[PATCH 5.10 384/452] md: fix an incorrect NULL check in does_sb_need_changing

[Greg Kroah-Hartman]

From: Xiaomeng Tong <xiam0nd.tong@xxxxxxxxx>

commit fc8738343eefc4ea8afb6122826dea48eacde514 upstream.

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>
Acked-by: Goldwyn Rodrigues <rgoldwyn@xxxxxxxx>
Signed-off-by: Song Liu <song@xxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
 drivers/md/md.c |    8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

--- a/drivers/md/md.c
+++ b/drivers/md/md.c
@@ -2648,14 +2648,16 @@ static void sync_sbs(struct mddev *mddev
 
 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)