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

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Message-ID: <13109.1260538879@redhat.com>
Date:	Fri, 11 Dec 2009 13:41:19 +0000
From:	David Howells <dhowells@...hat.com>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	dhowells@...hat.com, LKML <linux-kernel@...r.kernel.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Dipankar Sarma <dipankar@...ibm.com>,
	Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <peterz@...radead.org>,
	Oleg Nesterov <oleg@...sign.ru>,
	Al Viro <viro@...iv.linux.org.uk>,
	James Morris <jmorris@...ei.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	linux-security-module@...r.kernel.org
Subject: Re: [patch 1/9] sys: Fix missing rcu protection for __task_cred() access

Thomas Gleixner <tglx@...utronix.de> wrote:

> commit c69e8d9 (CRED: Use RCU to access another task's creds and to
> release a task's own creds) added non rcu_read_lock() protected access
> to task creds of the target task in set_prio_one().
> 
> The comment above the function says:
>  * - the caller must hold the RCU read lock
> 
> The calling code in sys_setpriority does read_lock(&tasklist_lock) but
> not rcu_read_lock(). This works only when CONFIG_TREE_PREEMPT_RCU=n.
> With CONFIG_TREE_PREEMPT_RCU=y the rcu_callbacks can run in the tick
> interrupt when they see no read side critical section.
> 
> There is another instance of __task_cred() in sys_setpriority() itself
> which is equally unprotected.
> 
> Wrap the whole code section into a rcu read side critical section to
> fix this quick and dirty.
> 
> Will be revisited in course of the read_lock(&tasklist_lock) -> rcu
> crusade.
> 
> Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
> Cc: James Morris <jmorris@...ei.org>
> Cc: linux-security-module@...r.kernel.org

Acked-by: David Howells <dhowells@...hat.com>
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