 static int kvm_vcpu_ready_for_interrupt_injection(struct kvm_vcpu *vcpu)
 {
+	/*
+	 * Do not cause an interrupt window exit if an exception
+	 * is pending or an event needs reinjection; userspace
+	 * might want to inject the interrupt manually using KVM_SET_REGS
+	 * or KVM_SET_SREGS.  For that to work, we must be at an
+	 * instruction boundary and with no events half-injected.
+	 */
 	return kvm_arch_interrupt_allowed(vcpu) &&
-		kvm_cpu_accept_dm_intr(vcpu);
+		kvm_cpu_accept_dm_intr(vcpu) &&
+	        !kvm_event_needs_reinjection(vcpu)
+	        !vcpu->arch.exception.pending;
+
 }
 
 static int kvm_vcpu_ioctl_interrupt(struct kvm_vcpu *vcpu,
-- 
2.27.0

Re: [PATCH v2] KVM: x86: accept userspace interrupt only if no event is injected

[Sean Christopherson]

On Tue, Jul 27, 2021, Paolo Bonzini wrote:
> Once an exception has been injected, any side effects related to
> the exception (such as setting CR2 or DR6) have been taked place.
> Therefore, once KVM sets the VM-entry interruption information
> field or the AMD EVENTINJ field, the next VM-entry must deliver that
> exception.
> 
> Pending interrupts are processed after injected exceptions, so
> in theory it would not be a problem to use KVM_INTERRUPT when
> an injected exception is present.  However, DOSEMU is using
> run->ready_for_interrupt_injection to detect interrupt windows
> and then using KVM_SET_SREGS/KVM_SET_REGS to inject the
> interrupt manually.  For this to work, the interrupt window
> must be delayed after the completion of the previous event
> injection.
> 
> Cc: stable@vger.kernel.org
> Reported-by: Stas Sergeev <stsp2@yandex.ru>
> Tested-by: Stas Sergeev <stsp2@yandex.ru>
> Fixes: 71cc849b7093 ("KVM: x86: Fix split-irqchip vs interrupt injection window request")
> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
> (cherry picked from commit 043264d97e9ab74cc9661c8b1f9c00c8ce24cad9)

?  I can't find 043264d97e9ab74cc9661c8b1f9c00c8ce24cad9 anywhere.

> ---
>  arch/x86/kvm/x86.c | 12 +++++++++++-
>  1 file changed, 11 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 4116567f3d44..5e921f1e00db 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -4358,8 +4358,18 @@ static int kvm_cpu_accept_dm_intr(struct kvm_vcpu *vcpu)
>  
>  static int kvm_vcpu_ready_for_interrupt_injection(struct kvm_vcpu *vcpu)
>  {
> +	/*
> +	 * Do not cause an interrupt window exit if an exception
> +	 * is pending or an event needs reinjection; userspace
> +	 * might want to inject the interrupt manually using KVM_SET_REGS
> +	 * or KVM_SET_SREGS.  For that to work, we must be at an
> +	 * instruction boundary and with no events half-injected.
> +	 */
>  	return kvm_arch_interrupt_allowed(vcpu) &&
> -		kvm_cpu_accept_dm_intr(vcpu);
> +		kvm_cpu_accept_dm_intr(vcpu) &&

Opportunistically align this indentation?

> +	        !kvm_event_needs_reinjection(vcpu)

Missing &&, apparently the mysterious cherry-pick didn't go so well :-)

> +	        !vcpu->arch.exception.pending;
> +
>  }
>  
>  static int kvm_vcpu_ioctl_interrupt(struct kvm_vcpu *vcpu,
> -- 
> 2.27.0
> 

Re: [PATCH v2] KVM: x86: accept userspace interrupt only if no event is injected

[Paolo Bonzini]

On 27/07/21 22:58, Sean Christopherson wrote:
>> ---
>>   arch/x86/kvm/x86.c | 12 +++++++++++-
>>   1 file changed, 11 insertions(+), 1 deletion(-)
>>
>> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
>> index 4116567f3d44..5e921f1e00db 100644
>> --- a/arch/x86/kvm/x86.c
>> +++ b/arch/x86/kvm/x86.c
>> @@ -4358,8 +4358,18 @@ static int kvm_cpu_accept_dm_intr(struct kvm_vcpu *vcpu)
>>   
>>   static int kvm_vcpu_ready_for_interrupt_injection(struct kvm_vcpu *vcpu)
>>   {
>> +	/*
>> +	 * Do not cause an interrupt window exit if an exception
>> +	 * is pending or an event needs reinjection; userspace
>> +	 * might want to inject the interrupt manually using KVM_SET_REGS
>> +	 * or KVM_SET_SREGS.  For that to work, we must be at an
>> +	 * instruction boundary and with no events half-injected.
>> +	 */
>>   	return kvm_arch_interrupt_allowed(vcpu) &&
>> -		kvm_cpu_accept_dm_intr(vcpu);
>> +		kvm_cpu_accept_dm_intr(vcpu) &&
> 
> Opportunistically align this indentation?

Yep, good idea.

>> +	        !kvm_event_needs_reinjection(vcpu)
> 
> Missing &&, apparently the mysterious cherry-pick didn't go so well :-)

Well, yeah.  The only way I can excuse myself, is by not being the kind 
of person that yells for such stupid things...

Paolo