the NULL deref on umount in the 3.9.0-rc7 kernel

FYI

-------- Original Message --------
Subject: the NULL deref on umount in the 3.9.0-rc7 kernel
Date: Thu, 18 Apr 2013 15:37:36 +0400
From: [email protected]
Organization: Oracle Corporation
To: Tejun Heo <[email protected]>, Li Zefan <[email protected]>
CC: Dan Carpenter <[email protected]>, [email protected], containers-cunTk1MwBs9QetFLy7KEm3xJsTq8ys+cHZ5vskTnxNA@public.gmane.org

Hi All

I would like to report the NULL deref on umount. Tested it in linux kernel 3.7.10 and it's still in the 3.9.0-rc7.

Test-case description:
Mount cgroup filesystem with xattr option and create inside root cgroup another hierarchy.
Then set extended attribute to any files within root hierarchy or sub hierarchie.
Then remove (rmdir) sub hierarchy and call umount cgroup filesystem. Afterthat, umount crash the kernel.

Also, if you don't remove sub hierarchy (steps 1.4& 2.9 in examples below), calling umount will produce nothing except that cgroup filesystem will be unmounted (no cgroup files in the directory) but with
error: cgroups continue working, while call mount again to get control access to running cgroups will produce error, such as filesystem is already mounted, but in /proc/mounts you don't have such mount point. And there is no way to get control access back to the running cgroups, except for reboot.

Here are some manual methods which will reproduce Linux crash.

1. One way to reproduce this fault:

1.1% mount -t cgroup cgroot_test -o xattr /sys/fs/cgroup

1.2% mkdir /sys/fs/cgroup/test_subsys

1.3% setfattr -n trusted.value -v test_value /sys/fs/cgroup/tasks

1.4% rmdir /sys/fs/cgroup/test_subsys

1.5% umount cgroot_test

2. Another way:

2.1% mount -t tmpfs cgroup_root /sys/fs/cgroup

2.2% mkdir /sys/fs/cgroup/rg1

2.3% mount -t cgroup -o cpuset,xattr hier1 /sys/fs/cgroup/rg1

2.4% cd /sys/fs/cgroup/rg1

2.5% mkdir test_subsys

2.6% setfattr -n trusted.value -v test_value ./tasks

2.7% setfattr -n trusted.value -v test_value ./test_subsys

2.8% setfattr -n trusted.value -v test_value ./test_subsys/tasks

2.9% rmdir test_subsys

2.10% cd ../

2.11% umount hier1

Thanks,
Alexey Kodanev