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

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Date:	Thu, 10 Dec 2009 15:44:28 +0100 (CET)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Oleg Nesterov <oleg@...hat.com>
cc:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	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 Thu, 10 Dec 2009, Oleg Nesterov wrote:
> 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.

Right, and that's what the problem is. commit_creds(), which rcu frees
the old creds, does not take tasklist lock write lock.

Thanks,

	tglx
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