Subject: Re: [PATCH] KVM: arm64: Reallocate the nested_mmus array under the mmu_lock

From: Oliver Upton <[email-masked]>
Date: Thu, 4 Jun 2026 15:27:16 -0700

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 <[email-masked]>

The diff itself LGTM

Reviewed-by: Oliver Upton <[email-masked]>

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
>