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

From: Peter Xu <peterx@redhat.com>
To: linux-kernel@vger.kernel.org,
	Linus Torvalds <torvalds@linux-foundation.org>,
	linux-mm@kvack.org
Cc: Andrew Morton <akpm@linux-foundation.org>,
	peterx@redhat.com,
	syzbot+a8c70b7f3579fc0587dc@syzkaller.appspotmail.com
Subject: [PATCH 2/2] mm/gup: Mark lock taken only after a successful retake
Date: Tue,  7 Apr 2020 21:40:10 -0400	[thread overview]
Message-ID: <20200408014010.80428-3-peterx@redhat.com> (raw)
In-Reply-To: <20200408014010.80428-1-peterx@redhat.com>

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


