On Mon, Apr 08, 2019 at 11:39:09PM -0400, Richard Guy Briggs wrote:

It is not permitted to unset the audit container identifier.
A child inherits its parent's audit container identifier.

...

 /**
+ * audit_set_contid - set current task's audit contid
+ * @contid: contid value
+ *
+ * Returns 0 on success, -EPERM on permission failure.
+ *
+ * Called (set) from fs/proc/base.c::proc_contid_write().
+ */
+int audit_set_contid(struct task_struct *task, u64 contid)
+{
+	u64 oldcontid;
+	int rc = 0;
+	struct audit_buffer *ab;
+	uid_t uid;
+	struct tty_struct *tty;
+	char comm[sizeof(current->comm)];
+
+	task_lock(task);
+	/* Can't set if audit disabled */
+	if (!task->audit) {
+		task_unlock(task);
+		return -ENOPROTOOPT;
+	}
+	oldcontid = audit_get_contid(task);
+	read_lock(&tasklist_lock);
+	/* Don't allow the audit containerid to be unset */
+	if (!audit_contid_valid(contid))
+		rc = -EINVAL;
+	/* if we don't have caps, reject */
+	else if (!capable(CAP_AUDIT_CONTROL))
+		rc = -EPERM;
+	/* if task has children or is not single-threaded, deny */
+	else if (!list_empty(&task->children))
+		rc = -EBUSY;
+	else if (!(thread_group_leader(task) && thread_group_empty(task)))
+		rc = -EALREADY;
+	read_unlock(&tasklist_lock);
+	if (!rc)
+		task->audit->contid = contid;
+	task_unlock(task);
+
+	if (!audit_enabled)
+		return rc;

...but it is allowed to change it (assuming
capable(CAP_AUDIT_CONTROL), of course)? Seems like this might be more
immediately useful since we still live in the world of majority
privileged containers if we didn't allow changing it, in addition to
un-setting it.

Tycho