[PATCH v3] KVM: arm64: nv: Keep the shadow S2 MMUs at fixed addresses

[PATCH v3] KVM: arm64: nv: Keep the shadow S2 MMUs at fixed addresses

[Karl Mehltretter]

kvm_vcpu_init_nested() can grow kvm->arch.nested_mmus while initialising
another vCPU: it copies the MMUs, publishes the new allocation, and frees
the old one. It updates pgt->mmu back-pointers, but not hw_mmu, leaving
already-running vCPUs with pointers to freed memory. hw_mmu cannot be
fixed up the same way: a running vCPU reads it without holding mmu_lock.
The nested S2 ptdump file's debugfs private data is also affected, as it
points into the freed array.

KASAN reports an access through the stale hw_mmu pointer as a
slab-use-after-free in kvm_handle_guest_abort().

Turn nested_mmus into a pointer table allocated once for the maximum
number of vCPUs during VM creation. Allocate the MMUs separately as
vCPUs are initialised and append their pointers to that table. The MMU
objects never move, so cached hw_mmu pointers, pgt->mmu back-pointers,
and ptdump private data remain valid.

Two issues in the old implementation are also fixed:

- The old failure path passed uninitialised MMUs to
  kvm_free_stage2_pgd(), which derives kvm from mmu->arch and can
  therefore dereference an invalid pointer. Only call
  kvm_free_stage2_pgd() for initialised MMUs.
- Previously, initialisation of the new MMUs was not ordered before
  publication of nested_mmus_size. Fix this by taking mmu_lock when
  increasing nested_mmus_size.

Fixes: 4f128f8e1aaa ("KVM: arm64: nv: Support multiple nested Stage-2 mmu structures")
Cc: stable@vger.kernel.org
Suggested-by: Marc Zyngier <maz@kernel.org>
Assisted-by: Claude:claude-fable-5
Signed-off-by: Karl Mehltretter <kmehltretter@gmail.com>
---
 arch/arm64/include/asm/kvm_host.h   |  6 +--
 arch/arm64/include/asm/kvm_nested.h |  2 +-
 arch/arm64/kvm/arm.c                |  7 ++-
 arch/arm64/kvm/nested.c             | 81 ++++++++++++++++-------------
 4 files changed, 54 insertions(+), 42 deletions(-)

Changes in v3:
- Tighten the commit message (Wei-Lin Chang).
- The code diff changes only arch/arm64/kvm/nested.c compared to v2:
  - Drop the redundant pointer-table allocation comment and use
    guard(write_lock) when publishing nested_mmus_size (Wei-Lin Chang).
  - Rewrite the error handling without gotos.

v2: https://lore.kernel.org/r/20260806062352.93489-1-kmehltretter@gmail.com/

Changes in v2:
- Allocate the fixed pointer table during VM creation, as suggested by Marc.
- Allocate the MMUs individually and simplify the error and teardown paths.
- Drop the selftest patch that triggered KASAN.
  It is not a good fit for the existing suite.

v1: https://lore.kernel.org/r/20260803224405.41468-1-kmehltretter@gmail.com/

diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
index bae2c4f92ef5..59d1d77ee116 100644
--- a/arch/arm64/include/asm/kvm_host.h
+++ b/arch/arm64/include/asm/kvm_host.h
@@ -319,10 +319,10 @@ struct kvm_arch {
 	u64 fgu[__NR_FGT_GROUP_IDS__];
 
 	/*
-	 * Stage 2 paging state for VMs with nested S2 using a virtual
-	 * VMID.
+	 * Stage 2 paging state for VMs with nested S2 using a virtual VMID.
+	 * MMUs are allocated separately to keep their addresses stable.
 	 */
-	struct kvm_s2_mmu *nested_mmus;
+	struct kvm_s2_mmu **nested_mmus;
 	size_t nested_mmus_size;
 	int nested_mmus_next;
 
diff --git a/arch/arm64/include/asm/kvm_nested.h b/arch/arm64/include/asm/kvm_nested.h
index 012d711034d1..d21be647ac57 100644
--- a/arch/arm64/include/asm/kvm_nested.h
+++ b/arch/arm64/include/asm/kvm_nested.h
@@ -66,7 +66,7 @@ static inline u64 translate_ttbr0_el2_to_ttbr0_el1(u64 ttbr0)
 
