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

From: Hyunwoo Kim <[email-masked]>
Date: Fri, 5 Jun 2026 14:35:20 +0900

On Thu, Jun 04, 2026 at 03:27:16PM -0700, Oliver Upton wrote:
> 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

heh, that's confusing indeed. I'll change it that way.

> 
> 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?

Yes. The MMU notifier path also walks nested_mmus[] under mmu_lock.
kvm_vcpu_init_nested() holds only config_lock, so if a notifier fires
during vCPU init, it races with the array realloc and free.

Here's the reworked changelog. Should I send v2?

  kvm->arch.nested_mmus[] is walked under kvm->mmu_lock, including from the
  MMU notifier path (kvm_unmap_gfn_range() -> kvm_nested_s2_unmap()), which
  can run at any time. kvm_vcpu_init_nested() reallocates the array and frees
  the old buffer while holding only kvm->arch.config_lock, so such a walker
  can reference the freed array.

  Allocate the new array outside of mmu_lock, as the allocation can sleep.
  Under the lock, copy the existing entries, fix up the back pointers and
  reassign the array. Free the old buffer after dropping the lock, as
  kvfree() can sleep as well.

> 
> > 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 for the review.

> 
> 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
> > 

Best regards,
Hyunwoo Kim