[kernel-hardening] Re: [PATCH v2 1/7] random: Simplify API for random address requests

[Kees Cook <keescook@chromium.org>]

On Sat, Jul 30, 2016 at 8:42 AM, Jason Cooper <jason@lakedaemon.net> wrote:
> To date, all callers of randomize_range() have set the length to 0, and
> check for a zero return value.  For the current callers, the only way
> to get zero returned is if end <= start.  Since they are all adding a
> constant to the start address, this is unnecessary.
>
> We can remove a bunch of needless checks by simplifying the API to do
> just what everyone wants, return an address between [start, start +
> range).
>
> While we're here, s/get_random_int/get_random_long/.  No current call
> site is adversely affected by get_random_int(), since all current range
> requests are < UINT_MAX.  However, we should match caller expectations
> to avoid coming up short (ha!) in the future.
>
> All current callers to randomize_range() chose to use the start address
> if randomize_range() failed.  Therefore, we simplify things by just
> returning the start address on error.
>
> randomize_range() will be removed once all callers have been converted
> over to randomize_addr().
>
> Signed-off-by: Jason Cooper <jason@lakedaemon.net>
> ---
> Changes from v1:
>  - Explicitly mention page_aligned start assumption (Yann Droneaud)
>  - pick random pages vice random addresses (Yann Droneaud)
>  - catch range=0 last
>
>  drivers/char/random.c  | 28 ++++++++++++++++++++++++++++
>  include/linux/random.h |  1 +
>  2 files changed, 29 insertions(+)
>
> diff --git a/drivers/char/random.c b/drivers/char/random.c
> index 0158d3bff7e5..3bedf69546d6 100644
> --- a/drivers/char/random.c
> +++ b/drivers/char/random.c
> @@ -1840,6 +1840,34 @@ randomize_range(unsigned long start, unsigned long end, unsigned long len)
>         return PAGE_ALIGN(get_random_int() % range + start);
>  }
>
> +/**
> + * randomize_addr - Generate a random, page aligned address
> + * @start:     The smallest acceptable address the caller will take.
> + * @range:     The size of the area, starting at @start, within which the
> + *             random address must fall.
> + *
> + * If @start + @range would overflow, @range is capped.
> + *
> + * NOTE: Historical use of randomize_range, which this replaces, presumed that
> + * @start was already page aligned.  This assumption still holds.
> + *
> + * Return: A page aligned address within [start, start + range).  On error,
> + * @start is returned.
> + */
> +unsigned long
> +randomize_addr(unsigned long start, unsigned long range)

Since we're changing other things about this, let's try to document
its behavior in its name too and call this "randomize_page" instead.
If it requires a page-aligned value, we should probably also BUG_ON
it, or adjust the start too.

-Kees

> +{
> +       if (start > ULONG_MAX - range)
> +               range = ULONG_MAX - start;
> +
> +       range >>= PAGE_SHIFT;
> +
> +       if (range == 0)
> +               return start;
> +
> +       return start + (get_random_long() % range << PAGE_SHIFT);
> +}
> +
>  /* Interface for in-kernel drivers of true hardware RNGs.
>   * Those devices may produce endless random bits and will be throttled
>   * when our pool is full.
> diff --git a/include/linux/random.h b/include/linux/random.h
> index e47e533742b5..f1ca2fa4c071 100644
> --- a/include/linux/random.h
> +++ b/include/linux/random.h
> @@ -35,6 +35,7 @@ extern const struct file_operations random_fops, urandom_fops;
>  unsigned int get_random_int(void);
>  unsigned long get_random_long(void);
>  unsigned long randomize_range(unsigned long start, unsigned long end, unsigned long len);
> +unsigned long randomize_addr(unsigned long start, unsigned long range);
>
>  u32 prandom_u32(void);
>  void prandom_bytes(void *buf, size_t nbytes);
> --
> 2.9.2
>



-- 
Kees Cook
Chrome OS & Brillo Security