md: fix an incorrect NULL check in md_reload_sb

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: 70bcecdb1534 ("md-cluster: Improve md_reload_sb to be less error prone")
Signed-off-by: Xiaomeng Tong <xiam0nd.tong@gmail.com>
Signed-off-by: Song Liu <song@kernel.org>

diff --git a/drivers/md/md.c b/drivers/md/md.c
index f07f007..4e3c314 100644 (file)
--- a/drivers/md/md.c
+++ b/drivers/md/md.c
@@ -9795,16 +9795,18 @@ 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;
        }