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

From: Andrew Morton
Date: Mon Oct 03 2011 - 18:03:17 EST


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

>
> From: Jiri Kosina <jkosina@xxxxxxx>
> 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@xxxxxxxxx> and
> Josh Boyer <jwboyer@xxxxxxxxxx>
>
> Cc: Ingo Molnar <mingo@xxxxxxx>
> Cc: Jiri Kosina <jkosina@xxxxxxx>
> Cc: Nicolas Pitre <nicolas.pitre@xxxxxxxxxx>
> Cc: Russell King <rmk@xxxxxxxxxxxxxxxx>
> Signed-off-by: Jiri Kosina <jkosina@xxxxxxx>
> ---
> 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?


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