chrony <= 3.5.1 data corruption through symlink vulnerability writing the pidfile

Miroslav Lichvar reports:

chrony-3.5.1 […] fixes a security issue in writing of the pidfile.
When chronyd is configured to save the pidfile in a directory where the
chrony user has write permissions (e.g. /var/run/chrony - the default
since chrony-3.4), an attacker that compromised the chrony user account
could create a symbolic link at the location of the pidfile to make
chronyd starting with root privileges follow the symlink and write its
process ID to a file for which the chrony user doesn’t have write
permissions, causing a denial of service, or data loss.
This issue was reported by Matthias Gerstner of SUSE.