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

[Jason Cooper <jason@lakedaemon.net>]

Hi Kees,

On Mon, Aug 01, 2016 at 12:47:59PM -0700, Kees Cook wrote:
> On Sun, Jul 31, 2016 at 1:56 PM, Jason Cooper <jason@lakedaemon.net> 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@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.
> >
> > 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.)

No, I think BUG_ON is appropriate.  afaict, the only time this will be
encountered is during the development process.

thx,

Jason.