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[3.5.y.z,extended,stable] Patch "cgroup: fix exit() vs rmdir() race" has been added to staging queue

Message ID 1361895280-29051-1-git-send-email-luis.henriques@canonical.com
State New
Headers show

Commit Message

Luis Henriques Feb. 26, 2013, 4:14 p.m. UTC 
This is a note to let you know that I have just added a patch titled

    cgroup: fix exit() vs rmdir() race

to the linux-3.5.y-queue branch of the 3.5.y.z extended stable tree 
which can be found at:

 http://kernel.ubuntu.com/git?p=ubuntu/linux.git;a=shortlog;h=refs/heads/linux-3.5.y-queue

If you, or anyone else, feels it should not be added to this tree, please 
reply to this email.

For more information about the 3.5.y.z tree, see
https://wiki.ubuntu.com/Kernel/Dev/ExtendedStable

Thanks.
-Luis

------

From a12ae690eb3bd2c414a294bedac7c4f66f7b348d Mon Sep 17 00:00:00 2001
From: Li Zefan <lizefan@huawei.com>
Date: Thu, 24 Jan 2013 14:43:28 +0800
Subject: [PATCH] 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: Luis Henriques <luis.henriques@canonical.com>
---
 kernel/cgroup.c | 8 ++++++++
 1 file changed, 8 insertions(+)

--
1.8.1.2
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Patch

diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 963c956..87fbb45 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -422,12 +422,20 @@  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);
 	}