 extern bool forward_smc_trap(struct kvm_vcpu *vcpu);
 extern bool forward_debug_exception(struct kvm_vcpu *vcpu);
-extern void kvm_init_nested(struct kvm *kvm);
+extern int kvm_init_nested(struct kvm *kvm);
 extern int kvm_vcpu_init_nested(struct kvm_vcpu *vcpu);
 extern void kvm_init_nested_s2_mmu(struct kvm_s2_mmu *mmu);
 extern struct kvm_s2_mmu *lookup_s2_mmu(struct kvm_vcpu *vcpu);
diff --git a/arch/arm64/kvm/arm.c b/arch/arm64/kvm/arm.c
index 50adfff75be8..e883e45382fb 100644
--- a/arch/arm64/kvm/arm.c
+++ b/arch/arm64/kvm/arm.c
@@ -223,8 +223,6 @@ int kvm_arch_init_vm(struct kvm *kvm, unsigned long type)
 	mutex_unlock(&kvm->lock);
 #endif
 
-	kvm_init_nested(kvm);
-
 	ret = kvm_share_hyp(kvm, kvm + 1);
 	if (ret)
 		return ret;
@@ -239,6 +237,10 @@ int kvm_arch_init_vm(struct kvm *kvm, unsigned long type)
 	if (ret)
 		goto err_free_cpumask;
 
+	ret = kvm_init_nested(kvm);
+	if (ret)
+		goto err_uninit_mmu;
+
 	if (is_protected_kvm_enabled()) {
 		/*
 		 * If any failures occur after this is successful, make sure to
@@ -267,6 +269,7 @@ int kvm_arch_init_vm(struct kvm *kvm, unsigned long type)
 
 err_uninit_mmu:
 	kvm_uninit_stage2_mmu(kvm);
+	kvfree(kvm->arch.nested_mmus);
 err_free_cpumask:
 	free_cpumask_var(kvm->arch.supported_cpus);
 err_unshare_kvm:
diff --git a/arch/arm64/kvm/nested.c b/arch/arm64/kvm/nested.c
index dfb96edbdc43..7dedb9f8ee86 100644
--- a/arch/arm64/kvm/nested.c
+++ b/arch/arm64/kvm/nested.c
@@ -44,11 +44,18 @@ struct vncr_tlb {
  */
 #define S2_MMU_PER_VCPU		2
 
-void kvm_init_nested(struct kvm *kvm)
+int kvm_init_nested(struct kvm *kvm)
 {
-	kvm->arch.nested_mmus = NULL;
+	kvm->arch.nested_mmus = kvcalloc(KVM_MAX_VCPUS * S2_MMU_PER_VCPU,
+					 sizeof(*kvm->arch.nested_mmus),
+					 GFP_KERNEL_ACCOUNT);
+	if (!kvm->arch.nested_mmus)
+		return -ENOMEM;
+
 	kvm->arch.nested_mmus_size = 0;
 	atomic_set(&kvm->arch.vncr_map_count, 0);
+
+	return 0;
 }
 
 static int init_nested_s2_mmu(struct kvm *kvm, struct kvm_s2_mmu *mmu)
@@ -66,11 +73,17 @@ static int init_nested_s2_mmu(struct kvm *kvm, struct kvm_s2_mmu *mmu)
 	return kvm_init_stage2_mmu(kvm, mmu, kvm_get_pa_bits(kvm));
 }
 
+static void free_nested_s2_mmu(struct kvm_s2_mmu *mmu)
+{
+	kvm_free_stage2_pgd(mmu);
+	kfree(mmu);
+}
+
 int kvm_vcpu_init_nested(struct kvm_vcpu *vcpu)
 {
 	struct kvm *kvm = vcpu->kvm;
-	struct kvm_s2_mmu *tmp;
-	int num_mmus, ret = 0;
+	struct kvm_s2_mmu *mmu;
+	int num_mmus, ret = 0, i;
 
 	if (test_bit(KVM_ARM_VCPU_HAS_EL2_E2H0, kvm->arch.vcpu_features) &&
 	    !cpus_have_final_cap(ARM64_HAS_HCR_NV1))
@@ -91,42 +104,38 @@ int kvm_vcpu_init_nested(struct kvm_vcpu *vcpu)
 	 */
 	num_mmus = atomic_read(&kvm->online_vcpus) * S2_MMU_PER_VCPU;
 
