CVE-2025-38621

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

md: make rdev_addable usable for rcu mode

Our testcase trigger panic:

BUG: kernel NULL pointer dereference, address: 00000000000000e0

...

Oops: Oops: 0000 [#1] SMP NOPTI

CPU: 2 UID: 0 PID: 85 Comm: kworker/2:1 Not tainted 6.16.0+ #94

PREEMPT(none)

Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS

1.16.1-2.fc37 04/01/2014

Workqueue: md_misc md_start_sync

RIP: 0010:rdev_addable+0x4d/0xf0

...

Call Trace:

<TASK>

md_start_sync+0x329/0x480

process_one_work+0x226/0x6d0

worker_thread+0x19e/0x340

kthread+0x10f/0x250

ret_from_fork+0x14d/0x180

ret_from_fork_asm+0x1a/0x30

</TASK>

Modules linked in: raid10

CR2: 00000000000000e0

---[ end trace 0000000000000000 ]---

RIP: 0010:rdev_addable+0x4d/0xf0

md_spares_need_change in md_start_sync will call rdev_addable which

protected by rcu_read_lock/rcu_read_unlock. This rcu context will help

protect rdev won't be released, but rdev->mddev will be set to NULL

before we call synchronize_rcu in md_kick_rdev_from_array. Fix this by

using READ_ONCE and check does rdev->mddev still alive.