Re: [PATCH] KVM: arm/arm64: Assign pmc->idx before kvm_pmu_stop_counter()

[Julien Thierry]

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@xxxxxxx>
> Cc: Marc Zyngier <maz@xxxxxxxxxx>
> Signed-off-by: Zenghui Yu <yuzenghui@xxxxxxxxxx>> ---
>  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).

> +		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@xxxxxxx>

>  	}
>  
>  	bitmap_zero(vcpu->arch.pmu.chained, ARMV8_PMU_MAX_COUNTER_PAIRS);
> 

Cheers,

-- 
Julien Thierry