-	if (num_mmus > kvm->arch.nested_mmus_size) {
-		tmp = kvcalloc(num_mmus, sizeof(*tmp), GFP_KERNEL_ACCOUNT);
-		if (!tmp)
-			return -ENOMEM;
-
-		write_lock(&kvm->mmu_lock);
+	if (num_mmus <= kvm->arch.nested_mmus_size)
+		return 0;
 
-		if (kvm->arch.nested_mmus_size) {
-			memcpy(tmp, kvm->arch.nested_mmus,
-			       size_mul(sizeof(*tmp), kvm->arch.nested_mmus_size));
+	lockdep_assert_held(&kvm->arch.config_lock);
 
-			for (int i = 0; i < kvm->arch.nested_mmus_size; i++)
-				tmp[i].pgt->mmu = &tmp[i];
+	for (i = 0; i < S2_MMU_PER_VCPU; i++) {
+		mmu = kzalloc_obj(*mmu, GFP_KERNEL_ACCOUNT);
+		if (!mmu) {
+			ret = -ENOMEM;
+			break;
 		}
 
-		swap(kvm->arch.nested_mmus, tmp);
-
-		write_unlock(&kvm->mmu_lock);
+		ret = init_nested_s2_mmu(kvm, mmu);
+		if (ret) {
+			kfree(mmu);
+			free_page((unsigned long)vcpu->arch.ctxt.vncr_array);
+			vcpu->arch.ctxt.vncr_array = NULL;
+			break;
+		}
 
-		kvfree(tmp);
+		kvm->arch.nested_mmus[kvm->arch.nested_mmus_size + i] = mmu;
 	}
 
-	for (int i = kvm->arch.nested_mmus_size; !ret && i < num_mmus; i++)
-		ret = init_nested_s2_mmu(kvm, &kvm->arch.nested_mmus[i]);
-
 	if (ret) {
-		for (int i = kvm->arch.nested_mmus_size; i < num_mmus; i++)
-			kvm_free_stage2_pgd(&kvm->arch.nested_mmus[i]);
-
-		free_page((unsigned long)vcpu->arch.ctxt.vncr_array);
-		vcpu->arch.ctxt.vncr_array = NULL;
+		while (i--)
+			free_nested_s2_mmu(kvm->arch.nested_mmus[kvm->arch.nested_mmus_size + i]);
 
 		return ret;
 	}
 
-	kvm->arch.nested_mmus_size = num_mmus;
+	guard(write_lock)(&kvm->mmu_lock);
+	kvm->arch.nested_mmus_size += S2_MMU_PER_VCPU;
 
 	return 0;
 }
@@ -725,7 +734,7 @@ void kvm_s2_mmu_iterate_by_vmid(struct kvm *kvm, u16 vmid,
 	write_lock(&kvm->mmu_lock);
 
 	for (int i = 0; i < kvm->arch.nested_mmus_size; i++) {
-		struct kvm_s2_mmu *mmu = &kvm->arch.nested_mmus[i];
+		struct kvm_s2_mmu *mmu = kvm->arch.nested_mmus[i];
 
 		if (!kvm_s2_mmu_valid(mmu))
 			continue;
@@ -767,7 +776,7 @@ struct kvm_s2_mmu *lookup_s2_mmu(struct kvm_vcpu *vcpu)
 	 *   if S2 translation is disabled.
 	 */
 	for (int i = 0; i < kvm->arch.nested_mmus_size; i++) {
-		struct kvm_s2_mmu *mmu = &kvm->arch.nested_mmus[i];
+		struct kvm_s2_mmu *mmu = kvm->arch.nested_mmus[i];
 
 		if (!kvm_s2_mmu_valid(mmu))
 			continue;
@@ -806,7 +815,7 @@ static struct kvm_s2_mmu *get_s2_mmu_nested(struct kvm_vcpu *vcpu)
 	for (i = kvm->arch.nested_mmus_next;
 	     i < (kvm->arch.nested_mmus_size + kvm->arch.nested_mmus_next);
 	     i++) {
-		s2_mmu = &kvm->arch.nested_mmus[i % kvm->arch.nested_mmus_size];
+		s2_mmu = kvm->arch.nested_mmus[i % kvm->arch.nested_mmus_size];
 
