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

["David Hildenbrand (Arm)" <david@kernel.org>]

On 9/21/26 04:49, Lance Yang wrote:
> 
> 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.
>> ---
>> mm/gup.c | 17 ++++++++++++++---
>> 1 file changed, 14 insertions(+), 3 deletions(-)
>>
>> diff --git a/mm/gup.c b/mm/gup.c
>> index eb898ea1ee22e..1681bea8eed7e 100644
>> --- a/mm/gup.c
>> +++ b/mm/gup.c
>> @@ -1659,6 +1659,10 @@ static __always_inline long __get_user_pages_locked(struct mm_struct *mm,
>> 	if (!nr_pages)
>> 		return 0;
>>
>> +	/* See the MMU variant: support the traditional behavior. */
>> +	if (pages && !(flags & FOLL_PIN))
>> +		flags |= FOLL_GET;
>> +
> 
> Wait ... isn't this the wrong __get_user_pages_locked()? The MMU version
> already does this a few lines below ...
> 

Yes, Sashiko complained as well. Will fix it up and send a v2. Lesson learned:
don't work on nommu patches on Saturday ;)

> Shouldn't the NOMMU version get the check instead, using foll_flags?

Yes, I'll fix that up and send a v2.

-- 
Cheers,

David