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Author :
cheloha
Date : 2020-07-09 02:17:07
Hash : 69657d9a
Message : adjfreq(2): limit adjustment to [-500000, +500000] ppm When we recompute the scaling factor during tc_windup() there is an opportunity for arithmetic overflow if the active timecounter's adjfreq(2) adjustment is too large. If we limit the adjustment to [-500000, +500000] ppm the statement in question cannot overflow. In particular, we are concerned with the following bit of code: scale = (u_int64_t)1 << 63; scale += \ ((th->th_adjustment + th->th_counter->tc_freq_adj) / 1024) * 2199; scale /= th->th_counter->tc_frequency; th->th_scale = scale * 2; where scale is an int64_t. Overflow when we do: scale += (...) / 1024 * 2199; as th->th_counter->tc_freq_adj is currently unbounded. th->th_adjustment is limited to [-5000ppm, 5000ppm]. To see that overflow is prevented with the new bounds, consider the new edge case where th->th_counter->tc_freq_adj is 500000ppm and th->th_adjustment is 5000ppm. Both are of type int64_t. We have: int64_t th_adjustment = (5000 * 1000) << 32; /* 21474836480000000 */ int64_t tc_freq_adj = 500000000LL << 32; /* 2147483648000000000 */ scale = (u_int64_t)1 << 63; /* 9223372036854775808 */ scale += (th_adjustment + tc_freq_adj) / 1024 * 2199; /* scale += 2168958484480000000 / 1024 * 2199; */ /* scale += 4657753620480000000; */ 9223372036854775808 + 4657753620480000000 = 13881125657334775808, which less than 18446744073709551616, so we don't have overflow. On the opposite end, if th->th_counter->tc_freq_adj is -500000ppm and th->th_adjustment is -5000ppm we would have -4657753620480000000. 9223372036854775808 - 4657753620480000000 = 4565618416374775808. Again, no overflow. 500000ppm and -500000ppm are extreme adjustments. otto@ says ntpd(8) would never arrive at them naturally, so we are not at risk of breaking a working setup by imposing these restrictions. Documentation input from kettenis@. No complaints from otto@.
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