[PATCH v3] md: fix an incorrect NULL check in does_sb_need_changing

[Xiaomeng Tong]

The bug is here:
	if (!rdev)

The list iterator value 'rdev' will *always* be set and non-NULL
by rdev_for_each(), so it is incorrect to assume that the iterator
value will be NULL if the list is empty or no element found.
Otherwise it will bypass the NULL check and lead to invalid memory
access passing the check.

To fix the bug, use a new variable 'iter' as the list iterator,
while using the original variable 'rdev' as a dedicated pointer to
point to the found element.

Cc: stable@xxxxxxxxxxxxxxx
Fixes: 2aa82191ac36 ("md-cluster: Perform a lazy update")
Acked-by: Guoqing Jiang <guoqing.jiang@xxxxxxxxx>
Signed-off-by: Xiaomeng Tong <xiam0nd.tong@xxxxxxxxx>
---

changes since v2:
 - fix typo (Song Liu)

changes since v1:
 - rephrase the subject (Guoqing Jiang)
 - add Acked-by: for Guoqing Jiang

v2:https://lore.kernel.org/lkml/20220328081127.26148-1-xiam0nd.tong@xxxxxxxxx/
v1:https://lore.kernel.org/lkml/20220327080002.11923-1-xiam0nd.tong@xxxxxxxxx/

---
 drivers/md/md.c | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/drivers/md/md.c b/drivers/md/md.c
index 4d38bd7dadd6..7476fc204172 100644
--- a/drivers/md/md.c
+++ b/drivers/md/md.c
@@ -2629,14 +2629,16 @@ static void sync_sbs(struct mddev *mddev, int nospares)
 
 static bool does_sb_need_changing(struct mddev *mddev)
 {
-	struct md_rdev *rdev;
+	struct md_rdev *rdev = NULL, *iter;
 	struct mdp_superblock_1 *sb;
 	int role;
 
 	/* Find a good rdev */
-	rdev_for_each(rdev, mddev)
-		if ((rdev->raid_disk >= 0) && !test_bit(Faulty, &rdev->flags))
+	rdev_for_each(iter, mddev)
+		if ((iter->raid_disk >= 0) && !test_bit(Faulty, &iter->flags)) {
+			rdev = iter;
 			break;
+		}
 
 	/* No good device found. */
 	if (!rdev)
-- 
2.17.1