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

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Date:   Tue,  7 Apr 2020 21:40:10 -0400
From:   Peter Xu <peterx@...hat.com>
To:     linux-kernel@...r.kernel.org,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        linux-mm@...ck.org
Cc:     Andrew Morton <akpm@...ux-foundation.org>, peterx@...hat.com,
        syzbot+a8c70b7f3579fc0587dc@...kaller.appspotmail.com
Subject: [PATCH 2/2] mm/gup: Mark lock taken only after a successful retake

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@...kaller.appspotmail.com
Fixes: 71335f37c5e8 ("mm/gup: allow to react to fatal signals")
Signed-off-by: Peter Xu <peterx@...hat.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