[PATCH 1/1] mm: fix a lockdep issue in vma_assert_write_locked

[Suren Baghdasaryan <surenb@google.com>]

__is_vma_write_locked() can be used only when mmap_lock is write-locked
to guarantee vm_lock_seq and mm_lock_seq stability during the check.
Therefore it asserts this condition before further checks. Because of
that it can't be used unless the user expects the mmap_lock to be
write-locked. vma_assert_locked() can't assume this before ensuring
that VMA is not read-locked.
Change the order of the checks in vma_assert_locked() to check if the
VMA is read-locked first and only then assert if it's not write-locked.

Fixes: 50b88b63e3e4 ("mm: handle userfaults under VMA lock")
Reported-by: Liam R. Howlett <liam.howlett@oracle.com>
Closes: https://lore.kernel.org/all/20230712022620.3yytbdh24b7i4zrn@revolver/
Reported-by: syzbot+339b02f826caafd5f7a8@syzkaller.appspotmail.com
Closes: https://lore.kernel.org/all/0000000000002db68f05ffb791bc@google.com/
Signed-off-by: Suren Baghdasaryan <surenb@google.com>
---
 include/linux/mm.h | 16 ++++++----------
 1 file changed, 6 insertions(+), 10 deletions(-)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index 9687b48dfb1b..e3b022a66343 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -668,6 +668,7 @@ static inline void vma_end_read(struct vm_area_struct *vma)
 	rcu_read_unlock();
 }
 
+/* WARNING! Can only be used if mmap_lock is expected to be write-locked */
 static bool __is_vma_write_locked(struct vm_area_struct *vma, int *mm_lock_seq)
 {
 	mmap_assert_write_locked(vma->vm_mm);
@@ -707,22 +708,17 @@ static inline bool vma_try_start_write(struct vm_area_struct *vma)
 	return true;
 }
 
-static inline void vma_assert_locked(struct vm_area_struct *vma)
+static inline void vma_assert_write_locked(struct vm_area_struct *vma)
 {
 	int mm_lock_seq;
 
-	if (__is_vma_write_locked(vma, &mm_lock_seq))
-		return;
-
-	lockdep_assert_held(&vma->vm_lock->lock);
-	VM_BUG_ON_VMA(!rwsem_is_locked(&vma->vm_lock->lock), vma);
+	VM_BUG_ON_VMA(!__is_vma_write_locked(vma, &mm_lock_seq), vma);
 }
 
-static inline void vma_assert_write_locked(struct vm_area_struct *vma)
+static inline void vma_assert_locked(struct vm_area_struct *vma)
 {
-	int mm_lock_seq;
-
-	VM_BUG_ON_VMA(!__is_vma_write_locked(vma, &mm_lock_seq), vma);
+	if (!rwsem_is_locked(&vma->vm_lock->lock))
+		vma_assert_write_locked(vma);
 }
 
 static inline void vma_mark_detached(struct vm_area_struct *vma, bool detached)
-- 
2.41.0.455.g037347b96a-goog