md: fix an incorrect NULL check in does_sb_need_changing

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@vger.kernel.org
Fixes: 2aa82191ac36 ("md-cluster: Perform a lazy update")
Acked-by: Guoqing Jiang <guoqing.jiang@linux.dev>
Signed-off-by: Xiaomeng Tong <xiam0nd.tong@gmail.com>
Acked-by: Goldwyn Rodrigues <rgoldwyn@suse.com>
Signed-off-by: Song Liu <song@kernel.org>

diff --git a/drivers/md/md.c b/drivers/md/md.c
index 9294f13e0c9db30c780a397c0c4499529fa798a1..f07f007ecae43d116dbb58f9481c0bdcdc3326c4 100644 (file)
--- a/drivers/md/md.c
+++ b/drivers/md/md.c
@@ -2627,14 +2627,16 @@ 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)