Re: [kernel-hardening] [RFC patch 1/6] random: Simplify API for random address requests

Yann Droneaud
2016-07-27T14:21:16Z

Hi,

Le mardi 26 juillet 2016 à 03:01 +0000, Jason Cooper a écrit :
> 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.
>

I agree.

> 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].
>

I agree.

For the record:

http://lkml.kernel.org/r/cover.1390770607.git.ydroneaud@opteya.com

> 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 < MAX_UINT.  However, we should match caller
> expectations to avoid coming up short (ha!) in the future.
>
> Signed-off-by: Jason Cooper <jason@lakedaemon.net>
> ---
>  drivers/char/random.c  | 17 ++++-------------
>  include/linux/random.h |  2 +-
>  2 files changed, 5 insertions(+), 14 deletions(-)
>
> diff --git a/drivers/char/random.c b/drivers/char/random.c
> index 0158d3bff7e5..1251cb2cbab2 100644
> --- a/drivers/char/random.c
> +++ b/drivers/char/random.c
> @@ -1822,22 +1822,13 @@ unsigned long get_random_long(void)
>  EXPORT_SYMBOL(get_random_long);
>
>  /*
> - * randomize_range() returns a start address such that
> - *
> - *    [...... <range> .....]
> - *  start                  end
> - *
> - * a <range> with size "len" starting at the return value is inside
> in the
> - * area defined by [start, end], but is otherwise randomized.
> + * randomize_addr() returns a page aligned address within [start,
> start +
> + * range]
>   */
>  unsigned long
> -randomize_range(unsigned long start, unsigned long end, unsigned
> long len)
> +randomize_addr(unsigned long start, unsigned long range)
>  {
> - unsigned long range = end - len - start;
> -
> - if (end <= start + len)
> - return 0;
> - return PAGE_ALIGN(get_random_int() % range + start);
> + return PAGE_ALIGN(get_random_long() % range + start);
>  }
>
>  /* Interface for in-kernel drivers of true hardware RNGs.
> diff --git a/include/linux/random.h b/include/linux/random.h
> index e47e533742b5..1ad877a98186 100644
> --- a/include/linux/random.h
> +++ b/include/linux/random.h
> @@ -34,7 +34,7 @@ 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);