Description of problem:
When using the timemaster service from the linuxptp package, a shared memory segment is used to exchange measurements between chronyd and ptp4l or phc2sys. The memory segment can be created by any of the three processes. There is a race. Usually it's chronyd what creates the memory segment and that's when it works, but if it's phc2sys or ptp4l, chronyd is not able to get the segment due to a SELinux rule.

type=AVC msg=audit(1525261575.377:77): avc:  denied  { unix_read unix_write } for  pid=1342 comm="chronyd" key=1314148400  scontext=system_u:system_r:chronyd_t:s0 tcontext=system_u:system_r:phc2sys_t:s0 tclass=shm

A similar issue is with segments created by ptp4l (using SW timestamping) when switching to phc2sys (using HW timestamping).

type=AVC msg=audit(1525263148.431:75): avc:  denied  { unix_read unix_write } for  pid=1366 comm="phc2sys" key=1314148401  scontext=system_u:system_r:phc2sys_t:s0 tcontext=system_u:system_r:ptp4l_t:s0 tclass=shm

Version-Release number of selected component (if applicable):
selinux-policy-3.13.1-192.el7_5.3.noarch

How reproducible:
sometimes

Steps to Reproduce:
1. remove existing segments (e.g. with ipcrm -M 0x4e545030)
1. start timemaster configured to use at least one PTP source, e.g.
     [ptp_domain 0]
     interfaces eth0
2. check the system log

Actual results:
chronyd[840]: Fatal error : shmget() failed

Expected results:
chronyd running with no AVCs reported in the logs.

Additional info: