linux-kernel - Re: [patch 1/9] sys: Fix missing rcu protection for __task

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Date:	Thu, 10 Dec 2009 22:17:31 +0100 (CET)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
cc:	oleg@...hat.com, linux-kernel@...r.kernel.org,
	paulmck@...ux.vnet.ibm.com, linux-security-module@...r.kernel.org
Subject: Re: [patch 1/9] sys: Fix missing rcu protection for
 __task_cred()access

On Fri, 11 Dec 2009, Tetsuo Handa wrote:
> > Usually tasklist gives enough protection, but if copy_process() fails
> > it calls free_pid() lockless and does call_rcu(delayed_put_pid().
> > This means, without rcu lock find_pid_ns() can't scan the hash table
> > safely.
> 
> So, we need to change below comment from "or" to "and" ?

No, both functions must be called with rcu_read_lock()

tasklist_lock read-held is not protecting the rcu lists and does not
protect against a concurrent update. It merily protects against tasks
going away or being added while we look up the lists.
 
> 378 /*
> 379  * Must be called under rcu_read_lock() or with tasklist_lock read-held.
> 380  */
> 381 struct task_struct *find_task_by_pid_ns(pid_t nr, struct pid_namespace *ns)
> 382 {
> 383         return pid_task(find_pid_ns(nr, ns), PIDTYPE_PID);
> 384 }
> 385
> 386 struct task_struct *find_task_by_vpid(pid_t vnr)
> 387 {
> 388         return find_task_by_pid_ns(vnr, current->nsproxy->pid_ns);
> 389 }
> 

Thanks,

	tglx
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