Audit: stop deadlock from signals under load

A deadlock is possible between kauditd and auditd under load if auditd
receives a signal.  When auditd receives a signal it sends a netlink
message to the kernel asking for information about the sender of the
signal.  In that same context the audit system will attempt to send a
netlink message back to the userspace auditd.  If kauditd has already
filled the socket buffer (see netlink_attachskb()) auditd will now put
itself to sleep waiting for room to send the message.  Since auditd is
responsible for draining that socket we have a deadlock.  The fix, since
the response from the kernel does not need to be synchronous is to send
the signal information back to auditd in a separate thread.  And thus
auditd can continue to drain the audit queue normally.
Signed-off-by: Eric Paris <eparis@redhat.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

kernel/audit.c

@@ -156,6 +156,11 @@ struct audit_buffer {
 	gfp_t		     gfp_mask;
 };
 
+struct audit_reply {
+	int pid;
+	struct sk_buff *skb;
+};
+
 static void audit_set_pid(struct audit_buffer *ab, pid_t pid)
 {
 	if (ab) {
@@ -528,6 +533,19 @@ struct sk_buff *audit_make_reply(int pid, int seq, int type, int done,
 	return NULL;
 }
 
+static int audit_send_reply_thread(void *arg)
+{
+	struct audit_reply *reply = (struct audit_reply *)arg;
+
+	mutex_lock(&audit_cmd_mutex);
+	mutex_unlock(&audit_cmd_mutex);
+
+	/* Ignore failure. It'll only happen if the sender goes away,
+	   because our timeout is set to infinite. */
+	netlink_unicast(audit_sock, reply->skb, reply->pid, 0);
+	kfree(reply);
+	return 0;
+}
 /**
  * audit_send_reply - send an audit reply message via netlink
  * @pid: process id to send reply to
@@ -545,13 +563,25 @@ void audit_send_reply(int pid, int seq, int type, int done, int multi,
 		      void *payload, int size)
 {
 	struct sk_buff *skb;
+	struct task_struct *tsk;
+	struct audit_reply *reply = kmalloc(sizeof(struct audit_reply),
+					    GFP_KERNEL);
+
+	if (!reply)
+		return;
+
 	skb = audit_make_reply(pid, seq, type, done, multi, payload, size);
 	if (!skb)
 		return;
-	/* Ignore failure. It'll only happen if the sender goes away,
-	   because our timeout is set to infinite. */
-	netlink_unicast(audit_sock, skb, pid, 0);
-	return;
+
+	reply->pid = pid;
+	reply->skb = skb;
+
+	tsk = kthread_run(audit_send_reply_thread, reply, "audit_send_reply");
+	if (IS_ERR(tsk)) {
+		kfree(reply);
+		kfree_skb(skb);
+	}
 }
 
 /*