linux-kernel - Re: [PATCH -tip] cpuacct: Make cpuacct hierarchy walk in cpuacct

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: <49BF42FB.4030103@cn.fujitsu.com>
Date:	Tue, 17 Mar 2009 14:28:11 +0800
From:	Li Zefan <lizf@...fujitsu.com>
To:	bharata@...ux.vnet.ibm.com
CC:	linux-kernel@...r.kernel.org,
	Dhaval Giani <dhaval@...ux.vnet.ibm.com>,
	Balbir Singh <balbir@...ux.vnet.ibm.com>,
	Paul Menage <menage@...gle.com>, Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Subject: Re: [PATCH -tip] cpuacct: Make cpuacct hierarchy walk in	cpuacct_charge()
 safe when rcupreempt is used.

Bharata B Rao wrote:
> cpuacct: Make cpuacct hierarchy walk in cpuacct_charge() safe when
> 	rcupreempt is used.
> 
> cpuacct_charge() obtains task's ca and does a hierarchy walk upwards.
> This can race with the task's movement between cgroups. This race
> can cause an access to freed ca pointer in cpuacct_charge(). This will not

Actually it can also end up access invalid tsk->cgroups. ;)

get tsk->cgroups (cg)
                         (move tsk to another cgroup) or (tsk exiting)
                         -> kfree(tsk->cgroups)
get cg->subsys[..]

> happen with rcu or tree rcu as cpuacct_charge() is called with preemption
> disabled. However if rcupreempt is used, the race still exists. Thanks to
> Li Zefan for explaining this.
> 
> Fix this race by explicitly protecting ca and the hierarchy walk with
> rcu_read_lock().
> 
> Signed-off-by: Bharata B Rao <bharata@...ux.vnet.ibm.com>
> ---
>  kernel/sched.c |    8 ++++++++
>  1 file changed, 8 insertions(+)
> 
> --- a/kernel/sched.c
> +++ b/kernel/sched.c
> @@ -9891,6 +9891,13 @@ static void cpuacct_charge(struct task_s
>  		return;
>  
>  	cpu = task_cpu(tsk);
> +
> +	/*
> +	 * preemption is already disabled here, but to be safe with
> +	 * rcupreempt, take rcu_read_lock(). This protects ca and
> +	 * hence the hierarchy walk.
> +	 */
> +	rcu_read_lock();
>  	ca = task_ca(tsk);
>  
>  	do {
> @@ -9898,6 +9905,7 @@ static void cpuacct_charge(struct task_s
>  		*cpuusage += cputime;
>  		ca = ca->parent;
>  	} while (ca);
> +	rcu_read_unlock();
>  }
>  
>  struct cgroup_subsys cpuacct_subsys = {
> 
> 
--
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/