[PATCH v3] KVM: x86: accept userspace interrupt only if no event is injected

[PATCH v3] 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>
---
 arch/x86/kvm/x86.c | 13 +++++++++++--
 1 file changed, 11 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 4116567f3d44..e5d5c5ed7dd4 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -4358,8 +4358,17 @@ static int kvm_cpu_accept_dm_intr(struct kvm_vcpu *vcpu)
 
 static int kvm_vcpu_ready_for_interrupt_injection(struct kvm_vcpu *vcpu)
 {
-	return kvm_arch_interrupt_allowed(vcpu) &&
-		kvm_cpu_accept_dm_intr(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_event_needs_reinjection(vcpu) &&
+		!vcpu->arch.exception.pending);
 }
 
 static int kvm_vcpu_ioctl_interrupt(struct kvm_vcpu *vcpu,
-- 
2.27.0

Re: [PATCH v3] 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>
> ---
>  arch/x86/kvm/x86.c | 13 +++++++++++--
>  1 file changed, 11 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 4116567f3d44..e5d5c5ed7dd4 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -4358,8 +4358,17 @@ static int kvm_cpu_accept_dm_intr(struct kvm_vcpu *vcpu)
>  
>  static int kvm_vcpu_ready_for_interrupt_injection(struct kvm_vcpu *vcpu)
>  {
> -	return kvm_arch_interrupt_allowed(vcpu) &&
> -		kvm_cpu_accept_dm_intr(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) &&

Ha, adding a '(' is one way to fix the indentation.

Reviewed-by: Sean Christopherson <seanjc@google.com> 

> +		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 v3] KVM: x86: accept userspace interrupt only if no event is injected

[stsp]

28.07.2021 00:09, Paolo Bonzini пишет:
> 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>
Acked-by: stsp2@yandex.ru