md: fix array_state=clear sysfs deadlock

Summary

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.