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

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On 07/03/2017 10:03 AM, Christoffer Dall wrote:
Hi Alex,

On Fri, Jun 23, 2017 at 05:21:59PM +0200, Alexander Graf wrote:
If we want to age an HVA while the VM is getting destroyed, we have a
tiny race window during which we may end up dereferencing an invalid
kvm->arch.pgd value.

    CPU0               CPU1

    kvm_age_hva()
                       kvm_mmu_notifier_release()
                       kvm_arch_flush_shadow_all()
                       kvm_free_stage2_pgd()
                       <grab mmu_lock>
    stage2_get_pmd()
    <wait for mmu_lock>
                       set kvm->arch.pgd = 0
                       <free mmu_lock>
    <grab mmu_lock>
    stage2_get_pud()
    <access kvm->arch.pgd>
    <use incorrect value>
I don't think this sequence, can happen, but I think kvm_age_hva() can
be called with the mmu_lock held and kvm->pgd already being NULL.

Is that possible for the mmu notifiers to be calling clear(_flush)_young
while also calling notifier_release?

I *think* the aging happens completely orthogonally to release. But let's ask Andrea - I'm sure he knows :).


Alex


If so, the patch below looks good to me.

Thanks,
-Christoffer


This patch adds a check for that case.

Signed-off-by: Alexander Graf <agraf@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 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;
+
  	pgd = kvm->arch.pgd + stage2_pgd_index(addr);
  	if (WARN_ON(stage2_pgd_none(*pgd))) {
  		if (!cache)
--
1.8.5.6

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