md: fix an incorrect NULL check in md_reload_sb

stable inclusion
from stable-4.19.247
commit 3a18aa6faa4c3b9b84d56188ac94099ba22a4f11
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I5FNPY
CVE: NA

--------------------------------

commit 64c54d92 upstream.

The bug is here:
	if (!rdev || rdev->desc_nr != nr) {

The list iterator value 'rdev' will *always* be set and non-NULL
by rdev_for_each_rcu(), so it is incorrect to assume that the
iterator value will be NULL if the list is empty or no element
found (In fact, it will be a bogus pointer to an invalid struct
object containing the HEAD). Otherwise it will bypass the 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 'pdev' as a dedicated pointer to
point to the found element.

Cc: stable@vger.kernel.org
Fixes: 70bcecdb ("md-cluster: Improve md_reload_sb to be less error prone")
Signed-off-by: NXiaomeng Tong <xiam0nd.tong@gmail.com>
Signed-off-by: NSong Liu <song@kernel.org>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: NYongqiang Liu <liuyongqiang13@huawei.com>
Signed-off-by: NLaibin Qiu <qiulaibin@huawei.com>

drivers/md/md.c

@@ -9402,16 +9402,18 @@ static int read_rdev(struct mddev *mddev, struct md_rdev *rdev)
 
 void md_reload_sb(struct mddev *mddev, int nr)
 {
-	struct md_rdev *rdev;
+	struct md_rdev *rdev = NULL, *iter;
 	int err;
 
 	/* Find the rdev */
-	rdev_for_each_rcu(rdev, mddev) {
-		if (rdev->desc_nr == nr)
+	rdev_for_each_rcu(iter, mddev) {
+		if (iter->desc_nr == nr) {
+			rdev = iter;
 			break;
+		}
 	}
 
-	if (!rdev || rdev->desc_nr != nr) {
+	if (!rdev) {
 		pr_warn("%s: %d Could not find rdev with nr %d\n", __func__, __LINE__, nr);
 		return;
 	}