Subject: Re: [PATCH 1/5] KVM: arm64: Grab KVM MMU write lock in kvm_arch_flush_shadow_all()

From: James Houghton <[email-masked]>
Date: Mon, 4 May 2026 23:10:47 +0000

On Mon, May 4, 2026 at 3:42 PM James Houghton <[email-masked]> wrote:
>
> kvm_arch_flush_shadow_all() may sometimes be called on the same `kvm`
> concurrently in the event that the KVM's `mm` is __mmput() at the
> same time that last reference to the KVM is being dropped.
>
> T1              T2
> KVM_CREATE_VM
>                 Get VM file from T1
> close VM
> exit_mm()       close VM
>
> T1: exit_mm() -> kvm_mmu_notifier_release() -> kvm_flush_shadow_all(),
>     with only the KVM srcu read lock held.
>
> T2: kvm_vm_release() ---> mmu_notifier_unregister() ->
>     kvm_mmu_notifier_release() -> kvm_flush_shadow_all(),
>     again, with only the KVM srcu read lock held.
>
> This leads to a potential double-free of
> kvm->arch.kvm_mmu_free_memory_cache and now with NV
> kvm->arch.nested_mmus.
>
> Cc: [email-masked]
> Fixes: e7bf7a490c68 ("KVM: arm64: Split huge pages when dirty logging is enabled")
> Signed-off-by: James Houghton <[email-masked]>
> ---
>  arch/arm64/include/asm/kvm_mmu.h |  1 +
>  arch/arm64/kvm/mmu.c             | 23 +++++++++++++++++++----
>  arch/arm64/kvm/nested.c          |  4 +++-
>  3 files changed, 23 insertions(+), 5 deletions(-)
>
> diff --git a/arch/arm64/include/asm/kvm_mmu.h b/arch/arm64/include/asm/kvm_mmu.h
> index 01e9c72d6aa7..30d5c24fcebb 100644
> --- a/arch/arm64/include/asm/kvm_mmu.h
> +++ b/arch/arm64/include/asm/kvm_mmu.h
> @@ -178,6 +178,7 @@ void stage2_unmap_vm(struct kvm *kvm);
>  int kvm_init_stage2_mmu(struct kvm *kvm, struct kvm_s2_mmu *mmu, unsigned long type);
>  void kvm_uninit_stage2_mmu(struct kvm *kvm);
>  void kvm_free_stage2_pgd(struct kvm_s2_mmu *mmu);
> +void kvm_free_stage2_pgd_locked(struct kvm_s2_mmu *mmu);
>  int kvm_phys_addr_ioremap(struct kvm *kvm, phys_addr_t guest_ipa,
>                           phys_addr_t pa, unsigned long size, bool writable);
>
> diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
> index d089c107d9b7..4bab407d43bb 100644
> --- a/arch/arm64/kvm/mmu.c
> +++ b/arch/arm64/kvm/mmu.c
> @@ -1021,7 +1021,9 @@ int kvm_init_stage2_mmu(struct kvm *kvm, struct kvm_s2_mmu *mmu, unsigned long t
>
>  void kvm_uninit_stage2_mmu(struct kvm *kvm)
>  {
> -       kvm_free_stage2_pgd(&kvm->arch.mmu);
> +       lockdep_assert_held_write(&kvm->mmu_lock);

*facepalm*.... this doesn't account for the other callers of
kvm_uninit_stage2_mmu(). They will get lockdep warnings.

I've attached a diff to the bottom of this reply that *does* deal with them.
:( Sorry.

I'm guessing Marc or Oliver will probably want this patch to look quite
different, so I'll wait to hear from them before actually sending a v2.

In the meantime, I'll properly retest with lockdep enabled.

> +
> +       kvm_free_stage2_pgd_locked(&kvm->arch.mmu);
>         kvm_mmu_free_memory_cache(&kvm->arch.mmu.split_page_cache);
>  }
>
> @@ -1095,12 +1097,14 @@ void stage2_unmap_vm(struct kvm *kvm)
>         srcu_read_unlock(&kvm->srcu, idx);
>  }
>
> -void kvm_free_stage2_pgd(struct kvm_s2_mmu *mmu)
> +static void __kvm_free_stage2_pgd(struct kvm_s2_mmu *mmu, bool locked)
>  {
>         struct kvm *kvm = kvm_s2_mmu_to_kvm(mmu);
>         struct kvm_pgtable *pgt = NULL;
>
> -       write_lock(&kvm->mmu_lock);
> +       if (!locked)
> +               write_lock(&kvm->mmu_lock);
> +
>         pgt = mmu->pgt;
>         if (pgt) {
>                 mmu->pgd_phys = 0;
> @@ -1111,7 +1115,8 @@ void kvm_free_stage2_pgd(struct kvm_s2_mmu *mmu)
>         if (kvm_is_nested_s2_mmu(kvm, mmu))
>                 kvm_init_nested_s2_mmu(mmu);
>
> -       write_unlock(&kvm->mmu_lock);
> +       if (!locked)
> +               write_unlock(&kvm->mmu_lock);
>
>         if (pgt) {
>                 kvm_stage2_destroy(pgt);
> @@ -1119,6 +1124,16 @@ void kvm_free_stage2_pgd(struct kvm_s2_mmu *mmu)
>         }
>  }
>
> +void kvm_free_stage2_pgd(struct kvm_s2_mmu *mmu)
> +{
> +       __kvm_free_stage2_pgd(mmu, false);
> +}
> +
> +void kvm_free_stage2_pgd_locked(struct kvm_s2_mmu *mmu)
> +{
> +       __kvm_free_stage2_pgd(mmu, true);
> +}
> +
>  static void hyp_mc_free_fn(void *addr, void *mc)
>  {
>         struct kvm_hyp_memcache *memcache = mc;
> diff --git a/arch/arm64/kvm/nested.c b/arch/arm64/kvm/nested.c
> index 883b6c1008fb..977598bff5e6 100644
> --- a/arch/arm64/kvm/nested.c
> +++ b/arch/arm64/kvm/nested.c
> @@ -1190,11 +1190,13 @@ void kvm_arch_flush_shadow_all(struct kvm *kvm)
>  {
>         int i;
>
> +       guard(write_lock)(&kvm->mmu_lock);
> +
>         for (i = 0; i < kvm->arch.nested_mmus_size; i++) {
>                 struct kvm_s2_mmu *mmu = &kvm->arch.nested_mmus[i];
>
>                 if (!WARN_ON(atomic_read(&mmu->refcnt)))
> -                       kvm_free_stage2_pgd(mmu);
> +                       kvm_free_stage2_pgd_locked(mmu);
>         }
>         kvfree(kvm->arch.nested_mmus);
>         kvm->arch.nested_mmus = NULL;
> --
> 2.54.0.545.g6539524ca2-goog

