Possible off-by-one bug in usr.sbin/rad/engine.c

Hi Theo and Florian,

I've started auditing the OpenBSD source code after the discussion on
arc4random_uniform(3) and my suggestion of arc4random_range() on the glibc
mailing list.

I found some cases where it seems like there's an off-by-one bug, which would be
solved by providing arc4random_range(). I'll show here one, to confirm that
it's a bug, and if you confirm it, I'll continue fixing similar bugs around the
OpenBSD tree.

Here's the first one I found, which I hope is fixed by my patch:


diff --git a/usr.sbin/rad/engine.c b/usr.sbin/rad/engine.c
index ceb11d574e3..a61ea3835a6 100644
--- a/usr.sbin/rad/engine.c
+++ b/usr.sbin/rad/engine.c
@@ -641,8 +641,7 @@ iface_timeout(int fd, short events, void *arg)
struct imsg_send_ra send_ra;
struct timeval tv;

- tv.tv_sec = MIN_RTR_ADV_INTERVAL +
- arc4random_uniform(MAX_RTR_ADV_INTERVAL - MIN_RTR_ADV_INTERVAL);
+ tv.tv_sec = arc4random_range(MIN_RTR_ADV_INTERVAL, MAX_RTR_ADV_INTERVAL);
tv.tv_usec = arc4random_uniform(1000000);

log_debug("%s new timeout in %lld", __func__, tv.tv_sec);


If I'm correct, it should have been 'min + (max - min + 1)' instead of 'min +
(max - min)'. Please confirm.


Cheers,

Alex

--
<http://www.alejandro-colomar.es/>