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

[Kees Cook <keescook@chromium.org>]

On Mon, Jul 25, 2016 at 8:30 PM, Jason Cooper <jason@lakedaemon.net> wrote:
> All,
>
> On Tue, Jul 26, 2016 at 03:01:55AM +0000, Jason Cooper 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 < 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);
>>  }
>
> bah!  old patch file.  This should have been:
>
> if (range == 0)
>         return start;
> else
>         return PAGE_ALIGN(get_random_long() % range + start);

I think range should be limited to start + range < UINTMAX, and it
should be very clear if the range is inclusive or exclusive.  start =
0, range = 4096. does this mean 1 page, or 2 pages possible?

-Kees

>
> sorry,
>
> Jason.
>
>>
>>  /* 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);
>> --
>> 2.9.2
>>



-- 
Kees Cook
Chrome OS & Brillo Security