linux-kernel - Re: [PATCH] sys_getppid oopses on debug kernel

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Date:	Tue, 8 Aug 2006 17:34:35 +0200
From:	Björn Steinbrink <B.Steinbrink@....de>
To:	Dave Hansen <haveblue@...ibm.com>
Cc:	Kirill Korotaev <dev@...ru>, Andrew Morton <akpm@...l.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] sys_getppid oopses on debug kernel

On 2006.08.08 08:26:57 -0700, Dave Hansen wrote:
> On Tue, 2006-08-08 at 14:22 +0400, Kirill Korotaev wrote:
> > sys_getppid() optimization can access a freed memory.
> > On kernels with DEBUG_SLAB turned ON, this results in
> > Oops.
> ...
> > +#else
> > +	/*
> > +	 * ->real_parent could be released before dereference and
> > +	 * we accessed freed kernel memory, which faults with debugging on.
> > +	 * Keep it simple and stupid.
> > +	 */
> > +	read_lock(&tasklist_lock);
> > +	pid = current->group_leader->real_parent->tgid;
> > +	read_unlock(&tasklist_lock);
> > +#endif
> >  	return pid;
> >  }
> 
> Accessing freed memory is a bug, always, not just *only* when slab
> debugging is on, right?  Doesn't this mean we could get junk, or that
> the reader could potentially run off a bad pointer?
> 
> It seems that this patch only papers over the problem in the case when
> it is observed, but doesn't really even fix the normal case.
> 
> Could we use a seqlock to determine when real_parent is in flux, and
> re-read real_parent until we get a consistent one?  We could use in in
> lieu of the existing for() loop.

See this thread: http://lkml.org/lkml/2006/8/8/215

Björn
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