Re: [PATCH v2 1/4] userfaultfd: correctly prevent registering VM_DROPPABLE regions

[Jason A. Donenfeld]

On Sat, Jun 07, 2025 at 02:40:00AM -0400, Tal Zussman wrote:
> vma_can_userfault() masks off non-userfaultfd VM flags from vm_flags.
> The vm_flags & VM_DROPPABLE test will then always be false, incorrectly
> allowing VM_DROPPABLE regions to be registered with userfaultfd.
> 
> Additionally, vm_flags is not guaranteed to correspond to the actual
> VMA's flags. Fix this test by checking the VMA's flags directly.
> 
> Link: https://lore.kernel.org/linux-mm/5a875a3a-2243-4eab-856f-bc53ccfec3ea@xxxxxxxxxx/
> Fixes: 9651fcedf7b9 ("mm: add MAP_DROPPABLE for designating always lazily freeable mappings")
> Acked-by: David Hildenbrand <david@xxxxxxxxxx>
> Acked-by: Peter Xu <peterx@xxxxxxxxxx>
> Signed-off-by: Tal Zussman <tz2294@xxxxxxxxxxxx>

Nice catch and thanks for fixing this.

Acked-by: Jason A. Donenfeld <Jason@xxxxxxxxx>