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 -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>