 		if (atomic_read(&s2_mmu->refcnt) == 0)
 			break;
@@ -1223,7 +1232,7 @@ void kvm_nested_s2_wp(struct kvm *kvm)
 		return;
 
 	for (i = 0; i < kvm->arch.nested_mmus_size; i++) {
-		struct kvm_s2_mmu *mmu = &kvm->arch.nested_mmus[i];
+		struct kvm_s2_mmu *mmu = kvm->arch.nested_mmus[i];
 
 		if (kvm_s2_mmu_valid(mmu))
 			kvm_stage2_wp_range(mmu, 0, kvm_phys_size(mmu));
@@ -1242,7 +1251,7 @@ void kvm_nested_s2_unmap(struct kvm *kvm, bool may_block)
 		return;
 
 	for (i = 0; i < kvm->arch.nested_mmus_size; i++) {
-		struct kvm_s2_mmu *mmu = &kvm->arch.nested_mmus[i];
+		struct kvm_s2_mmu *mmu = kvm->arch.nested_mmus[i];
 
 		if (kvm_s2_mmu_valid(mmu))
 			kvm_stage2_unmap_range(mmu, 0, kvm_phys_size(mmu), may_block);
@@ -1261,7 +1270,7 @@ void kvm_nested_s2_flush(struct kvm *kvm)
 		return;
 
 	for (i = 0; i < kvm->arch.nested_mmus_size; i++) {
-		struct kvm_s2_mmu *mmu = &kvm->arch.nested_mmus[i];
+		struct kvm_s2_mmu *mmu = kvm->arch.nested_mmus[i];
 
 		if (kvm_s2_mmu_valid(mmu))
 			kvm_stage2_flush_range(mmu, 0, kvm_phys_size(mmu));
@@ -1273,10 +1282,10 @@ void kvm_arch_flush_shadow_all(struct kvm *kvm)
 	int i;
 
 	for (i = 0; i < kvm->arch.nested_mmus_size; i++) {
-		struct kvm_s2_mmu *mmu = &kvm->arch.nested_mmus[i];
+		struct kvm_s2_mmu *mmu = kvm->arch.nested_mmus[i];
 
 		if (!WARN_ON(atomic_read(&mmu->refcnt)))
-			kvm_free_stage2_pgd(mmu);
+			free_nested_s2_mmu(mmu);
 	}
 	kvfree(kvm->arch.nested_mmus);
 	kvm->arch.nested_mmus = NULL;
-- 
2.39.5 (Apple Git-154)

Re: [PATCH v3] KVM: arm64: nv: Keep the shadow S2 MMUs at fixed addresses

[Wei-Lin Chang]

On Thu, Aug 06, 2026 at 09:24:51PM +0200, Karl Mehltretter wrote:
> kvm_vcpu_init_nested() can grow kvm->arch.nested_mmus while initialising
> another vCPU: it copies the MMUs, publishes the new allocation, and frees
> the old one. It updates pgt->mmu back-pointers, but not hw_mmu, leaving
> already-running vCPUs with pointers to freed memory. hw_mmu cannot be
> fixed up the same way: a running vCPU reads it without holding mmu_lock.
> The nested S2 ptdump file's debugfs private data is also affected, as it
> points into the freed array.
> 
> KASAN reports an access through the stale hw_mmu pointer as a
> slab-use-after-free in kvm_handle_guest_abort().
> 
> Turn nested_mmus into a pointer table allocated once for the maximum
> number of vCPUs during VM creation. Allocate the MMUs separately as
> vCPUs are initialised and append their pointers to that table. The MMU
> objects never move, so cached hw_mmu pointers, pgt->mmu back-pointers,
> and ptdump private data remain valid.
> 
> Two issues in the old implementation are also fixed:
> 
> - The old failure path passed uninitialised MMUs to
>   kvm_free_stage2_pgd(), which derives kvm from mmu->arch and can
>   therefore dereference an invalid pointer. Only call
>   kvm_free_stage2_pgd() for initialised MMUs.
> - Previously, initialisation of the new MMUs was not ordered before
>   publication of nested_mmus_size. Fix this by taking mmu_lock when
>   increasing nested_mmus_size.
> 
> Fixes: 4f128f8e1aaa ("KVM: arm64: nv: Support multiple nested Stage-2 mmu structures")
> Cc: stable@vger.kernel.org
> Suggested-by: Marc Zyngier <maz@kernel.org>
> Assisted-by: Claude:claude-fable-5
> Signed-off-by: Karl Mehltretter <kmehltretter@gmail.com>

[...]

