Cannot disable kernel's audit system via auditctl

[Steve Grubb]

On Thursday, June 29, 2023 6:34:03 PM EDT Samuel Bahr wrote:
> Hi linux-audit,
> 
> I'm running a fleet of Linux hosts with Red Canary Linux EDR (Endpoint
> Detection and Response) which uses eBPF for gathering telemetry in service
> ` cfsvcd.service`. In an older configuration, it gathered data from the
> kernel's audit system and everything was fine. However, when we switched
> cfsvcd to gathering data from eBPF instead, we noticed that the kernel
> ring buffer was flooded with audit messages. This is because
> cfsvcd.service now stops auditd.service, but leaves the kernel audit
> system enabled.
> 
> I've mitigated this issue by manually running `# auditctl -e 0` on our
> hosts (via Puppet). However, I'm running into a strange issue where _some_
> hosts (~0.5%) are still logging all audit events to the kernel ring buffer
> even after I have disabled the audit system via `# auditctl -e 0`. A `#
> auditctl -s` run shows `enabled 0`, yet audit logs continue to flood the
> kernel ring buffer.
> 
> I'm running Linux kernel 5.4.0-1063-aws on Ubuntu 18.04 with auditctl
> v2.8.2. `systemd-journald-audit.socket` is masked & inactive, `
> auditd.service` is disabled & inactive.
> 
> I cannot entirely disable the audit system via a kernel parameter because
> Red Canary Linux EDR fails to start cfsvcd.service as it fails to run some
> auditctl command due to no audit support in the kernel:
> 
> Jun 28 20:41:04 systemd[1]: Started Canary Forwarder Service.
> 
> > Jun 28 20:41:04 cfsvcd[105781]: Found config file at path
> > '"/opt/redcanary/config.json"', continuing execution...
> > Jun 28 20:41:12 cfsvcd[105781]: component: Sentry
> > Jun 28 20:41:12 cfsvcd[105781]:  Jun 28 20:41:12.055 INFO Initializing
> > Sentry Config { version: "1.4.17.release.[...]", https_proxy: None,
> > extra_tags: [("telemetry.source", "eBPF")], logging_enabled: true,
> > metrics_enabled: true, [...]}
> > Jun 28 20:41:12 cfsvcd[105781]: Failed to setup or configure host system:
> > Linux Audit watcher failure: Netlink Error: IO Error: Protocol not
> > supported (os error 93)
> > Jun 28 20:41:12 cfsvcd[105781]: Received stop. Exiting
> > unix::wait_for_signal. Signal: 0, done.is_signaled(): true
> > Jun 28 20:41:12 cfsvcd[105781]: Joining async_agent_thread
> > Jun 28 20:41:13 cfsvcd[105781]: Attempting to stop subscriber (Ebpf).
> > Jun 28 20:41:13 cfsvcd[105781]: Shutting down due to termination signal
> > [...]
> > Jun 28 20:41:15 cfsvcd[105781]: Subscriber stopped.
> > Jun 28 20:41:15 auditctl[106733]: Error - audit support not in kernel
> > Jun 28 20:41:15 auditctl[106733]: Cannot open netlink audit socket
> > Jun 28 20:41:15 systemd[1]: cfsvcd.service: Service hold-off time over,
> > scheduling restart.
> > Jun 28 20:41:15 systemd[1]: cfsvcd.service: Scheduled restart job,
> > restart
> > counter is at 301.
> > Jun 28 20:41:15 systemd[1]: Stopped Canary Forwarder Service.
> 
> Here's the unit file for cfsvcd.service:
> > [Unit]
> > Description=Canary Forwarder Service
> > 
> > DefaultDependencies=no
> > After=local-fs.target systemd-tmpfiles-setup.service auditd.service
> > Before=sysinit.target shutdown.target
> > # Replace the auditd service if it is running
> > Conflicts=auditd.service shutdown.target
> > 
> > [Service]
> > WorkingDirectory=/opt/redcanary
> > ExecStart=/opt/redcanary/cfsvcd
> > ExecStopPost=-/sbin/auditctl -D
> > Restart=always
> > TimeoutSec=15
> > 
> > [Install]
> > WantedBy=multi-user.target
> 
> Is this a known issue?

Not really. But this is a new capability. If you run auditctl -D, does it go 
away? Have any of the systems been setup with auditctl -e 2? This makes the 
configuration immutable.

-Steve

> Is there a workaround to stop the logging to the
> kernel ring buffer? Is there any more information I can provide to help
> debug?