[PATCH v3 1/4] userfaultfd: correctly prevent registering VM_DROPPABLE regions

[Tal Zussman <tz2294@columbia.edu>]

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@redhat.com/
Fixes: 9651fcedf7b9 ("mm: add MAP_DROPPABLE for designating always lazily freeable mappings")
Acked-by: David Hildenbrand <david@redhat.com>
Acked-by: Peter Xu <peterx@redhat.com>
Acked-by: Jason A. Donenfeld <Jason@zx2c4.com>
Signed-off-by: Tal Zussman <tz2294@columbia.edu>
---
 include/linux/userfaultfd_k.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/include/linux/userfaultfd_k.h b/include/linux/userfaultfd_k.h
index 75342022d144..f3b3d2c9dd5e 100644
--- a/include/linux/userfaultfd_k.h
+++ b/include/linux/userfaultfd_k.h
@@ -218,7 +218,7 @@ static inline bool vma_can_userfault(struct vm_area_struct *vma,
 {
 	vm_flags &= __VM_UFFD_FLAGS;
 
-	if (vm_flags & VM_DROPPABLE)
+	if (vma->vm_flags & VM_DROPPABLE)
 		return false;
 
 	if ((vm_flags & VM_UFFD_MINOR) &&

-- 
2.39.5