Re: [PATCH v2] mm/gup: honour FOLL_PIN in NOMMU __get_user_pages_locked()

David Hildenbrand (Arm)
2026-04-23T15:56:05Z

On 4/23/26 16:28, Greg Kroah-Hartman wrote:
> The !CONFIG_MMU implementation of __get_user_pages_locked() takes a bare
> get_page() reference for each page regardless of foll_flags:
> if (pages[i])
> get_page(pages[i]);
>
> This is reached from pin_user_pages*() with FOLL_PIN set.
> unpin_user_page() is shared between MMU and NOMMU configurations and
> unconditionally calls gup_put_folio(..., FOLL_PIN), which subtracts
> GUP_PIN_COUNTING_BIAS (1024) from the folio refcount.
>
> This means that pin adds 1, and then unpin will subtract 1024.
>
> If a user maps a page (refcount 1), registers it 1023 times as an
> io_uring fixed buffer (1023 pin_user_pages calls -> refcount 1024), then
> unregisters: the first unpin_user_page subtracts 1024, refcount hits 0,
> the page is freed and returned to the buddy allocator. The remaining
> 1022 unpins write into whatever was reallocated, and the user's VMA
> still maps the freed page (NOMMU has no MMU to invalidate it).
> Reallocating the page for an io_uring pbuf_ring then lets userspace
> corrupt the new owner's data through the stale mapping.
>
> Use try_grab_folio() which adds GUP_PIN_COUNTING_BIAS for FOLL_PIN and 1
> for FOLL_GET, mirroring the CONFIG_MMU path so pin and unpin are
> symmetric.
>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: David Hildenbrand <david@kernel.org>
> Cc: Jason Gunthorpe <jgg@ziepe.ca>
> Cc: John Hubbard <jhubbard@nvidia.com>
> Cc: Peter Xu <peterx@redhat.com>
> Reported-by: Anthropic
> Assisted-by: gkh_clanker_t1000

Assisted-by: David :(

(no, I'm not a tool! :) )

> Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> ---
> v2: - drop huge comment
> - rework error return value based on David's suggestion (heck,
> pretty much the full patch was written by him now)
> Link to v1: https://lore.kernel.org/r/2026042334-acutely-unadorned-e05c@gregkh
>
> mm/gup.c | 13 ++++++++++---
> 1 file changed, 10 insertions(+), 3 deletions(-)
>
> diff --git a/mm/gup.c b/mm/gup.c
> index ad9ded39609c..2f6f95a167af 100644
> --- a/mm/gup.c
> +++ b/mm/gup.c
> @@ -1983,6 +1983,7 @@ static long __get_user_pages_locked(struct mm_struct *mm, unsigned long start,
> struct vm_area_struct *vma;
> bool must_unlock = false;
> vm_flags_t vm_flags;
> + int ret, err = -EFAULT;
> long i;
>
> if (!nr_pages)
> @@ -2019,8 +2020,14 @@ static long __get_user_pages_locked(struct mm_struct *mm, unsigned long start,
>
> if (pages) {
> pages[i] = virt_to_page((void *)start);
> - if (pages[i])
> - get_page(pages[i]);
> + if (!pages[i])
> + break;

Best to mention that change in the patch description. I really think this is the
right thing to do (returning NULL in the page array is just very dubious).

Acked-by: David Hildenbrand (Arm) <david@kernel.org>

--
Cheers,

David