Re: [kernel-hardening] [PATCH 2/4] arm64: Reduce ELF_ET_DYN_BASE

[Ard Biesheuvel]

Hi Kees,

On 22 June 2017 at 18:06, Kees Cook <keescook@xxxxxxxxxxxx> wrote:
> Now that explicitly executed loaders are loaded in the mmap region,
> position PIE binaries lower in the address space to avoid possible
> collisions with mmap or stack regions. For 64-bit, align to 4GB to
> allow runtimes to use the entire 32-bit address space for 32-bit
> pointers.
>
> Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
> ---
>  arch/arm64/include/asm/elf.h | 13 ++++++-------
>  1 file changed, 6 insertions(+), 7 deletions(-)
>
> diff --git a/arch/arm64/include/asm/elf.h b/arch/arm64/include/asm/elf.h
> index 5d1700425efe..f742af8f7c42 100644
> --- a/arch/arm64/include/asm/elf.h
> +++ b/arch/arm64/include/asm/elf.h
> @@ -113,12 +113,13 @@
>  #define ELF_EXEC_PAGESIZE      PAGE_SIZE
>
>  /*
> - * This is the location that an ET_DYN program is loaded if exec'ed.  Typical
> - * use of this is to invoke "./ld.so someprog" to test out a new version of
> - * the loader.  We need to make sure that it is out of the way of the program
> - * that it will "exec", and that there is sufficient room for the brk.
> + * This is the base location for PIE (ET_DYN with INTERP) loads. On
> + * 64-bit, this is raised to 4GB to leave the entire 32-bit address
> + * space open for things that want to use the area for 32-bit pointers.
>   */
> -#define ELF_ET_DYN_BASE        (2 * TASK_SIZE_64 / 3)
> +#define ELF_ET_DYN_BASE                (test_thread_flag(TIF_32BIT) ?  \
> +                                       0x000400000UL :         \
> +                                       0x100000000UL)
>

Why are you merging this with the COMPAT definition?

>  #ifndef __ASSEMBLY__
>
> @@ -173,8 +174,6 @@ extern int arch_setup_additional_pages(struct linux_binprm *bprm,
>
>  #ifdef CONFIG_COMPAT
>
> -#define COMPAT_ELF_ET_DYN_BASE         (2 * TASK_SIZE_32 / 3)
> -
>  /* AArch32 registers. */
>  #define COMPAT_ELF_NGREG               18
>  typedef unsigned int                   compat_elf_greg_t;
> --
> 2.7.4
>