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From:   Vitaly Kuznetsov <vkuznets@redhat.com>
To:     Nicolas Saenz Julienne <nsaenz@amazon.com>, kvm@vger.kernel.org
Cc:     seanjc@google.com, pbonzini@redhat.com, tglx@linutronix.de,
        mingo@redhat.com, bp@alien8.de, dave.hansen@linux.intel.com,
        x86@kernel.org, hpa@zytor.com, graf@amazon.de, rkagan@amazon.de,
        linux-kernel@vger.kernel.org, anelkz@amazon.de,
        Nicolas Saenz Julienne <nsaenz@amazon.com>,
        stable@vger.kernel.org
Subject: Re: [PATCH v3] KVM: x86: hyper-v: Don't auto-enable stimer on write
 from user-space
In-Reply-To: <20231017155101.40677-1-nsaenz@amazon.com>
References: <20231017155101.40677-1-nsaenz@amazon.com>
Date:   Tue, 17 Oct 2023 18:02:24 +0200
Message-ID: <87bkcx6xv3.fsf@redhat.com>
MIME-Version: 1.0
Content-Type: text/plain
Precedence: bulk

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