Audit: save audit_backlog_limit audit messages in case auditd comes back

This patch causes the kernel audit subsystem to store up to
audit_backlog_limit messages for use by auditd if it ever appears
sometime in the future in userspace.  This is useful to collect audit
messages during bootup and even when auditd is stopped.  This is NOT a
reliable mechanism, it does not ever call audit_panic, nor should it.
audit_log_lost()/audit_panic() are called during the normal delivery
mechanism.  The messages are still sent to printk/syslog as usual and if
too many messages appear to be queued they will be silently discarded.

I liked doing it by default, but this patch only uses the queue in
question if it was booted with audit=1 or if the kernel was built
enabling audit by default.

Signed-off-by: Eric Paris <eparis@redhat.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

kernel/audit.c

@@ -126,6 +126,8 @@ static int	   audit_freelist_count;
 static LIST_HEAD(audit_freelist);
 
 static struct sk_buff_head audit_skb_queue;
+/* queue of skbs to send to auditd when/if it comes back */
+static struct sk_buff_head audit_skb_hold_queue;
 static struct task_struct *kauditd_task;
 static DECLARE_WAIT_QUEUE_HEAD(kauditd_wait);
 static DECLARE_WAIT_QUEUE_HEAD(audit_backlog_wait);
@@ -347,30 +349,83 @@ static int audit_set_failure(int state, uid_t loginuid, u32 sessionid, u32 sid)
 				      loginuid, sessionid, sid);
 }
 
+/*
+ * Queue skbs to be sent to auditd when/if it comes back.  These skbs should
+ * already have been sent via prink/syslog and so if these messages are dropped
+ * it is not a huge concern since we already passed the audit_log_lost()
+ * notification and stuff.  This is just nice to get audit messages during
+ * boot before auditd is running or messages generated while auditd is stopped.
+ * This only holds messages is audit_default is set, aka booting with audit=1
+ * or building your kernel that way.
+ */
+static void audit_hold_skb(struct sk_buff *skb)
+{
+	if (audit_default &&
+	    skb_queue_len(&audit_skb_hold_queue) < audit_backlog_limit)
+		skb_queue_tail(&audit_skb_hold_queue, skb);
+	else
+		kfree_skb(skb);
+}
+
+static void kauditd_send_skb(struct sk_buff *skb)
+{
+	int err;
+	/* take a reference in case we can't send it and we want to hold it */
+	skb_get(skb);
+	err = netlink_unicast(audit_sock, skb, audit_nlk_pid, 0);
+	if (err < 0) {
+		BUG_ON(err != -ECONNREFUSED); /* Shoudn't happen */
+		printk(KERN_ERR "audit: *NO* daemon at audit_pid=%d\n", audit_pid);
+		audit_log_lost("auditd dissapeared\n");
+		audit_pid = 0;
+		/* we might get lucky and get this in the next auditd */
+		audit_hold_skb(skb);
+	} else
+		/* drop the extra reference if sent ok */
+		kfree_skb(skb);
+}
+
 static int kauditd_thread(void *dummy)
 {
 	struct sk_buff *skb;
 
 	set_freezable();
 	while (!kthread_should_stop()) {
+		/*
+		 * if auditd just started drain the queue of messages already
+		 * sent to syslog/printk.  remember loss here is ok.  we already
+		 * called audit_log_lost() if it didn't go out normally.  so the
+		 * race between the skb_dequeue and the next check for audit_pid
+		 * doesn't matter.
+		 *
+		 * if you ever find kauditd to be too slow we can get a perf win
+		 * by doing our own locking and keeping better track if there
+		 * are messages in this queue.  I don't see the need now, but
+		 * in 5 years when I want to play with this again I'll see this
+		 * note and still have no friggin idea what i'm thinking today.
+		 */
+		if (audit_default && audit_pid) {
+			skb = skb_dequeue(&audit_skb_hold_queue);
+			if (unlikely(skb)) {
+				while (skb && audit_pid) {
+					kauditd_send_skb(skb);
+					skb = skb_dequeue(&audit_skb_hold_queue);
+				}
+			}
+		}
+
 		skb = skb_dequeue(&audit_skb_queue);
 		wake_up(&audit_backlog_wait);
 		if (skb) {
-			if (audit_pid) {
-				int err = netlink_unicast(audit_sock, skb, audit_nlk_pid, 0);
-				if (err < 0) {
-					BUG_ON(err != -ECONNREFUSED); /* Shoudn't happen */
-					printk(KERN_ERR "audit: *NO* daemon at audit_pid=%d\n", audit_pid);
-					audit_log_lost("auditd dissapeared\n");
-					audit_pid = 0;
-				}
-			} else {
+			if (audit_pid)
+				kauditd_send_skb(skb);
+			else {
 				if (printk_ratelimit())
-					printk(KERN_NOTICE "%s\n", skb->data +
-						NLMSG_SPACE(0));
+					printk(KERN_NOTICE "%s\n", skb->data + NLMSG_SPACE(0));
 				else
 					audit_log_lost("printk limit exceeded\n");
-				kfree_skb(skb);
+
+				audit_hold_skb(skb);
 			}
 		} else {
 			DECLARE_WAITQUEUE(wait, current);
@@ -885,6 +940,7 @@ static int __init audit_init(void)
 		audit_sock->sk_sndtimeo = MAX_SCHEDULE_TIMEOUT;
 
 	skb_queue_head_init(&audit_skb_queue);
+	skb_queue_head_init(&audit_skb_hold_queue);
 	audit_initialized = 1;
 	audit_enabled = audit_default;
 	audit_ever_enabled |= !!audit_default;
@@ -1363,19 +1419,23 @@ void audit_log_end(struct audit_buffer *ab)
 		audit_log_lost("rate limit exceeded");
 	} else {
 		struct nlmsghdr *nlh = nlmsg_hdr(ab->skb);
+		nlh->nlmsg_len = ab->skb->len - NLMSG_SPACE(0);
+
 		if (audit_pid) {
-			nlh->nlmsg_len = ab->skb->len - NLMSG_SPACE(0);
 			skb_queue_tail(&audit_skb_queue, ab->skb);
-			ab->skb = NULL;
 			wake_up_interruptible(&kauditd_wait);
-		} else if (nlh->nlmsg_type != AUDIT_EOE) {
-			if (printk_ratelimit()) {
-				printk(KERN_NOTICE "type=%d %s\n",
-					nlh->nlmsg_type,
-					ab->skb->data + NLMSG_SPACE(0));
-			} else
-				audit_log_lost("printk limit exceeded\n");
+		} else {
+			if (nlh->nlmsg_type != AUDIT_EOE) {
+				if (printk_ratelimit()) {
+					printk(KERN_NOTICE "type=%d %s\n",
+						nlh->nlmsg_type,
+						ab->skb->data + NLMSG_SPACE(0));
+				} else
+					audit_log_lost("printk limit exceeded\n");
+			}
+			audit_hold_skb(ab->skb);
 		}
+		ab->skb = NULL;
 	}
 	audit_buffer_free(ab);
 }