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

[Zenghui Yu]

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@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).
>
> 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@xxxxxxx>
> >
> > >   	}
> > >   
> > >   	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