Re: [PATCH 1/2] KVM: arm64: nv: Allocate the shadow S2 MMUs individually

[Marc Zyngier <maz@kernel.org>]

On Fri, 07 Aug 2026 18:08:17 +0100,
Karl Mehltretter <kmehltretter@gmail.com> wrote:
> 
> On Tue, Aug 04, 2026 at 03:31:13PM +0100, Marc Zyngier wrote:
> > See the hack below that seems to work OK.
> > 
> 
> Hi Marc,
> 
> Since error handling has come up again, I should clarify why
> v2 did not use this sketch verbatim.
> 
> > -	kvm_init_nested(kvm);
> > +	ret = kvm_init_nested(kvm);
> > +	if (ret)
> > +		return ret;
> >  
> >  	ret = kvm_share_hyp(kvm, kvm + 1);
> >  	if (ret)
> 
> Once kvm_init_nested() has allocated the pointer table, failure in
> kvm_share_hyp() or any subsequent VM initialisation step leaks that
> table.
>
> v2 therefore calls kvm_init_nested() after the earlier initialisation
> steps have succeeded and frees the table if a later step in
> kvm_arch_init_vm() fails.

https://lore.kernel.org/all/86ecgdaupe.wl-maz@kernel.org/

> 
> > +		for (int i = 0; !ret && i < S2_MMU_PER_VCPU; i++)
> > +			ret = init_nested_s2_mmu(kvm, &tmp[i]);
> 
> > +		if (ret) {
> > +			for (int i = 0; i < S2_MMU_PER_VCPU; i++)
> > +				kvm_free_stage2_pgd(&tmp[i]);
> 
> This cleanup includes the entry whose initialisation failed and any
> entries whose initialisation was never attempted. An uninitialised
> entry has no valid mmu->arch, but kvm_free_stage2_pgd() immediately
> derives kvm from mmu->arch before checking mmu->pgt. It can therefore
> dereference an invalid pointer.
> 
> This is why v2 tracks successfully initialised MMUs and frees only
> those entries.

Again, that's not exactly hard to fix.

Overall, I dislike the pointless helpers, the individual allocations
of S2 MMUs, the goto nest, and the inconsistency in freeing the vncr
array (a pre-existing condition).

My current patch is as follows, and so far, I haven't seen much that I
like better.

	M.

From e6c249a95db508ca036a00d36c4f90af4cbc3eba Mon Sep 17 00:00:00 2001
From: Marc Zyngier <maz@kernel.org>
Date: Sun, 9 Aug 2026 11:47:25 +0100
Subject: [PATCH] KVM: arm64: nv: Fix life cycle of the nested_mmus array

The nested_mmus array holds the shadow page tables that are used when
a guest is running a nested context. These structures are allocated on
VCPU_INIT for whole guest, which implies that they may have to be
relocated as the array grows.

Should a VCPU_INIT occur whilst a vcpu is actively running an L2 and
that the allocation requires relocation, that vcpu will still be
running with a pointer to the previous structure, which will have been
freed.

Fix this by turning the array of structures to an array of pointers,
which is now allocated at VM creation, sized to the absolute maximum
that KVM can handle.

In turn, each VCPU_INIT contributes S2_MMU_PER_VCPU to the pool. No
reallocation is ever performed, and the life cycle of each object is
much clearer:

- the nested_mmus array is allocated in kvm_init_nested(), and freed
  in kvm_arch_destroy_vm()

- s2_mmu structures are allocated in kvm_vcpu_init_nested(), and freed
  on kvm_arch_flush_shadow_all()

Finally, the freeing of vcpu->arch.vncr_array is made consistent
rather than being done on some failure paths, but not others.

Fixes: 4f128f8e1aaa ("KVM: arm64: nv: Support multiple nested Stage-2 mmu structures")
Reported-by: Shen Yongchao <grayhat@foxmail.com>
Reported-by: Karl Mehltretter <kmehltretter@gmail.com>
Suggested-by: Karl Mehltretter <kmehltretter@gmail.com>
Link: https://lore.kernel.org/r/20260803224405.41468-1-kmehltretter@gmail.com
Signed-off-by: Marc Zyngier <maz@kernel.org>
Cc: stable@vger.kernel.org
---
 arch/arm64/include/asm/kvm_host.h   |  2 +-
 arch/arm64/include/asm/kvm_nested.h |  2 +-
 arch/arm64/kvm/arm.c                |  8 ++-
 arch/arm64/kvm/nested.c             | 86 +++++++++++++----------------
 4 files changed, 45 insertions(+), 53 deletions(-)

diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
index 108966a9db12b..08b2f24dc3c79 100644
--- a/arch/arm64/include/asm/kvm_host.h
+++ b/arch/arm64/include/asm/kvm_host.h
@@ -322,7 +322,7 @@ struct kvm_arch {
 	 * Stage 2 paging state for VMs with nested S2 using a virtual
 	 * VMID.
 	 */
-	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 c83be6d0e79ac..21d0f4cbe07f1 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 50adfff75be82..7607173c1a40c 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:
@@ -324,6 +327,7 @@ void kvm_arch_destroy_vm(struct kvm *kvm)
 
 	kvm_unshare_hyp(kvm, kvm + 1);
 
