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SubjectRe: [RFC PATCH] binfmt_elf: Fix PIE execution with randomization disabled
On Thu, 29 Sep 2011 15:53:59 -0400
Josh Boyer <jwboyer@redhat.com> wrote:

> We've had a bug report[1] of some PIE programs getting a SIGKILL upon exec
> if you disable address randomization with:
>
> echo 0 > /proc/sys/kernel/randomize_va_space
>
> I tracked this down to get_unmapped_area_prot returning -ENOMEM because
> the address being passed in is larger than TASK_SIZE - len for the bss
> section of the test executable. That filters back to set_brk returning
> an error to load_elf_binary and the SIGKILL being sent around line 872
> of binfmt_elf.c.
>
> H.J. submitted an upstream bug report [2] as well, but got no feedback
> and we can't view it with kernel.org being down anyway. He came up with
> the patch below as well, which is what I'm sending on for comments. The
> changelog is my addition, so if that is wrong yell at me.
>
> I wanted to get some more eyes on this, because the current code sets
> load_bias to 0 unconditionally on CONFIG_X86 or CONFIG_ARM. I have no
> idea why that is. The original execshield patches had an #ifdef on
> __i386__ but the patch that was commited to add PIE support has the
> CONFIG_X86 setting.
>

It appears that Nicolas understood what's going on in there when he
wrote e4eab08d6050ad0 ("ARM: 6342/1: fix ASLR of PIE executables").
Alas, that patch's changelog is rather useless.

Help?

Also, please: review and test?

>
> [1] https://bugzilla.redhat.com/show_bug.cgi?id=708563
> [2] http://bugzilla.kernel.org/show_bug.cgi?id=36372
>
> josh
>
> ---
>
> From: H.J. Lu <hongjiu.lu@intel.com>
>
> Set the load_bias for PIE executables to a non-zero address if no virtual
> address is specified. This prevents us from running out of room for all
> the various loadable segments when ASLR is disabled.
>
> Signed-off-by: H.J. Lu <hongjiu.lu@intel.com>
> Signed-off-by: Josh Boyer <jwboyer@redhat.com>
>
> ---
>
> diff --git a/fs/binfmt_elf.c b/fs/binfmt_elf.c
> index 303983f..069ee29 100644
> --- a/fs/binfmt_elf.c
> +++ b/fs/binfmt_elf.c
> @@ -794,9 +794,14 @@ static int load_elf_binary(struct linux_binprm *bprm, struct pt_regs *regs)
> /* Try and get dynamic programs out of the way of the
> * default mmap base, as well as whatever program they
> * might try to exec. This is because the brk will
> - * follow the loader, and is not movable. */
> + * follow the loader, and is not movable. Don't use
> + * 0 load address since we may not have room for
> + * all loadable segements. */
> #if defined(CONFIG_X86) || defined(CONFIG_ARM)
> - load_bias = 0;
> + if (vaddr)
> + load_bias = 0;
> + else
> + load_bias = ELF_PAGESTART(ELF_ET_DYN_BASE);
> #else
> load_bias = ELF_PAGESTART(ELF_ET_DYN_BASE - vaddr);
> #endif


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