Patch "cgroup: fix exit() vs rmdir() race" has been added to the 3.4-stable tree

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This is a note to let you know that I've just added the patch titled

    cgroup: fix exit() vs rmdir() race

to the 3.4-stable tree which can be found at:
    http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
     cgroup-fix-exit-vs-rmdir-race.patch
and it can be found in the queue-3.4 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.


>From 71b5707e119653039e6e95213f00479668c79b75 Mon Sep 17 00:00:00 2001
From: Li Zefan <lizefan@xxxxxxxxxx>
Date: Thu, 24 Jan 2013 14:43:28 +0800
Subject: cgroup: fix exit() vs rmdir() race

From: Li Zefan <lizefan@xxxxxxxxxx>

commit 71b5707e119653039e6e95213f00479668c79b75 upstream.

In cgroup_exit() put_css_set_taskexit() is called without any lock,
which might lead to accessing a freed cgroup:

thread1                           thread2
---------------------------------------------
exit()
  cgroup_exit()
    put_css_set_taskexit()
      atomic_dec(cgrp->count);
                                   rmdir();
      /* not safe !! */
      check_for_release(cgrp);

rcu_read_lock() can be used to make sure the cgroup is alive.

Signed-off-by: Li Zefan <lizefan@xxxxxxxxxx>
Signed-off-by: Tejun Heo <tj@xxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

---
 kernel/cgroup.c |    8 ++++++++
 1 file changed, 8 insertions(+)

--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -378,12 +378,20 @@ static void __put_css_set(struct css_set
 		struct cgroup *cgrp = link->cgrp;
 		list_del(&link->cg_link_list);
 		list_del(&link->cgrp_link_list);
+
+		/*
+		 * We may not be holding cgroup_mutex, and if cgrp->count is
+		 * dropped to 0 the cgroup can be destroyed at any time, hence
+		 * rcu_read_lock is used to keep it alive.
+		 */
+		rcu_read_lock();
 		if (atomic_dec_and_test(&cgrp->count) &&
 		    notify_on_release(cgrp)) {
 			if (taskexit)
 				set_bit(CGRP_RELEASABLE, &cgrp->flags);
 			check_for_release(cgrp);
 		}
+		rcu_read_unlock();
 
 		kfree(link);
 	}


Patches currently in stable-queue which might be from lizefan@xxxxxxxxxx are

queue-3.4/cpuset-fix-cpuset_print_task_mems_allowed-vs-rename-race.patch
queue-3.4/cgroup-fix-exit-vs-rmdir-race.patch
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