linux-kernel - Re: brk() should check randomize_va_space rather then CONFIG_COMPAT

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Message-ID: <alpine.LNX.2.00.1102281530060.2160@pobox.suse.cz>
Date:	Mon, 28 Feb 2011 15:31:19 +0100 (CET)
From:	Jiri Kosina <jkosina@...e.cz>
To:	Frank Heckenbach <f.heckenbach@...soft.de>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: brk() should check randomize_va_space rather then
 CONFIG_COMPAT_BRK

On Mon, 28 Feb 2011, Jiri Kosina wrote:

> > The brk() syscall checks CONFIG_COMPAT_BRK to decide whether the brk
> > area starts just after the end of the code+bss.
> > 
> > Since CONFIG_COMPAT_BRK can be overridden by
> > /proc/sys/kernel/randomize_va_space, it should rather check this
> > variable (which is initialized in mm/memory.c from
> > CONFIG_COMPAT_BRK), see the patch below.
> > 
> > I've tested it with an old libc5 binary: Without the patch, if the
> > kernel was configured without CONFIG_COMPAT_BRK, the first malloc()
> > fails with any setting of randomize_va_space. With the patch
> > applied, the program runs correctly with randomize_va_space < 2, but
> > not with randomize_va_space == 2, as it should be.
> > 
> > --- linux-2.6.37.2/mm/mmap.c.orig	2011-02-25 00:09:00.000000000 +0100
> > +++ linux-2.6.37.2/mm/mmap.c	2011-02-26 01:13:53.000000000 +0100
> > @@ -252,11 +252,11 @@
> >  
> >  	down_write(&mm->mmap_sem);
> >  
> > -#ifdef CONFIG_COMPAT_BRK
> > -	min_brk = mm->end_code;
> > -#else
> > -	min_brk = mm->start_brk;
> > -#endif
> > +	if (randomize_va_space < 2)
> > +		min_brk = mm->end_code;
> > +	else
> > +		min_brk = mm->start_brk;
> > +
> >  	if (brk < min_brk)
> >  		goto out;
> 
> Hi Frank,
> 
> how do you avoid race here? 
> 
> More precisely -- randomize_va_space can be changed in runtime, when 
> already running processess have been started with different 
> randomize_va_space value (and thus the shifting of mm->brk already 
> happened in arch_randomize_brk())

Oh, and also see the commit

	commit 5520e89485252c759ee60d313e9422447659947b
	Author: Jiri Kosina <jkosina@...e.cz>
	Date:   Thu Jan 13 15:47:23 2011 -0800

	    brk: fix min_brk lower bound computation for COMPAT_BRK

which is quite relevant here as well (and you seem to be sending patch 
against code that doesn't have this patch applied).

-- 
Jiri Kosina
SUSE Labs, Novell Inc.
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