Re: [RFC 2/2] kvm: eventfd: detect integer overflow using check_*_overflow

[Sasha Levin <sasha.levin@oracle.com>]

On 11/26/2014 02:12 PM, Linus Torvalds wrote:
>> kernel/time/ntp.c:process_adjtimex_modes():
>> >
>> >         if (txc->modes & ADJ_FREQUENCY) {
>> >                 time_freq = txc->freq * PPM_SCALE;   <=== Undefined overflow
>> >                 time_freq = min(time_freq, MAXFREQ_SCALED);
>> >                 time_freq = max(time_freq, -MAXFREQ_SCALED);
>> >                 /* update pps_freq */
>> >                 pps_set_freq(time_freq);
>> >         }
>> >
>> > The multiplication is between signed integers, and it overflows (user triggerable).
> Well, we check that the end result - overflowed or not - is in a sane
> range. So this might fall under the heading of "user gets what he asks
> for".

I guess, though it wouldn't be clear to the user why it's broken since he passed a
seemingly valid looking value for txc->freq.


Thanks,
Sasha