linux-kernel - Re: [PATCH] KVM: arm/arm64: Assign pmc->idx before kvm_pmu_stop

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Date:   Thu, 18 Jul 2019 09:59:46 +0800
From:   Zenghui Yu <yuzenghui@...wei.com>
To:     Marc Zyngier <maz@...nel.org>,
        Julien Thierry <julien.thierry@....com>,
        <kvmarm@...ts.cs.columbia.edu>,
        <linux-arm-kernel@...ts.infradead.org>
CC:     <marc.zyngier@....com>, <james.morse@....com>,
        <suzuki.poulose@....com>, <julien.thierry.kdev@...il.com>,
        <linux-kernel@...r.kernel.org>, <wanghaibin.wang@...wei.com>,
        <andrew.murray@....com>
Subject: Re: [PATCH] KVM: arm/arm64: Assign pmc->idx before
 kvm_pmu_stop_counter()

Hi Julien, Marc,

On 2019/7/17 23:00, Marc Zyngier wrote:
> On 17/07/2019 14:44, Julien Thierry wrote:
>> Hi Zenghui,
>>
>> On 17/07/2019 13:20, Zenghui Yu wrote:
>>> We use "pmc->idx" and the "chained" bitmap to determine if the pmc is
>>> chained, in kvm_pmu_pmc_is_chained().  But idx might be uninitialized
>>> (and random) when we doing this decision, through a KVM_ARM_VCPU_INIT
>>> ioctl -> kvm_pmu_vcpu_reset(). And the test_bit() against this random
>>> idx will potentially hit a KASAN BUG [1].
>>>
>>> Fix it by moving the assignment of idx before kvm_pmu_stop_counter().
>>>
>>> [1] https://www.spinics.net/lists/kvm-arm/msg36700.html
>>>
>>> Fixes: 80f393a23be6 ("KVM: arm/arm64: Support chained PMU counters")
>>> Suggested-by: Andrew Murray <andrew.murray@....com>
>>> Cc: Marc Zyngier <maz@...nel.org>
>>> Signed-off-by: Zenghui Yu <yuzenghui@...wei.com>> ---
>>>   virt/kvm/arm/pmu.c | 2 +-
>>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>>
>>> diff --git a/virt/kvm/arm/pmu.c b/virt/kvm/arm/pmu.c
>>> index 3dd8238..521bfdd 100644
>>> --- a/virt/kvm/arm/pmu.c
>>> +++ b/virt/kvm/arm/pmu.c
>>> @@ -225,8 +225,8 @@ void kvm_pmu_vcpu_reset(struct kvm_vcpu *vcpu)
>>>   	struct kvm_pmu *pmu = &vcpu->arch.pmu;
>>>   
>>>   	for (i = 0; i < ARMV8_PMU_MAX_COUNTERS; i++) {
>>> -		kvm_pmu_stop_counter(vcpu, &pmu->pmc[i]);
>>>   		pmu->pmc[i].idx = i;
>>
>> Yes, this is kind of a static property that should really be part of a
>> "kvm_pmu_vcpu_init()" or "kvm_pmu_vcpu_create()" and is not expected to
>> be modified across resets...
>>
>> There is no such function at the time and I'm unsure whether this
>> warrants creating that separate function (I would still suggest creating
>> it to make things clearer).
> 
> Yup, that's pretty bad, now that you mention it. I'd be all for the
> introduction of kvm_pmu_vcpu_init(), given that we already have
> kvm_pmu_vcpu_destroy().
> 
>>
>>> +		kvm_pmu_stop_counter(vcpu, &pmu->pmc[i]);
>>
>> Whatever other opinions are on splitting pmu_vcpu_init/reset, that
>> change makes sense and fixes the issue:
>>
>> Acked-by: Julien Thierry <julien.thierry@....com>
>>
>>>   	}
>>>   
>>>   	bitmap_zero(vcpu->arch.pmu.chained, ARMV8_PMU_MAX_COUNTER_PAIRS);
>>>
>>
>> Cheers,
>>
> 
> Zenghui, could you please update your patch to take the above into account?

Sure. I will send a v2 with the new subject (may be "KVM: arm/arm64:
Introduce kvm_pmu_vcpu_init() to ...").

Thanks for your suggestions!


zenghui