linux-kernel - Re: [PATCH 04/17] cgroup: create directory before linking while creating a new cgroup

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <50A5D76D.70805@huawei.com>
Date:	Fri, 16 Nov 2012 14:04:29 +0800
From:	Li Zefan <lizefan@...wei.com>
To:	Tejun Heo <tj@...nel.org>
CC:	<containers@...ts.linux-foundation.org>, <cgroups@...r.kernel.org>,
	<linux-kernel@...r.kernel.org>, <mhocko@...e.cz>,
	<glommer@...allels.com>
Subject: Re: [PATCH 04/17] cgroup: create directory before linking while creating
 a new cgroup

On 2012/11/15 3:04, Tejun Heo wrote:
> Hello, Li.
> 
> On Wed, Nov 14, 2012 at 11:20:47AM +0800, Li Zefan wrote:
>>> It also removes the need to check whether cgroup->dentry is %NULL in
>>> cgroup_path.  If a cgroup is visible, its dentry is guaranteed to be
>>> visible too.
>>
>> I'm afraid this isn't safe.
>>
>> The cgroup is visible to a controller soon after ss->create(), and then
>> the controller might call cgroup_path() while cgrp->dentry is still NULL.
> 
> Hmmm... I can't find any case where ss->create() is calling
> cgroup_path().  Do you remember which one that was?
> 

I meant cgroup_create() may race with cgroup_path(). We used to have
this race:

(cpu0)                          (cpu1)
mkdir                           cat /proc/sched_debug

cgroup_create()
  cpu_cgroup_create()
    register_fair_sched_group()
      list_add_rcu(...)
				print_cfs_stats()
				  for_each_leaf_cfs_rq()
				    print_cfs_rq()
				      cgroup_path()
  css->cgroup = cgroup;
  cgroup->dentry = dentry;

In this case, cgroup_path() might access NULL cgroup or NULL dentry.

But I've looked the current scheduler code, no race anymore, because now the
cfs_rq won't be added to the gloal list until there's task in the cgroup.
(in the old time it was added to the list when creating a cgroup)

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/