userfaultfd: correctly prevent registering VM_DROPPABLE regions

userfaultfd: correctly prevent registering VM_DROPPABLE regions

Patch series "mm: userfaultfd: assorted fixes and cleanups", v3.

Two fixes and two cleanups for userfaultfd.

Note that the third patch yields a small change in the ABI, but we seem to
have concluded that it is acceptable in this case.

This patch (of 4):

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://lkml.kernel.org/r/20250619-uffd-fixes-v3-0-a7274d3bd5e4@columbia.edu
Link: https://lore.kernel.org/linux-mm/5a875a3a-2243-4eab-856f-bc53ccfec3ea@redhat.com/
Link: https://lkml.kernel.org/r/20250619-uffd-fixes-v3-1-a7274d3bd5e4@columbia.edu
Fixes: 9651fcedf7b9 ("mm: add MAP_DROPPABLE for designating always lazily freeable mappings")
Signed-off-by: Tal Zussman <tz2294@columbia.edu>
Acked-by: David Hildenbrand <david@redhat.com>
Acked-by: Peter Xu <peterx@redhat.com>
Acked-by: Jason A. Donenfeld <Jason@zx2c4.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Christian Brauner <brauner@kernel.org>
Cc: Jan Kara <jack@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>