[PATCH] audit: optionally print warning after waiting to enqueue record

[Max Englander]

On Wed, Jun 17, 2020 at 02:47:19PM -0400, Paul Moore wrote:
> On Tue, Jun 16, 2020 at 12:58 AM Max Englander <max.englander at gmail.com> wrote:
> >
> > In environments where security is prioritized, users may set
> > --backlog_wait_time to a high value in order to reduce the likelihood
> > that any audit event is lost, even though doing so may result in
> > unpredictable performance if the kernel schedules a timeout when the
> > backlog limit is exceeded. For these users, the next best thing to
> > predictable performance is the ability to quickly detect and react to
> > degraded performance. This patch proposes to aid the detection of kernel
> > audit subsystem pauses through the following changes:
> >
> > Add a variable named audit_backlog_warn_time. Enforce the value of this
> > variable to be no less than zero, and no more than the value of
> > audit_backlog_wait_time.
> >
> > If audit_backlog_warn_time is greater than zero and if the total time
> > spent waiting to enqueue an audit record is greater than or equal to
> > audit_backlog_warn_time, then print a warning with the total time
> > spent waiting.
> >
> > An example configuration:
> >
> >         auditctl --backlog_warn_time 50
> >
> > An example warning message:
> >
> >         audit: sleep_time=52 >= audit_backlog_warn_time=50
> >
> > Tested on Ubuntu 18.04.04 using complementary changes to the audit
> > userspace: https://github.com/linux-audit/audit-userspace/pull/131.
> >
> > Signed-off-by: Max Englander <max.englander at gmail.com>
> > ---
> >  include/uapi/linux/audit.h |  7 ++++++-
> >  kernel/audit.c             | 35 +++++++++++++++++++++++++++++++++++
> >  2 files changed, 41 insertions(+), 1 deletion(-)
> 
> If an admin is prioritizing security, aka don't loose any audit
> records, and there is a concern over variable system latency due to an
> audit queue backlog, why not simply disable the backlog limit?
> 
> -- 
> paul moore
> www.paul-moore.com

That’s good in some cases, but in other cases unbounded growth of the
backlog could result in memory issues. If the kernel runs out of memory
it would drop the audit event or possibly have other problems. It could
also also consume memory in a way that starves user workloads or causes
them to be killed by the OOMKiller.

To refine my motivating use case a bit, if a Kubernetes admin wants to
prioritize security, and also avoid unbounded growth of the audit
backlog, they may set -b and --backlog_wait_time in a way that limits
kernel memory usage and reduces the likelihood that any audit event is
lost. Occasional performance degradation may be acceptable to the admin,
but they would like a way to be alerted to prolonged kernel pauses, so
that they can investigate and take corrective action (increase backlog,
increase server capacity, move some workloads to other servers, etc.).

To state another way. The kernel currently can be configured to print a
message when the backlog limit is exceeded and it must discard the audit
event. This is a useful message for admins, which they can address with
corrective action. I think a message similar to the one proposed by this
patch would be equally useful when the backlog limit is exceeded and the
kernel is configured to wait for the backlog to drain. Admins could
address that message in the same way, but without the cost of lost audit
events.