md: don't dereference mddev after export_rdev()

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

md: don't dereference mddev after export_rdev()

Except for initial reference, mddev->kobject is referenced by

rdev->kobject, and if the last rdev is freed, there is no guarantee that

mddev is still valid. Hence mddev should not be used anymore after

export_rdev().

This problem can be triggered by following test for mdadm at very

low rate:

New file: mdadm/tests/23rdev-lifetime

devname=${dev0##*/}

devt=`cat /sys/block/$devname/dev`

pid=""

runtime=2

clean_up_test() {

pill -9 $pid

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

}

trap 'clean_up_test' EXIT

add_by_sysfs() {

while true; do

echo $devt > /sys/block/md0/md/new_dev

done

}

remove_by_sysfs(){

while true; do

echo remove > /sys/block/md0/md/dev-${devname}/state

done

}

echo md0 > /sys/module/md_mod/parameters/new_array || die "create md0 failed"

add_by_sysfs &

pid="$pid $!"

remove_by_sysfs &

pid="$pid $!"

sleep $runtime

exit 0

Test cmd:

./test --save-logs --logdir=/tmp/ --keep-going --dev=loop --tests=23rdev-lifetime

Test result:

general protection fault, probably for non-canonical address 0x6b6b6b6b6b6b6bcb: 0000 [#4] PREEMPT SMP

CPU: 0 PID: 1292 Comm: test Tainted: G D W 6.5.0-rc2-00121-g01e55c376936 #562

RIP: 0010:md_wakeup_thread+0x9e/0x320 [md_mod]

Call Trace:

<TASK>

mddev_unlock+0x1b6/0x310 [md_mod]

rdev_attr_store+0xec/0x190 [md_mod]

sysfs_kf_write+0x52/0x70

kernfs_fop_write_iter+0x19a/0x2a0

vfs_write+0x3b5/0x770

ksys_write+0x74/0x150

__x64_sys_write+0x22/0x30

do_syscall_64+0x40/0x90

entry_SYSCALL_64_after_hwframe+0x63/0xcd

Fix this problem by don't dereference mddev after export_rdev().