[PATCH v3] KVM: x86: hyper-v: Don't auto-enable stimer on write from user-space

[PATCH v3] KVM: x86: hyper-v: Don't auto-enable stimer on write from user-space

[Nicolas Saenz Julienne]

Don't apply the stimer's counter side effects when modifying its
value from user-space, as this may trigger spurious interrupts.

For example:
 - The stimer is configured in auto-enable mode.
 - The stimer's count is set and the timer enabled.
 - The stimer expires, an interrupt is injected.
 - The VM is live migrated.
 - The stimer config and count are deserialized, auto-enable is ON, the
   stimer is re-enabled.
 - The stimer expires right away, and injects an unwarranted interrupt.

Cc: stable@vger.kernel.org
Fixes: 1f4b34f825e8 ("kvm/x86: Hyper-V SynIC timers")
Signed-off-by: Nicolas Saenz Julienne <nsaenz@amazon.com>
---

Changes since v2: 
- reword commit message/subject.

Changes since v1:
- Cover all 'stimer->config.enable' updates.

 arch/x86/kvm/hyperv.c | 10 ++++++----
 1 file changed, 6 insertions(+), 4 deletions(-)

diff --git a/arch/x86/kvm/hyperv.c b/arch/x86/kvm/hyperv.c
index 7c2dac6824e2..238afd7335e4 100644
--- a/arch/x86/kvm/hyperv.c
+++ b/arch/x86/kvm/hyperv.c
@@ -727,10 +727,12 @@ static int stimer_set_count(struct kvm_vcpu_hv_stimer *stimer, u64 count,
 
 	stimer_cleanup(stimer);
 	stimer->count = count;
-	if (stimer->count == 0)
-		stimer->config.enable = 0;
-	else if (stimer->config.auto_enable)
-		stimer->config.enable = 1;
+	if (!host) {
+		if (stimer->count == 0)
+			stimer->config.enable = 0;
+		else if (stimer->config.auto_enable)
+			stimer->config.enable = 1;
+	}
 
 	if (stimer->config.enable)
 		stimer_mark_pending(stimer, false);
-- 
2.40.1

Re: [PATCH v3] KVM: x86: hyper-v: Don't auto-enable stimer on write from user-space

[Vitaly Kuznetsov]

Nicolas Saenz Julienne <nsaenz@amazon.com> writes:

> Don't apply the stimer's counter side effects when modifying its
> value from user-space, as this may trigger spurious interrupts.
>
> For example:
>  - The stimer is configured in auto-enable mode.
>  - The stimer's count is set and the timer enabled.
>  - The stimer expires, an interrupt is injected.
>  - The VM is live migrated.
>  - The stimer config and count are deserialized, auto-enable is ON, the
>    stimer is re-enabled.
>  - The stimer expires right away, and injects an unwarranted interrupt.
>
> Cc: stable@vger.kernel.org
> Fixes: 1f4b34f825e8 ("kvm/x86: Hyper-V SynIC timers")
> Signed-off-by: Nicolas Saenz Julienne <nsaenz@amazon.com>
> ---
>
> Changes since v2: 
> - reword commit message/subject.
>
> Changes since v1:
> - Cover all 'stimer->config.enable' updates.
>
>  arch/x86/kvm/hyperv.c | 10 ++++++----
>  1 file changed, 6 insertions(+), 4 deletions(-)
>
> diff --git a/arch/x86/kvm/hyperv.c b/arch/x86/kvm/hyperv.c
> index 7c2dac6824e2..238afd7335e4 100644
> --- a/arch/x86/kvm/hyperv.c
> +++ b/arch/x86/kvm/hyperv.c
> @@ -727,10 +727,12 @@ static int stimer_set_count(struct kvm_vcpu_hv_stimer *stimer, u64 count,
>  
>  	stimer_cleanup(stimer);
>  	stimer->count = count;
> -	if (stimer->count == 0)
> -		stimer->config.enable = 0;
> -	else if (stimer->config.auto_enable)
> -		stimer->config.enable = 1;
> +	if (!host) {
> +		if (stimer->count == 0)
> +			stimer->config.enable = 0;
> +		else if (stimer->config.auto_enable)
> +			stimer->config.enable = 1;
> +	}
>  
>  	if (stimer->config.enable)
>  		stimer_mark_pending(stimer, false);

LGTM, thanks!

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

-- 
Vitaly

Re: [PATCH v3] KVM: x86: hyper-v: Don't auto-enable stimer on write from user-space

[Sean Christopherson]

On Tue, 17 Oct 2023 15:51:02 +0000, Nicolas Saenz Julienne wrote:
> Don't apply the stimer's counter side effects when modifying its
> value from user-space, as this may trigger spurious interrupts.
> 
> For example:
>  - The stimer is configured in auto-enable mode.
>  - The stimer's count is set and the timer enabled.
>  - The stimer expires, an interrupt is injected.
>  - The VM is live migrated.
>  - The stimer config and count are deserialized, auto-enable is ON, the
>    stimer is re-enabled.
>  - The stimer expires right away, and injects an unwarranted interrupt.
> 
> [...]

Applied to kvm-x86 misc, thanks!

[1/1] KVM: x86: hyper-v: Don't auto-enable stimer on write from user-space
      https://github.com/kvm-x86/linux/commit/d6800af51c76

--
https://github.com/kvm-x86/linux/tree/next