mm: lock vma explicitly before doing vm_flags_reset and vm_flags_reset_once
Implicit vma locking inside vm_flags_reset() and vm_flags_reset_once() is
not obvious and makes it hard to understand where vma locking is happening.
Also in some cases (like in dup_userfaultfd()) vma should be locked earlier
than vma_flags modification. To make locking more visible, change these
functions to assert that the vma write lock is taken and explicitly lock
the vma beforehand. Fix userfaultfd functions which should lock the vma
earlier.
Link: https://lkml.kernel.org/r/20230804152724.3090321-5-surenb@google.com
Suggested-by: Linus Torvalds <torvalds@linuxfoundation.org>
Signed-off-by: Suren Baghdasaryan <surenb@google.com>
Cc: Jann Horn <jannh@google.com>
Cc: Liam R. Howlett <Liam.Howlett@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
diff --git a/mm/mlock.c b/mm/mlock.c
index 0a0c996..1746600 100644
--- a/mm/mlock.c
+++ b/mm/mlock.c
@@ -386,6 +386,7 @@ static void mlock_vma_pages_range(struct vm_area_struct *vma,
*/
if (newflags & VM_LOCKED)
newflags |= VM_IO;
+ vma_start_write(vma);
vm_flags_reset_once(vma, newflags);
lru_add_drain();
@@ -460,9 +461,9 @@ static int mlock_fixup(struct vma_iterator *vmi, struct vm_area_struct *vma,
* It's okay if try_to_unmap_one unmaps a page just after we
* set VM_LOCKED, populate_vma_page_range will bring it back.
*/
-
if ((newflags & VM_LOCKED) && (oldflags & VM_LOCKED)) {
/* No work to do, and mlocking twice would be wrong */
+ vma_start_write(vma);
vm_flags_reset(vma, newflags);
} else {
mlock_vma_pages_range(vma, start, end, newflags);
