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

[Kees Cook]

On Sun, Jul 31, 2016 at 1:56 PM, Jason Cooper <jason@xxxxxxxxxxxxxx> wrote:
> On Sun, Jul 31, 2016 at 09:46:53AM -0700, Kees Cook wrote:
>> On Sat, Jul 30, 2016 at 8:42 AM, Jason Cooper <jason@xxxxxxxxxxxxxx> 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@xxxxxxxxxxxxxx>
>> > ---
>> > 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.
>
> Ack.  Definitely more accurate.
>
>> If it requires a page-aligned value, we should probably also BUG_ON
>> it, or adjust the start too.
>
> merf.  So, this whole series started from a suggested cleanup by William
> to s/get_random_int/get_random_long/.
>
> The current users have all been stable the way they are for a long time.
> Like pre-git long.  So, if this is just a cleanup for those callers, I
> don't think we need to do more than we already are.
>
> However, if the intent is for this function to see wider use, then by
> all means, we need to handle start != PAGE_ALIGN(start).
>
> Do you have any new call sites in mind?

I have no new call sites in mind, but it seems safe to add a BUG_ON to
verify we don't gain callers that don't follow the correct
expectations. (Or maybe WARN and return start.)

-Kees

-- 
Kees Cook
Chrome OS & Brillo Security