md/bitmap: factor out a helper to set timeout

Register/unregister 'mddev->thread' are both under 'reconfig_mutex',
however, some context didn't hold the mutex to access mddev->thread,
which can cause null-ptr-deference:

1) md_bitmap_daemon_work() can be called from md_check_recovery() where
'reconfig_mutex' is not held, deference 'mddev->thread' might cause
null-ptr-deference, because md_unregister_thread() reset the pointer
before stopping the thread.

2) timeout_store() access 'mddev->thread' multiple times,
null-ptr-deference can be triggered if 'mddev->thread' is reset in the
middle.

This patch factor out a helper to set timeout, the new helper always
check if 'mddev->thread' is null first, so that problem 1 can be fixed.

Now that this helper only access 'mddev->thread' once, but it's possible
that 'mddev->thread' can be freed while this helper is still in progress,
hence the problem is not fixed yet. Follow up patches will fix this by
protecting md_thread with rcu.

Signed-off-by: Yu Kuai <yukuai3@huawei.com>
Signed-off-by: Song Liu <song@kernel.org>
Link: https://lore.kernel.org/r/20230523021017.3048783-5-yukuai1@huaweicloud.com

diff --git a/drivers/md/md-bitmap.c b/drivers/md/md-bitmap.c
index 4b5ba81c53be9c1f7f7baef1c366cc62d9a3ff89..23522df41ca5f1a08c9588df226788c0649383ed 100644 (file)
--- a/drivers/md/md-bitmap.c
+++ b/drivers/md/md-bitmap.c
@@ -1234,11 +1234,22 @@ static bitmap_counter_t *md_bitmap_get_counter(struct bitmap_counts *bitmap,
                                               sector_t offset, sector_t *blocks,
                                               int create);
 
+static void mddev_set_timeout(struct mddev *mddev, unsigned long timeout,
+                             bool force)
+{
+       struct md_thread *thread = mddev->thread;
+
+       if (!thread)
+               return;
+
+       if (force || thread->timeout < MAX_SCHEDULE_TIMEOUT)
+               thread->timeout = timeout;
+}
+
 /*
  * bitmap daemon -- periodically wakes up to clean bits and flush pages
  *                     out to disk
  */
-
 void md_bitmap_daemon_work(struct mddev *mddev)
 {
        struct bitmap *bitmap;
@@ -1262,7 +1273,7 @@ void md_bitmap_daemon_work(struct mddev *mddev)
 
        bitmap->daemon_lastrun = jiffies;
        if (bitmap->allclean) {
-               mddev->thread->timeout = MAX_SCHEDULE_TIMEOUT;
+               mddev_set_timeout(mddev, MAX_SCHEDULE_TIMEOUT, true);
                goto done;
        }
        bitmap->allclean = 1;
@@ -1359,8 +1370,7 @@ void md_bitmap_daemon_work(struct mddev *mddev)
 
  done:
        if (bitmap->allclean == 0)
-               mddev->thread->timeout =
-                       mddev->bitmap_info.daemon_sleep;
+               mddev_set_timeout(mddev, mddev->bitmap_info.daemon_sleep, true);
        mutex_unlock(&mddev->bitmap_info.mutex);
 }
 
@@ -1821,8 +1831,7 @@ void md_bitmap_destroy(struct mddev *mddev)
        mddev->bitmap = NULL; /* disconnect from the md device */
        spin_unlock(&mddev->lock);
        mutex_unlock(&mddev->bitmap_info.mutex);
-       if (mddev->thread)
-               mddev->thread->timeout = MAX_SCHEDULE_TIMEOUT;
+       mddev_set_timeout(mddev, MAX_SCHEDULE_TIMEOUT, true);
 
        md_bitmap_free(bitmap);
 }
@@ -1965,7 +1974,7 @@ int md_bitmap_load(struct mddev *mddev)
        /* Kick recovery in case any bits were set */
        set_bit(MD_RECOVERY_NEEDED, &bitmap->mddev->recovery);
 
-       mddev->thread->timeout = mddev->bitmap_info.daemon_sleep;
+       mddev_set_timeout(mddev, mddev->bitmap_info.daemon_sleep, true);
        md_wakeup_thread(mddev->thread);
 
        md_bitmap_update_sb(bitmap);
@@ -2470,17 +2479,11 @@ timeout_store(struct mddev *mddev, const char *buf, size_t len)
                timeout = MAX_SCHEDULE_TIMEOUT-1;
        if (timeout < 1)
                timeout = 1;
-       mddev->bitmap_info.daemon_sleep = timeout;
-       if (mddev->thread) {
-               /* if thread->timeout is MAX_SCHEDULE_TIMEOUT, then
-                * the bitmap is all clean and we don't need to
-                * adjust the timeout right now
-                */
-               if (mddev->thread->timeout < MAX_SCHEDULE_TIMEOUT)
-                       mddev->thread->timeout = timeout;
-       }
 
+       mddev->bitmap_info.daemon_sleep = timeout;
+       mddev_set_timeout(mddev, timeout, false);
        md_wakeup_thread(mddev->thread);
+
        return len;
 }