md-cluster: fix NULL pointer dereference in process_metadata_update

Overview

In the Linux kernel, the following vulnerability has been resolved:

md-cluster: fix NULL pointer dereference in process_metadata_update

The function process_metadata_update() blindly dereferences the 'thread' pointer (acquired via rcu_dereference_protected) within the wait_event() macro.

While the code comment states "daemon thread must exist", there is a valid race condition window during the MD array startup sequence (md_run):

1. bitmap_load() is called, which invokes md_cluster_ops->join().
2. join() starts the "cluster_recv" thread (recv_daemon).
3. At this point, recv_daemon is active and processing messages.
4. However, mddev->thread (the main MD thread) is not initialized until later in md_run().

If a METADATA_UPDATED message is received from a remote node during this specific window, process_metadata_update() will be called while mddev->thread is still NULL, leading to a kernel panic.

To fix this, we must validate the 'thread' pointer. If it is NULL, we release the held lock (no_new_dev_lockres) and return early, safely ignoring the update request as the array is not yet fully ready to process it.

Affected

• Linux >= 0ba959774e93911caff596de6391f085fb640ac4 < b65ea893290056bee380d0c8eeb47797c2bcb311
• Linux >= 0ba959774e93911caff596de6391f085fb640ac4 < a61c1bc84c4a0f1e7c2fe55b0f43d7d94af4adf1
• Linux >= 0ba959774e93911caff596de6391f085fb640ac4 < dec123825c1ed74d98fd5fc7571a851dea4f46ff
• Linux >= 0ba959774e93911caff596de6391f085fb640ac4 < 721599e837d3f4c0e6cc14da059612c017b6d3ec
• Linux >= 0ba959774e93911caff596de6391f085fb640ac4 < dceb5a843910004cb118148e267036104fc3ee43
• Linux >= 0ba959774e93911caff596de6391f085fb640ac4 < f150e753cb8dd756085f46e86f2c35ce472e0a3c
• Linux 4.12

Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.