From: Li Zefan <lizefan@huawei.com>
Date: Thu, 24 Jan 2013 06:43:28 +0000 (+0800)
Subject: cgroup: fix exit() vs rmdir() race
X-Git-Tag: v3.4.35~11
X-Git-Url: https://git.karo-electronics.de/?a=commitdiff_plain;h=916b6c67cd471fafa3e5e53ce8389c36749ed6f1;p=karo-tx-linux.git

cgroup: fix exit() vs rmdir() race

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@huawei.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---

diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index a5dccd4c2360..a4c47d1b3cec 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -378,12 +378,20 @@ static void __put_css_set(struct css_set *cg, int taskexit)
 		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);
 	}