Re: [PATCH 0/2] Fix mmap_base entropy for >31 bits.

[Kees Cook]

On Thu, Feb 4, 2016 at 2:06 PM, Daniel Cashman <dcashman@xxxxxxxxxxx> wrote:
> Upstream commit: d07e22597d1d355829b7b18ac19afa912cf758d1 added the
> ability to choose from a range of values to use for entropy count in
> generating the random offset to the mmap_base address.  The
> maximum value on this range was set to 32 bits for 64-bit x86 systems,
> but this value could be increased further, requiring more than the 32
> bits of randomness provided by get_random_int(), as is already possible
> for arm64. Add a new function: get_random_long() which more naturally
> fits with the mmap usage of get_random_int() but operates exactly the
> same as get_random_int().
>
> Also, fix the shifting constant in mmap_rnd() to be an unsigned long so
> that values greater than 31 bits generate an appropriate mask without
> overflow.  This is especially important on x86, as its shift instruction
> uses a 5-bit mask for the shift operand, which meant that any value for
> mmap_rnd_bits over 31 acts as a no-op and effectively disables mmap_base
> randomization.
>
> Finally, replace calls to get_random_int() with get_random_long() where
> appropriate.
>
> Daniel Cashman (2):
>   drivers: char: random: Add get_random_long().
>   use get_random_long().
>
>  arch/arm/mm/mmap.c               |  2 +-
>  arch/arm64/mm/mmap.c             |  4 ++--
>  arch/mips/mm/mmap.c              |  4 ++--
>  arch/powerpc/kernel/process.c    |  4 ++--
>  arch/powerpc/mm/mmap.c           |  4 ++--
>  arch/sparc/kernel/sys_sparc_64.c |  2 +-
>  arch/x86/mm/mmap.c               |  6 +++---
>  drivers/char/random.c            | 22 ++++++++++++++++++++++
>  fs/binfmt_elf.c                  |  2 +-
>  include/linux/random.h           |  1 +
>  10 files changed, 37 insertions(+), 14 deletions(-)

Thanks for fixing this!

Acked-by: Kees Cook <keescook@xxxxxxxxxxxx>

-Kees

-- 
Kees Cook
Chrome OS & Brillo Security