audit: queue netlink multicast sends just like we do for unicast sends

Sending audit netlink multicast messages is bad for all the same
reasons that sending audit netlink unicast messages is bad, so this
patch reworks things so that we don't do the multicast send in
audit_log_end(), we do it from the dedicated kauditd_thread thread just
as we do for unicast messages.

See the GitHub issues below for more information/history:

 * https://github.com/linux-audit/audit-kernel/issues/23
 * https://github.com/linux-audit/audit-kernel/issues/22Signed-off-by: Paul Moore <paul@paul-moore.com>

kernel/audit.c

@@ -511,26 +511,46 @@ static void flush_hold_queue(void)
 
 static int kauditd_thread(void *dummy)
 {
-	set_freezable();
-	while (!kthread_should_stop()) {
 	struct sk_buff *skb;
+	struct nlmsghdr *nlh;
 
+	set_freezable();
+	while (!kthread_should_stop()) {
 		flush_hold_queue();
 
 		skb = skb_dequeue(&audit_skb_queue);
-
 		if (skb) {
-			if (!audit_backlog_limit ||
-			    (skb_queue_len(&audit_skb_queue) <= audit_backlog_limit))
-				wake_up(&audit_backlog_wait);
+			nlh = nlmsg_hdr(skb);
+
+			/* if nlh->nlmsg_len is zero then we haven't attempted
+			 * to send the message to userspace yet, if nlmsg_len
+			 * is non-zero then we have attempted to send it to
+			 * the multicast listeners as well as auditd; keep
+			 * trying to send to auditd but don't repeat the
+			 * multicast send */
+			if (nlh->nlmsg_len == 0) {
+				nlh->nlmsg_len = skb->len;
+				kauditd_send_multicast_skb(skb, GFP_KERNEL);
+
+				/* see the note in kauditd_send_multicast_skb
+				 * regarding the nlh->nlmsg_len value and why
+				 * it differs between the multicast and unicast
+				 * clients */
+				nlh->nlmsg_len -= NLMSG_HDRLEN;
+			}
+
 			if (audit_pid)
 				kauditd_send_skb(skb);
 			else
 				audit_printk_skb(skb);
-			continue;
+		} else {
+			/* we have flushed the backlog so wake everyone up who
+			 * is blocked and go to sleep until we have something
+			 * in the backlog again */
+			wake_up(&audit_backlog_wait);
+			wait_event_freezable(kauditd_wait,
+					     skb_queue_len(&audit_skb_queue));
 		}
-
-		wait_event_freezable(kauditd_wait, skb_queue_len(&audit_skb_queue));
 	}
 	return 0;
 }
@@ -1969,10 +1989,10 @@ void audit_log_link_denied(const char *operation, struct path *link)
  * audit_log_end - end one audit record
  * @ab: the audit_buffer
  *
- * netlink_unicast() cannot be called inside an irq context because it blocks
- * (last arg, flags, is not set to MSG_DONTWAIT), so the audit buffer is placed
- * on a queue and a tasklet is scheduled to remove them from the queue outside
- * the irq context.  May be called in any context.
+ * We can not do a netlink send inside an irq context because it blocks (last
+ * arg, flags, is not set to MSG_DONTWAIT), so the audit buffer is placed on a
+ * queue and a tasklet is scheduled to remove them from the queue outside the
+ * irq context.  May be called in any context.
  */
 void audit_log_end(struct audit_buffer *ab)
 {
@@ -1981,28 +2001,8 @@ void audit_log_end(struct audit_buffer *ab)
 	if (!audit_rate_check()) {
 		audit_log_lost("rate limit exceeded");
 	} else {
-		struct nlmsghdr *nlh = nlmsg_hdr(ab->skb);
-
-		nlh->nlmsg_len = ab->skb->len;
-		kauditd_send_multicast_skb(ab->skb, ab->gfp_mask);
-
-		/*
-		 * The original kaudit unicast socket sends up messages with
-		 * nlmsg_len set to the payload length rather than the entire
-		 * message length.  This breaks the standard set by netlink.
-		 * The existing auditd daemon assumes this breakage.  Fixing
-		 * this would require co-ordinating a change in the established
-		 * protocol between the kaudit kernel subsystem and the auditd
-		 * userspace code.
-		 */
-		nlh->nlmsg_len -= NLMSG_HDRLEN;
-
-		if (audit_pid) {
 		skb_queue_tail(&audit_skb_queue, ab->skb);
 		wake_up_interruptible(&kauditd_wait);
-		} else {
-			audit_printk_skb(ab->skb);
-		}
 		ab->skb = NULL;
 	}
 	audit_buffer_free(ab);