+	kvfree(kvm->arch.nested_mmus);
 	kvm_arm_teardown_hypercalls(kvm);
 }
 
diff --git a/arch/arm64/kvm/nested.c b/arch/arm64/kvm/nested.c
index 20af94197a8a7..5b69a0f382320 100644
--- a/arch/arm64/kvm/nested.c
+++ b/arch/arm64/kvm/nested.c
@@ -44,11 +44,15 @@ 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 = kvmalloc_array(KVM_MAX_VCPUS * S2_MMU_PER_VCPU,
+					       sizeof(struct s2_mmu *),
+					       GFP_KERNEL_ACCOUNT);
 	kvm->arch.nested_mmus_size = 0;
 	atomic_set(&kvm->arch.vncr_tlb_count, 0);
+
+	return kvm->arch.nested_mmus ? 0 : -ENOMEM;
 }
 
 static int init_nested_s2_mmu(struct kvm *kvm, struct kvm_s2_mmu *mmu)
@@ -69,8 +73,7 @@ static int init_nested_s2_mmu(struct kvm *kvm, struct kvm_s2_mmu *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;
+	int num_mmus;
 
 	if (test_bit(KVM_ARM_VCPU_HAS_EL2_E2H0, kvm->arch.vcpu_features) &&
 	    !cpus_have_final_cap(ARM64_HAS_HCR_NV1))
@@ -83,51 +86,37 @@ int kvm_vcpu_init_nested(struct kvm_vcpu *vcpu)
 	if (!vcpu->arch.ctxt.vncr_array)
 		return -ENOMEM;
 
-	/*
-	 * Let's treat memory allocation failures as benign: If we fail to
-	 * allocate anything, return an error and keep the allocated array
-	 * alive. Userspace may try to recover by initializing the vcpu
-	 * again, and there is no reason to affect the whole VM for this.
-	 */
 	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);
+		struct kvm_s2_mmu *tmp;
+		int i, ret = 0;
+
+		tmp = kvcalloc(S2_MMU_PER_VCPU, sizeof(*tmp), GFP_KERNEL_ACCOUNT);
 		if (!tmp)
-			return -ENOMEM;
+			ret = -ENOMEM;
 
-		write_lock(&kvm->mmu_lock);
+		for (i = 0; !ret && i < S2_MMU_PER_VCPU; i++)
+			ret = init_nested_s2_mmu(kvm, &tmp[i]);
 
-		if (kvm->arch.nested_mmus_size) {
-			memcpy(tmp, kvm->arch.nested_mmus,
-			       size_mul(sizeof(*tmp), kvm->arch.nested_mmus_size));
+		if (ret) {
+			while (--i >= 0)
+				kvm_free_stage2_pgd(&tmp[i]);
 
-			for (int i = 0; i < kvm->arch.nested_mmus_size; i++)
-				tmp[i].pgt->mmu = &tmp[i];
+			kvfree(tmp);
+			free_page((unsigned long)vcpu->arch.ctxt.vncr_array);
+			vcpu->arch.ctxt.vncr_array = NULL;
+			return ret;
 		}
+		
+		guard(write_lock)(&kvm->mmu_lock);
 
-		swap(kvm->arch.nested_mmus, tmp);
+		for (i = 0; i < S2_MMU_PER_VCPU; i++)
+			kvm->arch.nested_mmus[i + kvm->arch.nested_mmus_size] = &tmp[i];
 
-		write_unlock(&kvm->mmu_lock);
-
-		kvfree(tmp);
+		kvm->arch.nested_mmus_size += S2_MMU_PER_VCPU;
 	}
 
-	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;
-
-		return ret;
-	}
-
-	kvm->arch.nested_mmus_size = num_mmus;
-
 	return 0;
 }
 
@@ -741,7 +730,7 @@ 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;
@@ -783,7 +772,7 @@ 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;
@@ -822,7 +811,7 @@ 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;
@@ -1269,7 +1258,7 @@ 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));
@@ -1288,7 +1277,7 @@ 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);
@@ -1307,7 +1296,7 @@ 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));
@@ -1316,16 +1305,15 @@ void kvm_nested_s2_flush(struct kvm *kvm)
 
 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];
+	for (int i = kvm->arch.nested_mmus_size - 1; i >= 0; i--) {
+		struct kvm_s2_mmu *mmu = kvm->arch.nested_mmus[i];
 
 		if (!WARN_ON(atomic_read(&mmu->refcnt)))
 			kvm_free_stage2_pgd(mmu);
+
+		if ((i % S2_MMU_PER_VCPU) == 0)
+			kvfree(mmu);
 	}
-	kvfree(kvm->arch.nested_mmus);
-	kvm->arch.nested_mmus = NULL;
 	kvm->arch.nested_mmus_size = 0;
 	kvm_uninit_stage2_mmu(kvm);
 }
-- 
2.47.3

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