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

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Message-ID: <20091210142915.GB8226@redhat.com>
Date:	Thu, 10 Dec 2009 15:29:15 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	LKML <linux-kernel@...r.kernel.org>,
	Dipankar Sarma <dipankar@...ibm.com>,
	Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <peterz@...radead.org>,
	Al Viro <viro@...iv.linux.org.uk>,
	James Morris <jmorris@...ei.org>,
	David Howells <dhowells@...hat.com>,
	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

On 12/09, Paul E. McKenney wrote:
>
> On Thu, Dec 10, 2009 at 12:52:51AM -0000, Thomas Gleixner 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.
>
> OK, I will bite...  Don't the corresponding updates write-hold
> tasklist_lock?  If so, then the fact that the code below is read-holding
> tasklist_lock would prevent any of the data from changing, which would
> remove the need to do the rcu_read_lock().
>
> Or are there updates that are carried out without write-holding
> tasklist_lock that I am missing?

Yes, commit_creds() is called lockless.

Or I misunderstood the question/patch...

Oleg.

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