[PATCH 4/4] mm/gup: Assert that the mmap lock is held in __get_user_pages()

[Jann Horn]

After having cleaned up all GUP callers (except for the atomisp staging
driver, which currently gets mmap locking completely wrong [1]) to always
ensure that they hold the mmap lock when calling into GUP (unless the mm is
not yet globally visible), add an assertion to make sure it stays that way
going forward.

[1] https://lore.kernel.org/lkml/CAG48ez3tZAb9JVhw4T5e-i=h2_DUZxfNRTDsagSRCVazNXx5qA@xxxxxxxxxxxxxx/

Signed-off-by: Jann Horn <jannh@xxxxxxxxxx>
---
 mm/gup.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/mm/gup.c b/mm/gup.c
index f11d39867cf5..3e5d843215b9 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -1020,6 +1020,8 @@ static long __get_user_pages(struct mm_struct *mm,
 	struct vm_area_struct *vma = NULL;
 	struct follow_page_context ctx = { NULL };

+	mmap_assert_locked(mm);
+
 	if (!nr_pages)
 		return 0;

-- 
2.28.0.709.gb0816b6eb0-goog