>  int kvm_vcpu_init_nested(struct kvm_vcpu *vcpu)
>  {
>  	struct kvm *kvm = vcpu->kvm;
> -	struct kvm_s2_mmu *tmp;
> -	int num_mmus, ret = 0;
> +	struct kvm_s2_mmu *mmu;
> +	int num_mmus, ret = 0, i;
>  
>  	if (test_bit(KVM_ARM_VCPU_HAS_EL2_E2H0, kvm->arch.vcpu_features) &&
>  	    !cpus_have_final_cap(ARM64_HAS_HCR_NV1))
> @@ -91,42 +104,38 @@ int kvm_vcpu_init_nested(struct kvm_vcpu *vcpu)
>  	 */
>  	num_mmus = atomic_read(&kvm->online_vcpus) * S2_MMU_PER_VCPU;
>  
> -	if (num_mmus > kvm->arch.nested_mmus_size) {
> -		tmp = kvcalloc(num_mmus, sizeof(*tmp), GFP_KERNEL_ACCOUNT);
> -		if (!tmp)
> -			return -ENOMEM;
> -
> -		write_lock(&kvm->mmu_lock);
> +	if (num_mmus <= kvm->arch.nested_mmus_size)
> +		return 0;
>  
> -		if (kvm->arch.nested_mmus_size) {
> -			memcpy(tmp, kvm->arch.nested_mmus,
> -			       size_mul(sizeof(*tmp), kvm->arch.nested_mmus_size));
> +	lockdep_assert_held(&kvm->arch.config_lock);
>  
> -			for (int i = 0; i < kvm->arch.nested_mmus_size; i++)
> -				tmp[i].pgt->mmu = &tmp[i];
> +	for (i = 0; i < S2_MMU_PER_VCPU; i++) {
> +		mmu = kzalloc_obj(*mmu, GFP_KERNEL_ACCOUNT);
> +		if (!mmu) {
> +			ret = -ENOMEM;
> +			break;
>  		}
>  
> -		swap(kvm->arch.nested_mmus, tmp);
> -
> -		write_unlock(&kvm->mmu_lock);
> +		ret = init_nested_s2_mmu(kvm, mmu);
> +		if (ret) {
> +			kfree(mmu);
> +			free_page((unsigned long)vcpu->arch.ctxt.vncr_array);
> +			vcpu->arch.ctxt.vncr_array = NULL;
> +			break;
> +		}
>  
> -		kvfree(tmp);
> +		kvm->arch.nested_mmus[kvm->arch.nested_mmus_size + i] = mmu;
>  	}
>  
> -	for (int i = kvm->arch.nested_mmus_size; !ret && i < num_mmus; i++)
> -		ret = init_nested_s2_mmu(kvm, &kvm->arch.nested_mmus[i]);
> -
>  	if (ret) {
> -		for (int i = kvm->arch.nested_mmus_size; i < num_mmus; i++)
> -			kvm_free_stage2_pgd(&kvm->arch.nested_mmus[i]);
> -
> -		free_page((unsigned long)vcpu->arch.ctxt.vncr_array);
> -		vcpu->arch.ctxt.vncr_array = NULL;
> +		while (i--)
> +			free_nested_s2_mmu(kvm->arch.nested_mmus[kvm->arch.nested_mmus_size + i]);
>  
>  		return ret;
>  	}
>  
> -	kvm->arch.nested_mmus_size = num_mmus;
> +	guard(write_lock)(&kvm->mmu_lock);
> +	kvm->arch.nested_mmus_size += S2_MMU_PER_VCPU;
>  
>  	return 0;
>  }

Hey sorry for nitpicking once more, I actually like v2 more, as this
version splits the error handling to two places. Marc's implementation
[1] is also good, but that requires a twist in kvm_arch_flush_shadow_all().
So I'd personally go with v2 (+my suggestions for it :)), but also fine if
others have other opinions.

