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

[Oscar Salvador <osalvador@suse.de>]

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 :-).
Maybe this is good as is as a quick fix, but we really need to
re-assest this situation
.
I will have a look once I managed to finish a couple of other things.

 

-- 
Oscar Salvador
SUSE Labs