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

[Jason Cooper <jason@lakedaemon.net>]

Hi Kees,

On Mon, Jul 25, 2016 at 09:39:58PM -0700, Kees Cook wrote:
> On Mon, Jul 25, 2016 at 8:30 PM, Jason Cooper <jason@lakedaemon.net> wrote:
> > 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

merf. UINT_MAX.

> >> 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,

ULONG_MAX?  I agree.

if (range == 0 || ULONG_MAX - range < start)
	return start;
else
	return PAGE_ALIGN(get_random_long() % range + start);

?

> and it should be very clear if the range is inclusive or exclusive.

Sorry, I was reading the original comment, '[start, end]'  with square
brackets denoting inclusive.

Regardless, the math in randomize_range() was just undoing the math at
each of the call sites.  This proposed change to randomize_addr()
doesn't alter the current state of affairs wrt inclusive, exclusive.

> start = 0, range = 4096. does this mean 1 page, or 2 pages possible?

ooh, good spot.  What we have right now is [start, start + range), which
is matching previous behavior.  But does not match the old comment,
[start, end].  It should have been [start, end).

So, you're correct, I need to clarify this in the comments.

thx,

Jason.