Subject: [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 22:42:08 +0000

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);
+
+	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