* [PATCH] KVM: arm64: Reallocate the nested_mmus array under the mmu_lock
@ 2026-06-04 18:30 Hyunwoo Kim
2026-06-04 22:27 ` Oliver Upton
0 siblings, 1 reply; 3+ messages in thread
From: Hyunwoo Kim @ 2026-06-04 18:30 UTC (permalink / raw)
To: maz, oupton, joey.gouly, seiden, suzuki.poulose, yuzenghui,
catalin.marinas, will, christoffer.dall
Cc: linux-arm-kernel, kvmarm, imv4bel
Code that walks kvm->arch.nested_mmus[] holds kvm->mmu_lock. By contrast,
kvm_vcpu_init_nested() reallocates the array and frees the old buffer while
holding only kvm->arch.config_lock, so a walker can reference the freed
array.
Allocate the new array outside the lock, as the allocation can sleep, and
do only the copy and the pointer swap under the mmu_lock. After the swap no
walker can reach the old buffer, so free it once the lock has been
released.
Fixes: 4f128f8e1aaac ("KVM: arm64: nv: Support multiple nested Stage-2 mmu structures")
Signed-off-by: Hyunwoo Kim <imv4bel@gmail•com>
---
arch/arm64/kvm/nested.c | 33 ++++++++++++++++++++-------------
1 file changed, 20 insertions(+), 13 deletions(-)
diff --git a/arch/arm64/kvm/nested.c b/arch/arm64/kvm/nested.c
index 38f672e940878..6f7bc9a9992e0 100644
--- a/arch/arm64/kvm/nested.c
+++ b/arch/arm64/kvm/nested.c
@@ -89,21 +89,28 @@ int kvm_vcpu_init_nested(struct kvm_vcpu *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;
- tmp = kvrealloc(kvm->arch.nested_mmus,
- size_mul(sizeof(*kvm->arch.nested_mmus), num_mmus),
- GFP_KERNEL_ACCOUNT | __GFP_ZERO);
- if (!tmp)
- return -ENOMEM;
- swap(kvm->arch.nested_mmus, tmp);
+ if (num_mmus > kvm->arch.nested_mmus_size) {
+ tmp = kvcalloc(num_mmus, sizeof(*tmp), GFP_KERNEL_ACCOUNT);
+ if (!tmp)
+ return -ENOMEM;
- /*
- * If we went through a realocation, adjust the MMU back-pointers in
- * the previously initialised kvm_pgtable structures.
- */
- if (kvm->arch.nested_mmus != tmp)
- for (int i = 0; i < kvm->arch.nested_mmus_size; i++)
- kvm->arch.nested_mmus[i].pgt->mmu = &kvm->arch.nested_mmus[i];
+ write_lock(&kvm->mmu_lock);
+
+ if (kvm->arch.nested_mmus_size) {
+ memcpy(tmp, kvm->arch.nested_mmus,
+ size_mul(sizeof(*tmp), kvm->arch.nested_mmus_size));
+
+ for (int i = 0; i < kvm->arch.nested_mmus_size; i++)
+ tmp[i].pgt->mmu = &tmp[i];
+ }
+
+ swap(kvm->arch.nested_mmus, tmp);
+
+ write_unlock(&kvm->mmu_lock);
+
+ kvfree(tmp);
+ }
for (int i = kvm->arch.nested_mmus_size; !ret && i < num_mmus; i++)
ret = init_nested_s2_mmu(kvm, &kvm->arch.nested_mmus[i]);
--
2.43.0
^ permalink raw reply related [flat|nested] 3+ messages in thread* Re: [PATCH] KVM: arm64: Reallocate the nested_mmus array under the mmu_lock
2026-06-04 18:30 [PATCH] KVM: arm64: Reallocate the nested_mmus array under the mmu_lock Hyunwoo Kim
@ 2026-06-04 22:27 ` Oliver Upton
2026-06-04 22:58 ` Sean Christopherson
0 siblings, 1 reply; 3+ messages in thread
From: Oliver Upton @ 2026-06-04 22:27 UTC (permalink / raw)
To: Hyunwoo Kim
Cc: maz, joey.gouly, seiden, suzuki.poulose, yuzenghui,
catalin.marinas, will, christoffer.dall, linux-arm-kernel, kvmarm
Hi,
The shortlog is very confusing, since "allocate behind $LOCK" is usually
something alarming. Maybe instead:
KVM: arm64: Reassign nested_mmus array behind mmu_lock
On Fri, Jun 05, 2026 at 03:30:00AM +0900, Hyunwoo Kim wrote:
> Code that walks kvm->arch.nested_mmus[] holds kvm->mmu_lock. By contrast,
> kvm_vcpu_init_nested() reallocates the array and frees the old buffer while
> holding only kvm->arch.config_lock, so a walker can reference the freed
> array.
