Re: [PATCH] mm: fix copy_vma() error handling for hugetlb mappings

[Lorenzo Stoakes <lorenzo.stoakes@oracle.com>]

On Fri, May 23, 2025 at 10:54:24AM +0200, Oscar Salvador wrote:
> On Fri, May 23, 2025 at 09:56:18AM +0200, Ricardo Cañuelo Navarro wrote:
> > If, during a mremap() operation for a hugetlb-backed memory mapping,
> > copy_vma() fails after the source vma has been duplicated and
> > opened (ie. vma_link() fails), the error is handled by closing the new
> > vma. This updates the hugetlbfs reservation counter of the reservation
> > map which at this point is referenced by both the source vma and the new
> > copy. As a result, once the new vma has been freed and copy_vma()
> > returns, the reservation counter for the source vma will be incorrect.
> >
> > This patch addresses this corner case by clearing the hugetlb private
> > page reservation reference for the new vma and decrementing the
> > reference before closing the vma, so that vma_close() won't update the
> > reservation counter.
> >
> > The issue was reported by a private syzbot instance, see the error
> > report log [1] and reproducer [2]. Possible duplicate of public syzbot
> > report [3].
> >
> > Signed-off-by: Ricardo Cañuelo Navarro <rcn@igalia.com>
> > Cc: stable@vger.kernel.org # 6.12+
> > Link: https://people.igalia.com/rcn/kernel_logs/20250422__WARNING_in_page_counter_cancel.txt [1]
> > Link: https://people.igalia.com/rcn/kernel_logs/20250422__WARNING_in_page_counter_cancel__repro.c [2]
> > Link: https://lore.kernel.org/all/67000a50.050a0220.49194.048d.GAE@google.com/ [3]
> > ---
> >  mm/vma.c | 2 ++
> >  1 file changed, 2 insertions(+)
> >
> > diff --git a/mm/vma.c b/mm/vma.c
> > index 839d12f02c885d3338d8d233583eb302d82bb80b..9d9f699ace977c9c869e5da5f88f12be183adcfb 100644
> > --- a/mm/vma.c
> > +++ b/mm/vma.c
> > @@ -1834,6 +1834,8 @@ struct vm_area_struct *copy_vma(struct vm_area_struct **vmap,
> >  	return new_vma;
> >
> >  out_vma_link:
> > +	if (is_vm_hugetlb_page(new_vma))
> > +		clear_vma_resv_huge_pages(new_vma);
> >  	vma_close(new_vma);
> >
> >  	if (new_vma->vm_file)
>
> Sigh, I do not think Lorenzo will be happy about having yet another
> hugetlb check around vma code :-).

NAAAA... nah only joking ;)

But you do know me well sir, and indeed I hate doing this here.

> Maybe this is good as is as a quick fix, but we really need to
> re-assest this situation

Yes. We really need to find a way to avoid arbitrarily putting in branches like:

if ([hugetlb/uffd/dax/blah blah])
	some_seemingly_random_thing_that_is_an_implementation_detail();

> .
> I will have a look once I managed to finish a couple of other things.

Thanks.

>
>
>
> --
> Oscar Salvador
> SUSE Labs

Let me have a look at this fix-wise as obviously that's a priority here and we
might need to live with some ugliness for that reason.

But I want to make sure this is correct (I mean it kind of rings true).

Can you confirm at least on the theoretical side this makes sense? I was
actually going to cc you if you weren't already (hadn't checked yet :)

Cheers, Lorenzo