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

[Andrew Morton <akpm00@gmail.com>]

On Mon, 3 Oct 2011 17:11:47 +0200 (CEST)
Jiri Kosina <jkosina@suse.cz> wrote:

> 
> From: Jiri Kosina <jkosina@suse.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@intel.com> and
> Josh Boyer <jwboyer@redhat.com>
> 
> Cc: Ingo Molnar <mingo@elte.hu>
> Cc: Jiri Kosina <jkosina@suse.cz>
> Cc: Nicolas Pitre <nicolas.pitre@linaro.org>
> Cc: Russell King <rmk@arm.linux.org.uk>
> Signed-off-by: Jiri Kosina <jkosina@suse.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.

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?