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

[Suzuki K Poulose]

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.


----8>----

kvm-arm: Handle hva aging while destroying vm

The mmu_notifier_release() callback of KVM triggers cleaning up
the stage2 page table on kvm-arm. However there could be other
notifier callbacks in parallel with the mmu_notifier_release(),
which could cause the call backs ending up in an empty stage2
page table. Make sure we check it for all the notifier callbacks.

Reported-by: Alex Graf <agraf@xxxxxxx>
Cc: Christoffer Dall <cdall@xxxxxxxxxx>
Cc: Marc Zyngier <marc.zyngier@xxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx
Fixes: commit 293f29363 ("kvm-arm: Unmap shadow pagetables properly")
Signed-off-by: Suzuki K Poulose <suzuki.poulose@xxxxxxx>
---
 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 e2e5eff..db1c7b2 100644
--- a/virt/kvm/arm/mmu.c
+++ b/virt/kvm/arm/mmu.c
@@ -1665,12 +1665,16 @@ static int kvm_test_age_hva_handler(struct kvm *kvm, gpa_t gpa, u64 size, void *
 
 int kvm_age_hva(struct kvm *kvm, unsigned long start, unsigned long end)
 {
+	if (!kvm->arch.pgd)
+		return 0;
 	trace_kvm_age_hva(start, end);
 	return handle_hva_to_gpa(kvm, start, end, kvm_age_hva_handler, NULL);
 }
 
 int kvm_test_age_hva(struct kvm *kvm, unsigned long hva)
 {
+	if (!kvm->arch.pgd)
+		return 0;
 	trace_kvm_test_age_hva(hva);
 	return handle_hva_to_gpa(kvm, hva, hva, kvm_test_age_hva_handler, NULL);
 }
-- 
2.7.5