And here is the diff that should fix this patch. (Sorry!!)

diff --git a/arch/arm64/include/asm/kvm_mmu.h b/arch/arm64/include/asm/kvm_mmu.h
index 30d5c24fcebb..e32e844943be 100644
--- a/arch/arm64/include/asm/kvm_mmu.h
+++ b/arch/arm64/include/asm/kvm_mmu.h
@@ -177,6 +177,7 @@ void kvm_stage2_wp_range(struct kvm_s2_mmu *mmu, phys_addr_t addr, phys_addr_t e
 void stage2_unmap_vm(struct kvm *kvm);
 int kvm_init_stage2_mmu(struct kvm *kvm, struct kvm_s2_mmu *mmu, unsigned long type);
 void kvm_uninit_stage2_mmu(struct kvm *kvm);
+void kvm_uninit_stage2_mmu_locked(struct kvm *kvm);
 void kvm_free_stage2_pgd(struct kvm_s2_mmu *mmu);
 void kvm_free_stage2_pgd_locked(struct kvm_s2_mmu *mmu);
 int kvm_phys_addr_ioremap(struct kvm *kvm, phys_addr_t guest_ipa,
diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
index 4bab407d43bb..98ba8116676c 100644
--- a/arch/arm64/kvm/mmu.c
+++ b/arch/arm64/kvm/mmu.c
@@ -1019,14 +1019,6 @@ int kvm_init_stage2_mmu(struct kvm *kvm, struct kvm_s2_mmu *mmu, unsigned long t
 	return err;
 }
 
-void kvm_uninit_stage2_mmu(struct kvm *kvm)
-{
-	lockdep_assert_held_write(&kvm->mmu_lock);
-
-	kvm_free_stage2_pgd_locked(&kvm->arch.mmu);
-	kvm_mmu_free_memory_cache(&kvm->arch.mmu.split_page_cache);
-}
-
 static void stage2_unmap_memslot(struct kvm *kvm,
 				 struct kvm_memory_slot *memslot)
 {
@@ -1134,6 +1126,24 @@ void kvm_free_stage2_pgd_locked(struct kvm_s2_mmu *mmu)
 	__kvm_free_stage2_pgd(mmu, true);
 }
 
+static void __kvm_uninit_stage2_mmu(struct kvm *kvm, bool locked)
+{
+	__kvm_free_stage2_pgd(&kvm->arch.mmu, locked);
+	kvm_mmu_free_memory_cache(&kvm->arch.mmu.split_page_cache);
+}
+
+void kvm_uninit_stage2_mmu(struct kvm *kvm)
+{
+	__kvm_uninit_stage2_mmu(kvm, false);
+}
+
+void kvm_uninit_stage2_mmu_locked(struct kvm *kvm)
+{
+	lockdep_assert_held_write(&kvm->mmu_lock);
+
+	__kvm_uninit_stage2_mmu(kvm, true);
+}
+
 static void hyp_mc_free_fn(void *addr, void *mc)
 {
 	struct kvm_hyp_memcache *memcache = mc;
diff --git a/arch/arm64/kvm/nested.c b/arch/arm64/kvm/nested.c
index 977598bff5e6..f61f0244f0fb 100644
--- a/arch/arm64/kvm/nested.c
+++ b/arch/arm64/kvm/nested.c
@@ -1201,7 +1201,7 @@ void kvm_arch_flush_shadow_all(struct kvm *kvm)
 	kvfree(kvm->arch.nested_mmus);
 	kvm->arch.nested_mmus = NULL;
 	kvm->arch.nested_mmus_size = 0;
-	kvm_uninit_stage2_mmu(kvm);
+	kvm_uninit_stage2_mmu_locked(kvm);
 }
 
 /*