linux-kernel - Re: [PATCH v3] 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.2.00.1110040051440.31654@pobox.suse.cz>
Date:	Tue, 4 Oct 2011 00:56:50 +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 v3] binfmt_elf: Fix PIE execution with randomization
 disabled (was Re: [RFC PATCH] binfmt_elf: Fix PIE execution with randomization
 disabled)

On Tue, 4 Oct 2011, Jiri Kosina wrote:

> > 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?

Andrew,

find the updated patch below. Please consider applying.

I believe Cc to stable is appropriate as well.

Thanks a lot Josh for verification of the fix.



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: Russell King <rmk@....linux.org.uk>
Cc: stable@...nel.org
Tested-by: Josh Boyer <jwboyer@...hat.com>
Acked-by: Nicolas Pitre <nicolas.pitre@...aro.org>
Signed-off-by: Jiri Kosina <jkosina@...e.cz>
---
 fs/binfmt_elf.c |   11 ++++++++++-
 1 files changed, 10 insertions(+), 1 deletions(-)

diff --git a/fs/binfmt_elf.c b/fs/binfmt_elf.c
index dd0fdfc..21ac5ee 100644
--- a/fs/binfmt_elf.c
+++ b/fs/binfmt_elf.c
@@ -795,7 +795,16 @@ 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;
+			/* Memory randomization might have been switched off
+			 * in runtime via sysctl.
+			 * If that is the case, retain the original non-zero
+			 * load_bias value in order to establish proper
+			 * non-randomized mappings.
+			 */
+			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

-- 
Jiri Kosina
SUSE Labs
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