[PATCH 1/2] powerpc: mm: support ARCH_MMAP_RND_BITS

[Kees Cook]

On Wed, Feb 1, 2017 at 9:42 PM, Bhupesh Sharma <bhsharma at redhat.com> wrote:
> powerpc: arch_mmap_rnd() uses hard-coded values, (23-PAGE_SHIFT) for
> 32-bit and (30-PAGE_SHIFT) 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.
>
> This patch makes sure that now powerpc mmap arch_mmap_rnd() approach
> is similar to other ARCHs like x86, arm64 and arm.
>
> Cc: Alexander Graf <agraf at suse.com>
> Cc: Benjamin Herrenschmidt <benh at kernel.crashing.org>
> Cc: Paul Mackerras <paulus at samba.org>
> Cc: Michael Ellerman <mpe at ellerman.id.au>
> Cc: Anatolij Gustschin <agust at denx.de>
> Cc: Alistair Popple <alistair at popple.id.au>
> Cc: Matt Porter <mporter at kernel.crashing.org>
> Cc: Vitaly Bordug <vitb at kernel.crashing.org>
> Cc: Scott Wood <oss at buserror.net>
> Cc: Kumar Gala <galak at kernel.crashing.org>
> Cc: Daniel Cashman <dcashman at android.com>
> Cc: Kees Cook <keescook at chromium.org>
> Signed-off-by: Bhupesh Sharma <bhsharma at redhat.com>
> ---
>  arch/powerpc/Kconfig   | 34 ++++++++++++++++++++++++++++++++++
>  arch/powerpc/mm/mmap.c |  7 ++++---
>  2 files changed, 38 insertions(+), 3 deletions(-)
>
> diff --git a/arch/powerpc/Kconfig b/arch/powerpc/Kconfig
> index a8ee573fe610..b4a843f68705 100644
> --- a/arch/powerpc/Kconfig
> +++ b/arch/powerpc/Kconfig
> @@ -22,6 +22,38 @@ config MMU
>         bool
>         default y
>
> +config ARCH_MMAP_RND_BITS_MIN
> +       default 5 if PPC_256K_PAGES && 32BIT
> +       default 12 if PPC_256K_PAGES && 64BIT
> +       default 7 if PPC_64K_PAGES && 32BIT
> +       default 14 if PPC_64K_PAGES && 64BIT
> +       default 9 if PPC_16K_PAGES && 32BIT
> +       default 16 if PPC_16K_PAGES && 64BIT
> +       default 11 if PPC_4K_PAGES && 32BIT
> +       default 18 if PPC_4K_PAGES && 64BIT
> +
> +# max bits determined by the following formula:
> +#  VA_BITS - PAGE_SHIFT - 4
> +#  for e.g for 64K page and 64BIT = 48 - 16 - 4 = 28
> +config ARCH_MMAP_RND_BITS_MAX
> +       default 10 if PPC_256K_PAGES && 32BIT
> +       default 26 if PPC_256K_PAGES && 64BIT
> +       default 12 if PPC_64K_PAGES && 32BIT
> +       default 28 if PPC_64K_PAGES && 64BIT
> +       default 14 if PPC_16K_PAGES && 32BIT
> +       default 30 if PPC_16K_PAGES && 64BIT
> +       default 16 if PPC_4K_PAGES && 32BIT
> +       default 32 if PPC_4K_PAGES && 64BIT
> +
> +config ARCH_MMAP_RND_COMPAT_BITS_MIN
> +       default 5 if PPC_256K_PAGES
> +       default 7 if PPC_64K_PAGES
> +       default 9 if PPC_16K_PAGES
> +       default 11
> +
> +config ARCH_MMAP_RND_COMPAT_BITS_MAX
> +       default 16
> +
>  config HAVE_SETUP_PER_CPU_AREA
>         def_bool PPC64
>
> @@ -100,6 +132,8 @@ config PPC
>         select HAVE_EFFICIENT_UNALIGNED_ACCESS if !(CPU_LITTLE_ENDIAN && POWER7_CPU)
>         select HAVE_KPROBES
>         select HAVE_ARCH_KGDB
> +       select HAVE_ARCH_MMAP_RND_BITS
> +       select HAVE_ARCH_MMAP_RND_COMPAT_BITS if COMPAT
>         select HAVE_KRETPROBES
>         select HAVE_ARCH_TRACEHOOK
>         select HAVE_MEMBLOCK
> diff --git a/arch/powerpc/mm/mmap.c b/arch/powerpc/mm/mmap.c
> index 2f1e44362198..babf59faab3b 100644
> --- a/arch/powerpc/mm/mmap.c
> +++ b/arch/powerpc/mm/mmap.c
> @@ -60,11 +60,12 @@ unsigned long arch_mmap_rnd(void)
>  {
>         unsigned long rnd;
>
> -       /* 8MB for 32bit, 1GB for 64bit */
> +#ifdef CONFIG_COMPAT
>         if (is_32bit_task())
> -               rnd = get_random_long() % (1<<(23-PAGE_SHIFT));
> +               rnd = get_random_long() & ((1UL << mmap_rnd_compat_bits) - 1);
>         else
> -               rnd = get_random_long() % (1UL<<(30-PAGE_SHIFT));
> +#endif
> +               rnd = get_random_long() & ((1UL << mmap_rnd_bits) - 1);
>
>         return rnd << PAGE_SHIFT;
>  }

Awesome! This looks good to me based on my earlier analysis.

Reviewed-by: Kees Cook <keescook at chromium.org>

-Kees

-- 
Kees Cook
Pixel Security