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

Alexander Graf
2017-06-23T15:21:39Z

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>

This patch adds a check for that case.

Signed-off-by: Alexander Graf <agraf@suse.de>
---
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