Backport patch to fix CVE-2019-6471.

Ref:
https://security-tracker.debian.org/tracker/CVE-2019-6471

CVE: CVE-2019-6471
Upstream-Status: Backport [https://gitlab.isc.org/isc-projects/bind9/commit/3a9c7bb]

Signed-off-by: Kai Kang <kai.kang@windriver.com>

From 3a9c7bb80d4a609b86427406d9dd783199920b5b Mon Sep 17 00:00:00 2001
From: Mark Andrews <marka@isc.org>
Date: Tue, 19 Mar 2019 14:14:21 +1100
Subject: [PATCH] move item_out test inside lock in dns_dispatch_getnext()

(cherry picked from commit 60c42f849d520564ed42e5ed0ba46b4b69c07712)
---
 lib/dns/dispatch.c | 12 ++++++++----
 1 file changed, 8 insertions(+), 4 deletions(-)

diff --git a/lib/dns/dispatch.c b/lib/dns/dispatch.c
index 408beda367..3278db4a07 100644
--- a/lib/dns/dispatch.c
+++ b/lib/dns/dispatch.c
@@ -134,7 +134,7 @@ struct dns_dispentry {
 	isc_task_t		       *task;
 	isc_taskaction_t		action;
 	void			       *arg;
-	bool			item_out;
+	bool				item_out;
 	dispsocket_t			*dispsocket;
 	ISC_LIST(dns_dispatchevent_t)	items;
 	ISC_LINK(dns_dispentry_t)	link;
@@ -3422,13 +3422,14 @@ dns_dispatch_getnext(dns_dispentry_t *resp, dns_dispatchevent_t **sockevent) {
 	disp = resp->disp;
 	REQUIRE(VALID_DISPATCH(disp));
 
-	REQUIRE(resp->item_out == true);
-	resp->item_out = false;
-
 	ev = *sockevent;
 	*sockevent = NULL;
 
 	LOCK(&disp->lock);
+
+	REQUIRE(resp->item_out == true);
+	resp->item_out = false;
+
 	if (ev->buffer.base != NULL)
 		free_buffer(disp, ev->buffer.base, ev->buffer.length);
 	free_devent(disp, ev);
@@ -3573,6 +3574,9 @@ dns_dispatch_removeresponse(dns_dispentry_t **resp,
 		isc_task_send(disp->task[0], &disp->ctlevent);
 }
 
+/*
+ * disp must be locked.
+ */
 static void
 do_cancel(dns_dispatch_t *disp) {
 	dns_dispatchevent_t *ev;
-- 
2.20.1