Re: [PATCH 4/4] x86: mm: support ARCH_MMAP_RND_BITS.

[Kees Cook <keescook@chromium.org>]

On Wed, Nov 18, 2015 at 2:48 PM, Daniel Cashman <dcashman@android.com> wrote:
> From: dcashman <dcashman@google.com>
>
> x86: arch_mmap_rnd() uses hard-coded values, 8 for 32-bit and 28 for
> 64-bit, 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 default
> values as new minimums.
>
> Signed-off-by: Daniel Cashman <dcashman@google.com>
> ---
>  arch/x86/Kconfig   | 16 ++++++++++++++++
>  arch/x86/mm/mmap.c | 12 ++++++------
>  2 files changed, 22 insertions(+), 6 deletions(-)
>
> diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
> index db3622f..12768c4 100644
> --- a/arch/x86/Kconfig
> +++ b/arch/x86/Kconfig
> @@ -82,6 +82,8 @@ config X86
>         select HAVE_ARCH_KASAN                  if X86_64 && SPARSEMEM_VMEMMAP
>         select HAVE_ARCH_KGDB
>         select HAVE_ARCH_KMEMCHECK
> +       select HAVE_ARCH_MMAP_RND_BITS
> +       select HAVE_ARCH_MMAP_RND_COMPAT_BITS if COMPAT
>         select HAVE_ARCH_SECCOMP_FILTER
>         select HAVE_ARCH_SOFT_DIRTY             if X86_64
>         select HAVE_ARCH_TRACEHOOK
> @@ -183,6 +185,20 @@ config HAVE_LATENCYTOP_SUPPORT
>  config MMU
>         def_bool y
>
> +config ARCH_MMAP_RND_BITS_MIN
> +       default 28 if 64BIT
> +       default 8
> +
> +config ARCH_MMAP_RND_BITS_MAX
> +       default 32 if 64BIT
> +       default 16
> +
> +config ARCH_MMAP_RND_COMPAT_BITS_MIN
> +       default 8
> +
> +config ARCH_MMAP_RND_COMPAT_BITS_MAX
> +       default 16
> +
>  config SBUS
>         bool
>
> diff --git a/arch/x86/mm/mmap.c b/arch/x86/mm/mmap.c
> index 844b06d..647fecf 100644
> --- a/arch/x86/mm/mmap.c
> +++ b/arch/x86/mm/mmap.c
> @@ -69,14 +69,14 @@ unsigned long arch_mmap_rnd(void)
>  {
>         unsigned long rnd;
>
> -       /*
> -        *  8 bits of randomness in 32bit mmaps, 20 address space bits
> -        * 28 bits of randomness in 64bit mmaps, 40 address space bits
> -        */
>         if (mmap_is_ia32())
> -               rnd = (unsigned long)get_random_int() % (1<<8);
> +#ifdef CONFIG_COMPAT
> +               rnd = (unsigned long)get_random_int() % (1 << mmap_rnd_compat_bits);
> +#else
> +               rnd = (unsigned long)get_random_int() % (1 << mmap_rnd_bits);
> +#endif

Oh, that is a weird side-effect of x86's compat: a 32-bit build isn't
compat, so it needs the native bits. I'm trying to resist bikeshedding
the names again, so probably this should stay as-is.

-Kees

>         else
> -               rnd = (unsigned long)get_random_int() % (1<<28);
> +               rnd = (unsigned long)get_random_int() % (1 << mmap_rnd_bits);
>
>         return rnd << PAGE_SHIFT;
>  }
> --
> 2.6.0.rc2.230.g3dd15c0
>



-- 
Kees Cook
Chrome OS Security

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