md: fix array_state=clear sysfs deadlock

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

md: fix array_state=clear sysfs deadlock

When "clear" is written to array_state, md_attr_store() breaks sysfs

active protection so the array can delete itself from its own sysfs

store method.

However, md_attr_store() currently drops the mddev reference before

calling sysfs_unbreak_active_protection(). Once do_md_stop(..., 0)

has made the mddev eligible for delayed deletion, the temporary

kobject reference taken by sysfs_break_active_protection() can become

the last kobject reference protecting the md kobject.

That allows sysfs_unbreak_active_protection() to drop the last

kobject reference from the current sysfs writer context. kobject

teardown then recurses into kernfs removal while the current sysfs

node is still being unwound, and lockdep reports recursive locking on

kn->active with kernfs_drain() in the call chain.

Reproducer on an existing level:

1. Create an md0 linear array and activate it:

mknod /dev/md0 b 9 0

echo none > /sys/block/md0/md/metadata_version

echo linear > /sys/block/md0/md/level

echo 1 > /sys/block/md0/md/raid_disks

echo "$(cat /sys/class/block/sdb/dev)" > /sys/block/md0/md/new_dev

echo "$(($(cat /sys/class/block/sdb/size) / 2))" > \

/sys/block/md0/md/dev-sdb/size

echo 0 > /sys/block/md0/md/dev-sdb/slot

echo active > /sys/block/md0/md/array_state

2. Wait briefly for the array to settle, then clear it:

sleep 2

echo clear > /sys/block/md0/md/array_state

The warning looks like:

WARNING: possible recursive locking detected

bash/588 is trying to acquire lock:

(kn->active#65) at __kernfs_remove+0x157/0x1d0

but task is already holding lock:

(kn->active#65) at sysfs_unbreak_active_protection+0x1f/0x40

...

Call Trace:

kernfs_drain

__kernfs_remove

kernfs_remove_by_name_ns

sysfs_remove_group

sysfs_remove_groups

__kobject_del

kobject_put

md_attr_store

kernfs_fop_write_iter

vfs_write

ksys_write

Restore active protection before mddev_put() so the extra sysfs

kobject reference is dropped while the mddev is still held alive. The

actual md kobject deletion is then deferred until after the sysfs

write path has fully returned.