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    Subject[patch 19/94] cgroups: fix a race between cgroup_clone and umount
    2.6.28-stable review patch.  If anyone has any objections, please let us know.

    ------------------

    From: Li Zefan <lizf@cn.fujitsu.com>

    commit 7b574b7b0124ed344911f5d581e9bc2d83bbeb19 upstream.

    The race is calling cgroup_clone() while umounting the ns cgroup subsys,
    and thus cgroup_clone() might access invalid cgroup_fs, or kill_sb() is
    called after cgroup_clone() created a new dir in it.

    The BUG I triggered is BUG_ON(root->number_of_cgroups != 1);

    ------------[ cut here ]------------
    kernel BUG at kernel/cgroup.c:1093!
    invalid opcode: 0000 [#1] SMP
    ...
    Process umount (pid: 5177, ti=e411e000 task=e40c4670 task.ti=e411e000)
    ...
    Call Trace:
    [<c0493df7>] ? deactivate_super+0x3f/0x51
    [<c04a3600>] ? mntput_no_expire+0xb3/0xdd
    [<c04a3ab2>] ? sys_umount+0x265/0x2ac
    [<c04a3b06>] ? sys_oldumount+0xd/0xf
    [<c0403911>] ? sysenter_do_call+0x12/0x31
    ...
    EIP: [<c0456e76>] cgroup_kill_sb+0x23/0xe0 SS:ESP 0068:e411ef2c
    ---[ end trace c766c1be3bf944ac ]---

    Cc: Serge E. Hallyn <serue@us.ibm.com>
    Signed-off-by: Li Zefan <lizf@cn.fujitsu.com>
    Cc: Paul Menage <menage@google.com>
    Cc: "Serge E. Hallyn" <serue@us.ibm.com>
    Cc: Balbir Singh <balbir@in.ibm.com>
    Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
    Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
    Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

    ---
    kernel/cgroup.c | 6 +++++-
    1 file changed, 5 insertions(+), 1 deletion(-)

    --- a/kernel/cgroup.c
    +++ b/kernel/cgroup.c
    @@ -2942,7 +2942,11 @@ int cgroup_clone(struct task_struct *tsk
    parent = task_cgroup(tsk, subsys->subsys_id);

    /* Pin the hierarchy */
    - atomic_inc(&parent->root->sb->s_active);
    + if (!atomic_inc_not_zero(&parent->root->sb->s_active)) {
    + /* We race with the final deactivate_super() */
    + mutex_unlock(&cgroup_mutex);
    + return 0;
    + }

    /* Keep the cgroup alive */
    get_css_set(cg);


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