the NULL deref on umount in the 3.9.0-rc7 kernel

[alexey.kodanev-QHcLZuEGTsvQT0dZR+AlfA@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.
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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

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