Re: [PATCH v2] mm/memfd: fix information leak in hugetlb folios

[Oscar Salvador]

On Wed, Nov 12, 2025 at 08:20:34PM +0530, Deepanshu Kartikey wrote:
> When allocating hugetlb folios for memfd, three initialization steps
> are missing:
> 
> 1. Folios are not zeroed, leading to kernel memory disclosure to userspace
> 2. Folios are not marked uptodate before adding to page cache
> 3. hugetlb_fault_mutex is not taken before hugetlb_add_to_page_cache()
> 
> The memfd allocation path bypasses the normal page fault handler
> (hugetlb_no_page) which would handle all of these initialization steps.
> This is problematic especially for udmabuf use cases where folios are
> pinned and directly accessed by userspace via DMA.
> 
> Fix by matching the initialization pattern used in hugetlb_no_page():
> - Zero the folio using folio_zero_user() which is optimized for huge pages
> - Mark it uptodate with folio_mark_uptodate()
> - Take hugetlb_fault_mutex before adding to page cache to prevent races
> 
> The folio_zero_user() change also fixes a potential security issue where
> uninitialized kernel memory could be disclosed to userspace through
> read() or mmap() operations on the memfd.
> 
> Reported-by: syzbot+f64019ba229e3a5c411b@xxxxxxxxxxxxxxxxxxxxxxxxx
> Link: https://lore.kernel.org/all/20251112031631.2315651-1-kartikey406@xxxxxxxxx/ [v1]
> Closes: https://syzkaller.appspot.com/bug?extid=f64019ba229e3a5c411b
> Fixes: 89c1905d9c14 ("mm/gup: introduce memfd_pin_folios() for pinning memfd folios")
> Cc: stable@xxxxxxxxxxxxxxx
> Suggested-by: Oscar Salvador <osalvador@xxxxxxx>
> Suggested-by: David Hildenbrand <david@xxxxxxxxxx>
> Tested-by: syzbot+f64019ba229e3a5c411b@xxxxxxxxxxxxxxxxxxxxxxxxx
> Signed-off-by: Deepanshu Kartikey <kartikey406@xxxxxxxxx>

As David mentioned, we can drop the comment wrt. __folio_mark_uptodate.
As for the addr_hint in folio_zero_user, I do not think it makes a
difference in here.
AFAIK, it serves the purpose that subpages belong to the addr_hint will
be zeroed the latest to keep them in cache, but here it does not really
apply, so '0' should just work?

Acked-by: Oscar Salvador <osalvador@xxxxxxx>


-- 
Oscar Salvador
SUSE Labs