Re: [PATCH v2 1/2] mm: prevent droppable mappings from being locked

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

On 3/12/26 03:16, anthony.yznaga@oracle.com wrote:
> 
> On 3/11/26 3:36 AM, Lorenzo Stoakes (Oracle) wrote:
>> On Tue, Mar 10, 2026 at 08:58:20AM -0700, Anthony Yznaga wrote:
>>> Droppable mappings must not be lockable. There is a check for VMAs with
>>> VM_DROPPABLE set in mlock_fixup() along with checks for other types of
>>> unlockable VMAs which ensures this when calling mlock()/mlock2().
>>>
>>> For mlockall(MCL_FUTURE), the check for unlockable VMAs is different.
>>> In apply_mlockall_flags(), if the flags parameter has MCL_FUTURE set,
>>> the
>>> current task's mm's default VMA flag field mm->def_flags has VM_LOCKED
>>> applied to it. VM_LOCKONFAULT is also applied if MCL_ONFAULT is also
>>> set.
>>> When these flags are set as default in this manner they are cleared in
>>> __mmap_complete() for new mappings that do not support mlock. A check
>>> for
>>> VM_DROPPABLE in __mmap_complete() is missing resulting in droppable
>>> mappings created with VM_LOCKED set. To fix this and reduce that
>>> chance of
>>> similar bugs in the future, introduce and use vma_supports_mlock().
>>>
>>> Fixes: 9651fcedf7b9 ("mm: add MAP_DROPPABLE for designating always
>>> lazily freeable mappings")
>> We should definitely cc: stable I think.
>>
>> It might result in some backport pain since it'll probably pre-date the
>> __mmap_region() stuff :)) sorry.
> 
> I could add a patch at the beginning that does the cheap fix followed by
> this patch.

Any backport conflicts should be easy and fast too resolve (if any). No
need to uglify our mainline patches :)

-- 
Cheers,

David