linux-kernel - Re: [PATCH v2] binfmt_elf: Fix PIE execution with randomization disabled (was Re: [RFC PATCH] binfmt_elf: Fix PIE execution with randomization disabled)

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Message-ID: <alpine.LNX.2.00.1110040004330.31654@pobox.suse.cz>
Date:	Tue, 4 Oct 2011 00:06:27 +0200 (CEST)
From:	Jiri Kosina <jkosina@...e.cz>
To:	Andrew Morton <akpm00@...il.com>
Cc:	Josh Boyer <jwboyer@...hat.com>,
	Nicolas Pitre <nicolas.pitre@...aro.org>,
	Ingo Molnar <mingo@...e.hu>, hongjiu.lu@...el.com,
	linux-kernel@...r.kernel.org, Russell King <rmk@....linux.org.uk>
Subject: Re: [PATCH v2] binfmt_elf: Fix PIE execution with randomization
 disabled (was Re: [RFC PATCH] binfmt_elf: Fix PIE execution with randomization
 disabled)

On Mon, 3 Oct 2011, Andrew Morton wrote:

> > From: Jiri Kosina <jkosina@...e.cz>
> > Subject: [PATCH] binfmt_elf: fix PIE execution with randomization disabled
> > 
> > The case of address space randomization being disabled in runtime through
> > randomize_va_space sysctl is not treated properly in load_elf_binary(),
> > resulting in SIGKILL coming at exec() time for certain PIE-linked binaries
> > in case the randomization has been disabled at runtime prior to calling
> > exec().
> > 
> > Handle the randomize_va_space == 0 case the same way as if we were not
> > supporting .text randomization at all.
> > 
> > Based on original patch by H.J. Lu <hongjiu.lu@...el.com> and
> > Josh Boyer <jwboyer@...hat.com>
> > 
> > Cc: Ingo Molnar <mingo@...e.hu>
> > Cc: Jiri Kosina <jkosina@...e.cz>
> > Cc: Nicolas Pitre <nicolas.pitre@...aro.org>
> > Cc: Russell King <rmk@....linux.org.uk>
> > Signed-off-by: Jiri Kosina <jkosina@...e.cz>
> > ---
> >  fs/binfmt_elf.c |    5 ++++-
> >  1 files changed, 4 insertions(+), 1 deletions(-)
> > 
> > diff --git a/fs/binfmt_elf.c b/fs/binfmt_elf.c
> > index dd0fdfc..bb11fe4 100644
> > --- a/fs/binfmt_elf.c
> > +++ b/fs/binfmt_elf.c
> > @@ -795,7 +795,10 @@ static int load_elf_binary(struct linux_binprm *bprm, struct pt_regs *regs)
> >  			 * might try to exec.  This is because the brk will
> >  			 * follow the loader, and is not movable.  */
> >  #if defined(CONFIG_X86) || defined(CONFIG_ARM)
> > -			load_bias = 0;
> > +			if (current->flags & PF_RANDOMIZE)
> > +				load_bias = 0;
> > +			else
> > +				load_bias = ELF_PAGESTART(ELF_ET_DYN_BASE - vaddr);
> >  #else
> >  			load_bias = ELF_PAGESTART(ELF_ET_DYN_BASE - vaddr);
> >  #endif
> 
> Guys, it took several people several days and 10+ emails to work out
> what's happening in there, and the first attempt to fix it was buggy. 
> This is all a huuuuge signal that the code is unobvious, hard to
> understand, hard to maintain.

It's unfortunately true. Rewriting the ELF binary loader is on my list of 
very-long-term TODOs. It has became quite a monster over decades.

> Please, let's get a good code comment in there while it's fresh in your 
> minds.  So the next person who comes along doesn't have the same amount 
> of difficulty?

I have to run now, I will resend the patch to you with an explanatory 
comment added in about 8 hours from now.

Thanks,

-- 
Jiri Kosina
SUSE Labs
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