Re: [PATCH v2 2/2] arm: mm: support ARCH_MMAP_RND_BITS.

[Kees Cook <keescook@chromium.org>]

On Tue, Nov 3, 2015 at 10:10 AM, Daniel Cashman <dcashman@android.com> wrote:
> From: dcashman <dcashman@google.com>
>
> arm: arch_mmap_rnd() uses a hard-code value of 8 to generate the
> random offset for the mmap base address.  This value represents a
> compromise between increased ASLR effectiveness and avoiding
> address-space fragmentation. Replace it with a Kconfig option, which
> is sensibly bounded, so that platform developers may choose where to
> place this compromise. Keep 8 as the minimum acceptable value.
>
> Signed-off-by: Daniel Cashman <dcashman@google.com>

Acked-by: Kees Cook <keescook@chromium.org>

Russell, if you don't see any problems here, it might make sense not
to put this through the ARM patch tracker since it depends on the 1/2,
and I think x86 and arm64 (and possibly other arch) changes are coming
too.

> ---
> Changes in v2:
>   - Changed arch/arm/Kconfig and arch/arm/mm/mmap.c to reflect changes
>   in [PATCH v2 1/2], specifically the movement of variables to global
>   rather than arch-specific files.
>
>  arch/arm/Kconfig   | 10 ++++++++++
>  arch/arm/mm/mmap.c |  3 +--
>  2 files changed, 11 insertions(+), 2 deletions(-)
>
> diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig
> index 639411f..47d7561 100644
> --- a/arch/arm/Kconfig
> +++ b/arch/arm/Kconfig
> @@ -35,6 +35,7 @@ config ARM
>         select HAVE_ARCH_BITREVERSE if (CPU_32v7M || CPU_32v7) && !CPU_32v6
>         select HAVE_ARCH_JUMP_LABEL if !XIP_KERNEL && !CPU_ENDIAN_BE32
>         select HAVE_ARCH_KGDB if !CPU_ENDIAN_BE32
> +       select HAVE_ARCH_MMAP_RND_BITS
>         select HAVE_ARCH_SECCOMP_FILTER if (AEABI && !OABI_COMPAT)
>         select HAVE_ARCH_TRACEHOOK
>         select HAVE_BPF_JIT
> @@ -306,6 +307,15 @@ config MMU
>           Select if you want MMU-based virtualised addressing space
>           support by paged memory management. If unsure, say 'Y'.
>
> +config ARCH_MMAP_RND_BITS_MIN
> +       default 8
> +
> +config ARCH_MMAP_RND_BITS_MAX
> +       default 14 if MMU && PAGE_OFFSET=0x40000000
> +       default 15 if MMU && PAGE_OFFSET=0x80000000
> +       default 16 if MMU
> +       default 8
> +
>  #
>  # The "ARM system type" choice list is ordered alphabetically by option
>  # text.  Please add new entries in the option alphabetic order.
> diff --git a/arch/arm/mm/mmap.c b/arch/arm/mm/mmap.c
> index 407dc78..c938693 100644
> --- a/arch/arm/mm/mmap.c
> +++ b/arch/arm/mm/mmap.c
> @@ -173,8 +173,7 @@ unsigned long arch_mmap_rnd(void)
>  {
>         unsigned long rnd;
>
> -       /* 8 bits of randomness in 20 address space bits */
> -       rnd = (unsigned long)get_random_int() % (1 << 8);
> +       rnd = (unsigned long)get_random_int() % (1 << mmap_rnd_bits);
>
>         return rnd << PAGE_SHIFT;
>  }

I like this getting pulled closer and closer to having arch_mmap_rnd()
be identical across all architectures, and then we can just pull it
out and leave the true variable: the entropy size.

Do you have patches for x86 and arm64?

-Kees

> --
> 2.6.0.rc2.230.g3dd15c0
>



-- 
Kees Cook
Chrome OS Security