Some security updates in RHEL 7.4

RHEL 7.4 has been out for a little while now. And with Centos build 1708 recently released, there are a couple new security features that I would like to take a moment to highlight.

yama ptrace control
The first item is a new sysctl setting, kernel.yama.ptrace_scope. This is used to control ptracing of processes. If you allow ptracing of processes, this also allows processes or in memory data to be altered. This can be used to do something referred to as process hollowing or process injection. This means the process starts up but gets modified so that it doesn't do what its supposed to do.

To prevent this form of attack, we can use  kernel.yama.ptrace_scope to set who can ptrace. The different values have the following meaning:

# 0 - Default attach security permissions.
# 1 - Restricted attach. Only child processes plus normal permissions.
# 2 - Admin-only attach. Only executables with CAP_SYS_PTRACE.
# 3 - No attach. No process may call ptrace at all. Irrevocable until next boot.

You can temporarily set it like this:
echo 2 > /proc/sys/kernel/yama/ptrace_scope

To permanently set it, edit /etc/sysctl.conf. If you have a system in a DMZ or with sensitive information, I'd recommend a value of 3. I'd go with a 2 for production machines and a 1 for everyone else. Also while you are in there you might want to see if you are setting any of these other security related sysctls:

kernel.kptr_restrict = 1
kernel.dmesg_restrict = 1
kernel.perf_event_paranoid = 2
kernel.kexec_load_disabled = 1


jitter entropy source
RHEL 7.4 also picked up the jitter entropy source. This entropy source mines the natural jitter that exists in the execution of CPU instructions. This helps Linux a whole lot because typically Linux kernel are starved for entropy. There
is one catch ... some people in the upstream community think that jitter from the CPU leans towards being deterministic. So, they do not want to automatically stir it into the entropy pool. This means that you must run rngd to get the benefit of this new entropy source. Also, note that rngd only moves entropy from hardware generators to the kernel entropy pool. It in no way creates entropy.


audit events as text
If you read my blog then you know that there have been improvements for being able to understand what the events mean. You can take the ugly and nearly unreadable audit events and have them turned into english sentences. To do
this you just pass --format=text to the ausearch command.


proctitle added to audit events
The new RHEL 7.4 kernel now includes a proctitle record. The proctitle record gives the command line and its arguments for any syscall filter originating event. This is useful to see how commands were invoked in case the arguments are important to an investigation.