I also think the comment in kvm_vcpu_init_nested() is worth adjusting
slightly, too.

Thanks,
Wei-Lin Chang

[1]: https://lore.kernel.org/kvmarm/86mrv2arf2.wl-maz@kernel.org/

[...]

> @@ -1273,10 +1282,10 @@ void kvm_arch_flush_shadow_all(struct kvm *kvm)
>  	int i;
>  
>  	for (i = 0; i < kvm->arch.nested_mmus_size; i++) {
> -		struct kvm_s2_mmu *mmu = &kvm->arch.nested_mmus[i];
> +		struct kvm_s2_mmu *mmu = kvm->arch.nested_mmus[i];
>  
>  		if (!WARN_ON(atomic_read(&mmu->refcnt)))
> -			kvm_free_stage2_pgd(mmu);
> +			free_nested_s2_mmu(mmu);
>  	}
>  	kvfree(kvm->arch.nested_mmus);
>  	kvm->arch.nested_mmus = NULL;
> -- 
> 2.39.5 (Apple Git-154)
> 

Re: [PATCH v3] KVM: arm64: nv: Keep the shadow S2 MMUs at fixed addresses

[Karl Mehltretter]

On Thu, Aug 06, 2026 at 10:54:03PM +0100, Wei-Lin Chang wrote:
> 
> Hey sorry for nitpicking once more, I actually like v2 more, as this
> version splits the error handling to two places. Marc's implementation
> [1] is also good, but that requires a twist in kvm_arch_flush_shadow_all().
> So I'd personally go with v2 (+my suggestions for it :)), but also fine if
> others have other opinions.
> 
> I also think the comment in kvm_vcpu_init_nested() is worth adjusting
> slightly, too.
> 

No worries, thanks for taking another look.

I was following the guidance in include/linux/cleanup.h:

  the expectation is that usage of "goto" and cleanup helpers is never
  mixed in the same function.

But maybe keeping goto is better. I'll wait a bit for others
to comment.

For the comment, I propose replacing the current text with:

	/*
	 * If allocating an MMU object fails, keep the VNCR page alive so
	 * that userspace can retry vCPU initialisation without affecting
	 * the VM.
	 */

Is that the adjustment you had in mind?

Thanks,
Karl

Re: [PATCH v3] KVM: arm64: nv: Keep the shadow S2 MMUs at fixed addresses

[Marc Zyngier]

On Fri, 07 Aug 2026 02:13:17 +0100,
Karl Mehltretter <kmehltretter@gmail.com> wrote:
> 
> On Thu, Aug 06, 2026 at 10:54:03PM +0100, Wei-Lin Chang wrote:
> > 
> > Hey sorry for nitpicking once more, I actually like v2 more, as this
> > version splits the error handling to two places. Marc's implementation
> > [1] is also good, but that requires a twist in kvm_arch_flush_shadow_all().
> > So I'd personally go with v2 (+my suggestions for it :)), but also fine if
> > others have other opinions.
> > 
> > I also think the comment in kvm_vcpu_init_nested() is worth adjusting
> > slightly, too.
> > 
> 
> No worries, thanks for taking another look.
> 
> I was following the guidance in include/linux/cleanup.h:
> 
>   the expectation is that usage of "goto" and cleanup helpers is never
>   mixed in the same function.
> 
> But maybe keeping goto is better. I'll wait a bit for others
> to comment.

The version I proposed in response to your first version had no goto,
and trivial error handling. Can we *please* stop the bike shedding and
settle on the simplest possible fix? Or *any* fix at all?

> 
> For the comment, I propose replacing the current text with:
> 
> 	/*
> 	 * If allocating an MMU object fails, keep the VNCR page alive so
> 	 * that userspace can retry vCPU initialisation without affecting
> 	 * the VM.
> 	 */
> 
> Is that the adjustment you had in mind?

I don't think we need to change any comment. If anything, it should be
deleted altogether. This is a security fix, not a literary exercise.

	M.

-- 
Jazz isn't dead. It just smells funny.