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

[Andrew Morton]

On Sat, 07 Jun 2025 02:40:00 -0400 Tal Zussman <tz2294@xxxxxxxxxxxx> 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.

Wondering if we should backport this.  afaict we don't know the
userspace impact of this because nobody has tried it!