Re: [PATCH 20/46] mm/gup: fix clearing folios in memfd_pin_folios()

[David Hildenbrand (Arm)]

On 10/27/26 16:00, Pratyush Yadav wrote:
> On Tue, Oct 27 2026, David Hildenbrand (Arm) wrote:
> 
>> From: Chris Mason <mason@xxxxxxxxxx>
>>
>> memfd_pin_folios() pins folios obtained directly from the page cache
>> without checking whether they are uptodate. However, shmem fallocate()
>> can leave newly allocated folios uncleared and not uptodate. Callers
>> such as udmabuf can then expose uninitialized memory to userspace.
>>
>> Clear non-uptodate folios and mark them uptodate under the folio lock
>> before returning them to the caller.
>>
>> Fix generated by an LLM and sent by Chris Mason in private.
>>
>> Fixes: 89c1905d9c14 ("mm/gup: introduce memfd_pin_folios() for pinning memfd folios")
>> Cc: stable@xxxxxxxxxxxxxxx
>> Signed-off-by: Chris Mason <mason@xxxxxxxxxx>
>> [ david: simplify description and comment ]
>> Signed-off-by: David Hildenbrand (Arm) <david@xxxxxxxxxx>
>> ---
>>  mm/gup.c | 14 ++++++++++++++
>>  1 file changed, 14 insertions(+)
>>
>> diff --git a/mm/gup.c b/mm/gup.c
>> index efc9aa2654625..47e7ae152cdf1 100644
>> --- a/mm/gup.c
>> +++ b/mm/gup.c
>> @@ -3516,6 +3516,20 @@ long memfd_pin_folios(struct file *memfd, loff_t start, loff_t end,
>>  					goto err;
>>  				}
>>  
>> +				/*
>> +				 * fallocate() can leave folios in the
>> +				 * page cache that were never cleared.
>> +				 */
>> +				if (!folio_test_uptodate(folio)) {
>> +					folio_lock(folio);
>> +					if (!folio_test_uptodate(folio)) {
>> +						folio_zero_range(folio, 0, folio_size(folio));
>> +						flush_dcache_folio(folio);
>> +						folio_mark_uptodate(folio);
>> +					}
>> +					folio_unlock(folio);
>> +				}
>> +
> 
> Nit: this logic is repeated in a couple places in shmem.c and in
> memfd_luo.c. Perhaps it deserves a helper?
> 
> Anyway, sounds like an important fix so don't make my nitpick stop you
> from landing this if you don't want to bother.

Thanks! yeah, I had the same thought when trying to improve this. We should have
an easier way to just not hand out unzeroed folios outside of shmem core
functions in the first place ...

-- 
Cheers,

David