The decision to log a seccomp action will always be subject to the
value of the kernel.seccomp.actions_logged sysctl, even for processes
that are being inspected via the audit subsystem, in an upcoming patch.
Therefore, we need to emit an audit record on attempts at writing to the
actions_logged sysctl when auditing is enabled.

This patch updates the write handler for the actions_logged sysctl to
emit an audit record on attempts to write to the sysctl. Successful
writes to the sysctl will result in a record that includes a normalized
list of logged actions in the "actions" field and a "res" field equal to
1. Unsuccessful writes to the sysctl will result in a record that
doesn't include the "actions" field and has a "res" field equal to 0.

Not all unsuccessful writes to the sysctl are audited. For example, an
audit record will not be emitted if an unprivileged process attempts to
open the sysctl file for reading since that access control check is not
part of the sysctl's write handler.

Below are some example audit records when writing various strings to the
actions_logged sysctl.

Writing "not-a-real-action", when the kernel.seccomp.actions_logged
sysctl previously was "kill_process kill_thread trap errno trace log",
emits this audit record:

 type=CONFIG_CHANGE msg=audit(1525392371.454:120): op=seccomp-logging
 actions=? old-actions=kill_process,kill_thread,trap,errno,trace,log
 res=0

If you then write "kill_process kill_thread errno trace log", this audit
record is emitted:

 type=CONFIG_CHANGE msg=audit(1525392401.645:126): op=seccomp-logging
 actions=kill_process,kill_thread,errno,trace,log
 old-actions=kill_process,kill_thread,trap,errno,trace,log res=1

If you then write "log log errno trace kill_process kill_thread", which
is unordered and contains the log action twice, it results in the same
actions value as the previous record:

 type=CONFIG_CHANGE msg=audit(1525392436.354:132): op=seccomp-logging
 actions=kill_process,kill_thread,errno,trace,log
 old-actions=kill_process,kill_thread,errno,trace,log res=1

If you then write an empty string to the sysctl, this audit record is
emitted:

 type=CONFIG_CHANGE msg=audit(1525392494.413:138): op=seccomp-logging
 actions=(none) old-actions=kill_process,kill_thread,errno,trace,log
 res=1

No audit records are generated when reading the actions_logged sysctl.

Suggested-by: Steve Grubb <sgrubb@redhat.com>
Signed-off-by: Tyler Hicks <tyhicks@canonical.com>
Acked-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Paul Moore <paul@paul-moore.com>