It wouldn't hurt to share slightly more information here. Are you
dealing with a concurrent MMU notifier?
> Allocate the new array outside the lock, as the allocation can sleep, and
> do only the copy and the pointer swap under the mmu_lock. After the swap no
> walker can reach the old buffer, so free it once the lock has been
> released.
>
> Fixes: 4f128f8e1aaac ("KVM: arm64: nv: Support multiple nested Stage-2 mmu structures")
> Signed-off-by: Hyunwoo Kim <imv4bel@gmail•com>
The diff itself LGTM
Reviewed-by: Oliver Upton <oupton@kernel•org>
Thanks,
Oliver
> ---
> arch/arm64/kvm/nested.c | 33 ++++++++++++++++++++-------------
> 1 file changed, 20 insertions(+), 13 deletions(-)
>
> diff --git a/arch/arm64/kvm/nested.c b/arch/arm64/kvm/nested.c
> index 38f672e940878..6f7bc9a9992e0 100644
> --- a/arch/arm64/kvm/nested.c
> +++ b/arch/arm64/kvm/nested.c
> @@ -89,21 +89,28 @@ int kvm_vcpu_init_nested(struct kvm_vcpu *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;
> - tmp = kvrealloc(kvm->arch.nested_mmus,
> - size_mul(sizeof(*kvm->arch.nested_mmus), num_mmus),
> - GFP_KERNEL_ACCOUNT | __GFP_ZERO);
> - if (!tmp)
> - return -ENOMEM;
>
> - swap(kvm->arch.nested_mmus, tmp);
> + if (num_mmus > kvm->arch.nested_mmus_size) {
> + tmp = kvcalloc(num_mmus, sizeof(*tmp), GFP_KERNEL_ACCOUNT);
> + if (!tmp)
> + return -ENOMEM;
>
> - /*
> - * If we went through a realocation, adjust the MMU back-pointers in
> - * the previously initialised kvm_pgtable structures.
> - */
> - if (kvm->arch.nested_mmus != tmp)
> - for (int i = 0; i < kvm->arch.nested_mmus_size; i++)
> - kvm->arch.nested_mmus[i].pgt->mmu = &kvm->arch.nested_mmus[i];
> + write_lock(&kvm->mmu_lock);
> +
> + if (kvm->arch.nested_mmus_size) {
> + memcpy(tmp, kvm->arch.nested_mmus,
> + size_mul(sizeof(*tmp), kvm->arch.nested_mmus_size));
> +
> + for (int i = 0; i < kvm->arch.nested_mmus_size; i++)
> + tmp[i].pgt->mmu = &tmp[i];
> + }
> +
> + swap(kvm->arch.nested_mmus, tmp);
> +
> + write_unlock(&kvm->mmu_lock);
> +
> + kvfree(tmp);
> + }
>
> for (int i = kvm->arch.nested_mmus_size; !ret && i < num_mmus; i++)
> ret = init_nested_s2_mmu(kvm, &kvm->arch.nested_mmus[i]);
> --
> 2.43.0
>
^ permalink raw reply [flat|nested] 3+ messages in thread* Re: [PATCH] KVM: arm64: Reallocate the nested_mmus array under the mmu_lock
2026-06-04 22:27 ` Oliver Upton
@ 2026-06-04 22:58 ` Sean Christopherson
0 siblings, 0 replies; 3+ messages in thread
From: Sean Christopherson @ 2026-06-04 22:58 UTC (permalink / raw)
To: Oliver Upton
Cc: Hyunwoo Kim, maz, joey.gouly, seiden, suzuki.poulose, yuzenghui,
catalin.marinas, will, christoffer.dall, linux-arm-kernel, kvmarm
On Thu, Jun 04, 2026, Oliver Upton wrote:
> The shortlog is very confusing, since "allocate behind $LOCK" is usually
> something alarming. Maybe instead:
>
> KVM: arm64: Reassign nested_mmus array behind mmu_lock
+1 from the peanut gallery. After reading the shortlog, I was about to grab my
pitchfork :-)
^ permalink raw reply [flat|nested] 3+ messages in thread
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2026-06-04 18:30 [PATCH] KVM: arm64: Reallocate the nested_mmus array under the mmu_lock Hyunwoo Kim
2026-06-04 22:27 ` Oliver Upton
2026-06-04 22:58 ` Sean Christopherson
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