Re: [PATCH v3] mm/gup: honour FOLL_PIN in NOMMU __get_user_pages_locked()

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

On Sat, Sep 19, 2026 at 12:04:36PM +0200, David Hildenbrand (arm) wrote:
> From: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> 
> 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. Keep supporting the traditional behavior where users specify
> a pages array but don't set FOLL_GET.
> 
> While at it, don't return NULL pointers in the page array,
> as this is really not expected for GUP users; instead, just fail and return
> -EFAULT.
> 
> [ david: support traditional behavior with no FOLL_GET, extend
>   description ]
> 
> 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
> Fixes: 3faa52c03f44 ("mm/gup: track FOLL_PIN pages")
> Assisted-by: gkh_clanker_t1000
> Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> Signed-off-by: David Hildenbrand (Arm) <david@kernel.org>
> ---
> I added a best-guess fixes tag. Given that this has been around for a
> while ... and it's nommu, I think this is fine just going into the next
> merge window. In that case, I will pick this up myself. If we want this
> as a hotfix, Andrew please pick it up.

Which ever you feel is fine, as it's nommu, obviously not many care
about it, including me :)

thanks,

greg k-h