Re: [BUG] WARNING in unlink_anon_vmas()

["Lorenzo Stoakes (Oracle)" <ljs@kernel.org>]

+cc Sasha

On Wed, Mar 18, 2026 at 10:59:33AM +0000, Lorenzo Stoakes (Oracle) wrote:
> (-cc old email)
>
> On Wed, Mar 18, 2026 at 06:42:49PM +0800, Jiakai Xu wrote:
> > Hi all,
> >
> > While fuzzing the KVM subsystem on RISC-V, I stumbled upon a kernel WARNING
> > that triggers in unlink_anon_vmas().
>
> Thanks!
>
> Will have a look at this.
>
> >
> >  WARNING: mm/rmap.c:528 at unlink_anon_vmas+0x562/0x768 mm/rmap.c:528
> >  unlink_anon_vmas+0x562/0x768 mm/rmap.c:528
>
> Assuming there's not some big mismatch with kernel versions this is:
>
> VM_WARN_ON(anon_vma->num_active_vmas);

OK so this _was_ reported by Sasha (via an AI assessment), but it completely
dropped off my radar sorry about that!

https://lore.kernel.org/linux-mm/20260302151547.2389070-1-sashal@kernel.org/

I want to fix this a slightly different way though.

SO what's happening is in dup_anon_vma() we do:

static int dup_anon_vma(struct vm_area_struct *dst,
			struct vm_area_struct *src, struct vm_area_struct **dup)
{
	...

		dst->anon_vma = src->anon_vma;
		ret = anon_vma_clone(dst, src, VMA_OP_MERGE_UNFAULTED);
		if (ret)
			return ret; <-- fault injection error here

		*dup = dst;         <-- NOT set

	...
}

Then:

int anon_vma_clone(struct vm_area_struct *dst, struct vm_area_struct *src,
		   enum vma_operation operation)
{
	...

	list_for_each_entry(pavc, &src->anon_vma_chain, same_vma) {
		avc = anon_vma_chain_alloc(GFP_KERNEL);  <-- FAULT INJECTED HERE
		if (!avc)
			goto enomem_failure;             <-- so we do this

		anon_vma_chain_assign(dst, avc, pavc->anon_vma);
	}

	...

	if (operation != VMA_OP_FORK)
		dst->anon_vma->num_active_vmas++;        <-- does NOT get run

	...

 enomem_failure:
	cleanup_partial_anon_vmas(dst);
	return -ENOMEM;
}

We only clear down the partially allocated anon_vma_chain objects:

static void cleanup_partial_anon_vmas(struct vm_area_struct *vma)
{
	struct anon_vma_chain *avc, *next;

	list_for_each_entry_safe(avc, next, &vma->anon_vma_chain, same_vma) {
		list_del(&avc->same_vma);
		anon_vma_chain_free(avc);
	}
}

But, crucially, dst->anon_vma is LEFT IN PLACE.

So on process exit, we call into unlink_anon_vmas() for that VMA:

void unlink_anon_vmas(struct vm_area_struct *vma)
{
	...
	struct anon_vma *active_anon_vma = vma->anon_vma; <-- is SET

	...

	/* Unfaulted is a no-op. */
	if (!active_anon_vma) {                           <-- is not called
		VM_WARN_ON_ONCE(!list_empty(&vma->anon_vma_chain));
		return;
	}

	...

	active_anon_vma->num_active_vmas--;               <-- Incorrect (*)

	...

	list_for_each_entry_safe(avc, next, &vma->anon_vma_chain, same_vma) {
		...

		VM_WARN_ON(anon_vma->num_active_vmas);    <-- triggers
		put_anon_vma(anon_vma);

		...
	}
}

* If anon_vma->num_active_vmas was 0, it underflows, but otherwise it'll get
  decrement one time too many, and so will _eventually underflow_ guaranteed and
  trigger the bug for for VMAs associated with this anon_vma.

The fix is to set vma->anon_vma = NULL in this situation, which I think is best
done in the cleanup code as I said at
https://lore.kernel.org/linux-mm/a709c736-fd76-4bc9-a1d2-e1351742b321@lucifer.local/
but then... didn't do as it dropped off my radar (oops!)

Will send a fix + cc, attribute Reported-by etc., thanks very much for reporting
this Jiakai, was very useful!

Cheers, Lorenzo