75_10-Fix-crash-from-SRV-lookup-hitting-a-CNAME.patch

From a189eb636256833f3053d8f2fbb95e51dc0f936c Mon Sep 17 00:00:00 2001
From: Jeremy Harris <jgh146exb@wizmail.org>
Date: Thu, 14 Mar 2019 12:26:34 +0000
Subject: [PATCH 10/29] Fix crash from SRV lookup hitting a CNAME

(cherry picked from commit 14bc9cf085aff7bd5147881e5b7068769a29b026)
(cherry picked from commit 09720dd9506176294154dad7152f5f40554046a4)
---
 doc/ChangeLog |  4 ++++
 src/dns.c         | 10 +++++++---
 2 files changed, 11 insertions(+), 3 deletions(-)

diff --git a/doc/ChangeLog b/doc/ChangeLog
index 26221318..585ccffc 100644
--- a/doc/ChangeLog
+++ b/doc/ChangeLog
@@ -27,6 +27,10 @@ JH/07 GnuTLS: Our use of late (post-handshake) certificate verification, under
 JB/01 Bug 2375: fix expansions of 822 addresses having comments in local-part
       and/or domain.  Found and fixed by Jason Betts.
 
+JH/08 Add hardening against SRV & TLSA lookups the hit CNAMEs (a nonvalid
+      configuration).  If a CNAME target was not a wellformed name pattern, a
+      crash could result.
+
 
 Exim version 4.92
 -----------------
diff --git a/src/dns.c b/src/dns.c
index 0f0b435d..b7978c52 100644
--- a/src/dns.c
+++ b/src/dns.c
@@ -716,7 +716,11 @@ lookup, which constructs the names itself, so they should be OK. Besides,
 bitstring labels don't conform to normal name syntax. (But the aren't used any
 more.)
 
-For SRV records, we omit the initial _smtp._tcp. components at the start. */
+For SRV records, we omit the initial _smtp._tcp. components at the start.
+The check has been seen to bite on the destination of a SRV lookup that
+initiall hit a CNAME, for which the next name had only two components.
+RFC2782 makes no mention of the possibiility of CNAMES, but the Wikipedia
+article on SRV says they are not a valid configuration. */
 
 #ifndef STAND_ALONE   /* Omit this for stand-alone tests */
 
@@ -732,8 +736,8 @@ if (check_dns_names_pattern[0] != 0 && type != T_PTR && type != T_TXT)
 
   if (type == T_SRV || type == T_TLSA)
     {
-    while (*checkname++ != '.');
-    while (*checkname++ != '.');
+    while (*checkname && *checkname++ != '.') ;
+    while (*checkname && *checkname++ != '.') ;
     }
 
   if (pcre_exec(regex_check_dns_names, NULL, CCS checkname, Ustrlen(checkname),
-- 
2.20.1