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

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Date:	Fri, 11 Dec 2009 14:52:10 +0100 (CET)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	David Howells <dhowells@...hat.com>
cc:	Oleg Nesterov <oleg@...hat.com>,
	"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>,
	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 Fri, 11 Dec 2009, David Howells wrote:

> Thomas Gleixner <tglx@...utronix.de> wrote:
> 
> > > > 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.
> 
> commit_creds() does not need to hold a write lock, because it is implicitly
> write-locked by only being permitted to run in the thread to which it is
> committing.
> 
> I don't think commit_creds() needs to take the RCU read lock as no-one else
> can alter/delete the creds it is dealing with.

commit_cred() is not required to take anything, but the reader side
needs to take rcu_read_lock() when accessing __task_cred() of another
task as that task could update its own creds right at that point.

The point is that read_lock(tasklist_lock) is not sufficient for the
reader side.

Thanks,

	tglx
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