[PATCH 2/2] mm/gup: Mark lock taken only after a successful retake

Peter Xu
2020-04-08T01:40:29Z

It's definitely incorrect to mark the lock as taken even if
down_read_killable() failed. It's overlooked when we switched from
down_read() to down_read_killable() because down_read() won't fail
while down_read_killable() could.

Reported-by: syzbot+a8c70b7f3579fc0587dc@syzkaller.appspotmail.com
Fixes: 71335f37c5e8 ("mm/gup: allow to react to fatal signals")
Signed-off-by: Peter Xu <peterx@redhat.com>
---
mm/gup.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/gup.c b/mm/gup.c
index da3e03185144..1f9a9b3a5869 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -1328,7 +1328,6 @@ static __always_inline long __get_user_pages_locked(struct task_struct *tsk,
if (fatal_signal_pending(current))
break;

- *locked = 1;
ret = down_read_killable(&mm->mmap_sem);
if (ret) {
BUG_ON(ret > 0);
@@ -1337,6 +1336,7 @@ static __always_inline long __get_user_pages_locked(struct task_struct *tsk,
break;
}

+ *locked = 1;
ret = __get_user_pages(tsk, mm, start, 1, flags | FOLL_TRIED,
pages, NULL, locked);
if (!*locked) {
--
2.24.1