Re: [PATCH v2] KVM: arm/arm64: Handle hva aging while destroying the vm

[Alexander Graf]

On 05.07.17 10:57, Suzuki K Poulose wrote:
> Hi Alex,
>
> On Wed, Jul 05, 2017 at 08:20:31AM +0200, Alexander Graf wrote:
> > The kvm_age_hva callback may be called all the way concurrently while
> > kvm_mmu_notifier_release() is running.
> >
> > The release function sets kvm->arch.pgd = NULL which the aging function
> > however implicitly relies on in stage2_get_pud(). That means they can
> > race and the aging function may dereference a NULL pgd pointer.
> >
> > This patch adds a check for that case, so that we leave the aging
> > function silently.
> >
> > Cc: stable@xxxxxxxxxxxxxxx
> > Fixes: 293f29363 ("kvm-arm: Unmap shadow pagetables properly")
> > Signed-off-by: Alexander Graf <agraf@xxxxxxx>
> >
> > ---
> >
> > v1 -> v2:
> >
> >    - Fix commit message
> >    - Add Fixes and stable tags
> > ---
> >   virt/kvm/arm/mmu.c | 4 ++++
> >   1 file changed, 4 insertions(+)
> >
> > diff --git a/virt/kvm/arm/mmu.c b/virt/kvm/arm/mmu.c
> > index f2d5b6c..227931f 100644
> > --- a/virt/kvm/arm/mmu.c
> > +++ b/virt/kvm/arm/mmu.c
> > @@ -861,6 +861,10 @@ static pud_t *stage2_get_pud(struct kvm *kvm, struct kvm_mmu_memory_cache *cache
> >   	pgd_t *pgd;
> >   	pud_t *pud;
> >   
> > +	/* Do we clash with kvm_free_stage2_pgd()? */
> > +	if (!kvm->arch.pgd)
> > +		return NULL;
> > +
>
> I think this check should be moved up in the chain. We call kvm_age_hva(), with
> the kvm->mmu_lock held and we don't release it till we reach here. So, ideally,
> if we find the PGD is null when we reach kvm_age_hva(), we could simply return
> there, like we do for other call backs from the KVM mmu_notifier.

That probably works too - I'm not sure which version is more consistent 
as well as more maintainable in the long run. I'll leave the call here 
to Christoffer.


Alex