[PATCH v2] md/raid5: fix possible null-pointer dereferences in raid5_store_group_thread_cnt()

[Tuo Li <islituo@gmail.com>]

The variable mddev->private is first assigned to conf and then checked:

   conf = mddev->private;
    if (!conf) ...

If conf is NULL, then mddev->private is also NULL. In this case,
null-pointer dereferences can occur when calling raid5_quiesce():

  raid5_quiesce(mddev, true);
  raid5_quiesce(mddev, false);

since mddev->private is assigned to conf again in raid5_quiesce(), and conf
is dereferenced in several places, for example:

  conf->quiesce = 0;
  wake_up(&conf->wait_for_quiescent);

To fix this issue, the function should unlock mddev and return before
invoking raid5_quiesce() when conf is NULL, following the existing pattern
in raid5_change_consistency_policy().

Fixes: fa1944bbe622 ("md/raid5: Wait sync io to finish before changing group cnt")
Signed-off-by: Tuo Li <islituo@gmail.com>
---
v2:
* Move the NULL check and early return ahead of the first call to
  raid5_quiesce().
  Thanks to Yu Kuai for helpful advice.
---
 drivers/md/raid5.c | 10 ++++++----
 1 file changed, 6 insertions(+), 4 deletions(-)

diff --git a/drivers/md/raid5.c b/drivers/md/raid5.c
index e57ce3295292..8dc98f545969 100644
--- a/drivers/md/raid5.c
+++ b/drivers/md/raid5.c
@@ -7187,12 +7187,14 @@ raid5_store_group_thread_cnt(struct mddev *mddev, const char *page, size_t len)
 	err = mddev_suspend_and_lock(mddev);
 	if (err)
 		return err;
+	conf = mddev->private;
+	if (!conf) {
+		mddev_unlock_and_resume(mddev);
+		return -ENODEV;
+	}
 	raid5_quiesce(mddev, true);
 
-	conf = mddev->private;
-	if (!conf)
-		err = -ENODEV;
-	else if (new != conf->worker_cnt_per_group) {
+	if (new != conf->worker_cnt_per_group) {
 		old_groups = conf->worker_groups;
 		if (old_groups)
 			flush_workqueue(raid5_wq);
-- 
2.43.0