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

On 2022-12-31 23:54 +01, Ingo Schwarze <schwarze@usta.de> wrote:
> Hi Alejandro,
>
> Alejandro Colomar wrote on Sat, Dec 31, 2022 at 05:56:27PM +0100:
>
>> 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);
>
> Currently, the code puts a number in the range [200, 600) in tv_sec
> and a random number of microseconds into tv_usec,
> i.e. the timeout will be greater than or equal to 200 seconds
> and strictly less than 600 seconds with a uniform distribution.
>
> Isn't that exactly what is intended?
>
>> 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.
>
> With your change, the timeout could go up to 600.999999, i.e. almost 601
> seconds. I don't know the protocol and can't say whether the change
> would matter, but naively, exceeding the MAX_ feels surprising to me.
>
> Really, this doesn't look like a bug to me...

Unfortunately the OP did not explain why they think this is a bug.

>
> Yours,
> Ingo

--
I'm not entirely sure you are real.