Sources that are not specified as a pool and have a name (i.e. not
specified by an IP address or added from chronyc) will be replaced with
a newly resolved address of the name when they become unreachable or
falseticker too.
In UTI_IsTimeOffsetSane() consider time in one year interval before
32-bit time_t overflow (in 2038) as invalid. Hopefully everything will
be using 64-bit time_t when that time comes.
Different systems may consider different time values to be valid.
Don't exit on settimeofday()/adjtimex() error in case the check in
UTI_IsTimeOffsetSane() isn't restrictive enough.
When allocating memory to save unacknowledged replies to authenticated
command requests, the last "next" pointer was not initialized to NULL.
When all allocated reply slots were used, the next reply could be
written to an invalid memory instead of allocating a new slot for it.
An attacker that has the command key and is allowed to access cmdmon
(only localhost is allowed by default) could exploit this to crash
chronyd or possibly execute arbitrary code with the privileges of the
chronyd process.
When NTP or cmdmon access was configured (from chrony.conf or via
authenticated cmdmon) with a subnet size that is indivisible by 4 and
an address that has nonzero bits in the 4-bit subnet remainder (e.g.
192.168.15.0/22 or f000::/3), the new setting was written to an
incorrect location, possibly outside the allocated array.
An attacker that has the command key and is allowed to access cmdmon
(only localhost is allowed by default) could exploit this to crash
chronyd or possibly execute arbitrary code with the privileges of the
chronyd process.
Time smoothing determines an offset that needs to be applied to the
cooked time to make it smooth for external observers. Observed offset
and frequency change slowly and there are no discontinuities. This can
be used on an NTP server to make it easier for the clients to track the
time and keep their clocks close together even when large offset or
frequency corrections are applied to the server's clock (e.g. after
being offline for longer time).
Accumulated offset and frequency are smoothed out in three stages. In
the first stage, the frequency is changed at a constant rate (wander) up
to a maximum, in the second stage the frequency stays at the maximum for
as long as needed and in the third stage the frequency is brought back
to zero.
Time smoothing is configured by the smoothtime directive. It takes two
arguments, maximum frequency offset and maximum wander. It's disabled by
default.
An attacker knowing that NTP hosts A and B are peering with each other
(symmetric association) can send a packet with random timestamps to host
A with source address of B which will set the NTP state variables on A
to the values sent by the attacker. Host A will then send on its next
poll to B a packet with originate timestamp that doesn't match the
transmit timestamp of B and the packet will be dropped. If the attacker
does this periodically for both hosts, they won't be able to synchronize
to each other. It is a denial-of-service attack.
According to [1], NTP authentication is supposed to protect symmetric
associations against this attack, but in the NTPv3 (RFC 1305) and NTPv4
(RFC 5905) specifications the state variables are updated before the
authentication check is performed, which means the association is
vulnerable to the attack even when authentication is enabled.
To fix this problem in chrony, save the originate and local timestamps
only when the authentication check (test5) passed.
[1] https://www.eecis.udel.edu/~mills/onwire.html
