Fix CVE-2012-6702 and CVE-2016-5300.

https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2012-6702
https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2016-5300

Patch copied from:
https://sources.debian.net/src/expat/2.1.0-6%2Bdeb8u3/debian/patches/cve-2012-6702-plus-cve-2016-5300-v1.patch/

From cb31522769d11a375078a073cba94e7176cb48a4 Mon Sep 17 00:00:00 2001
From: Sebastian Pipping <sebastian@pipping.org>
Date: Wed, 16 Mar 2016 15:30:12 +0100
Subject: [PATCH] Resolve call to srand, use more entropy (patch version 1.0)

Squashed backport against vanilla Expat 2.1.1, addressing:
* CVE-2012-6702 -- unanticipated internal calls to srand
* CVE-2016-5300 -- use of too little entropy

Since commit e3e81a6d9f0885ea02d3979151c358f314bf3d6d
(released with Expat 2.1.0) Expat called srand by itself
from inside generate_hash_secret_salt for an instance
of XML_Parser if XML_SetHashSalt was either (a) not called
for that instance or if (b) salt 0 was passed to XML_SetHashSalt
prior to parsing.  That call to srand passed (rather litle)
entropy extracted from the current time as a seed for srand.

That call to srand (1) broke repeatability for code calling
srand with a non-random seed prior to parsing with Expat,
and (2) resulted in a rather small set of hashing salts in
Expat in total.

For a short- to mid-term fix, the new approach avoids calling
srand altogether, extracts more entropy out of the clock and
other sources, too.

For a long term fix, we may want to read sizeof(long) bytes
from a source like getrandom(..) on Linux, and from similar
sources on other supported architectures.

https://bugzilla.redhat.com/show_bug.cgi?id=1197087
---
 CMakeLists.txt |  3 +++
 lib/xmlparse.c | 48 +++++++++++++++++++++++++++++++++++++++++-------
 2 files changed, 44 insertions(+), 7 deletions(-)

diff --git a/CMakeLists.txt b/CMakeLists.txt
index 353627e..524d514 100755
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -41,6 +41,9 @@ include_directories(${CMAKE_BINARY_DIR} ${CMAKE_SOURCE_DIR}/lib)
 if(MSVC)
     add_definitions(-D_CRT_SECURE_NO_WARNINGS -wd4996)
 endif(MSVC)
+if(WIN32)
+    add_definitions(-DCOMPILED_FROM_DSP)
+endif(WIN32)
 
 set(expat_SRCS
     lib/xmlparse.c
diff --git a/lib/xmlparse.c b/lib/xmlparse.c
index e308c79..c5f942f 100644
--- a/lib/xmlparse.c
+++ b/lib/xmlparse.c
@@ -6,7 +6,14 @@
 #include <string.h>                     /* memset(), memcpy() */
 #include <assert.h>
 #include <limits.h>                     /* UINT_MAX */
-#include <time.h>                       /* time() */
+
+#ifdef COMPILED_FROM_DSP
+#define getpid GetCurrentProcessId
+#else
+#include <sys/time.h>                   /* gettimeofday() */
+#include <sys/types.h>                  /* getpid() */
+#include <unistd.h>                     /* getpid() */
+#endif
 
 #define XML_BUILDING_EXPAT 1
 
@@ -432,7 +439,7 @@ static ELEMENT_TYPE *
 getElementType(XML_Parser parser, const ENCODING *enc,
                const char *ptr, const char *end);
 
-static unsigned long generate_hash_secret_salt(void);
+static unsigned long generate_hash_secret_salt(XML_Parser parser);
 static XML_Bool startParsing(XML_Parser parser);
 
 static XML_Parser
@@ -691,11 +698,38 @@ static const XML_Char implicitContext[] = {
 };
 
 static unsigned long
-generate_hash_secret_salt(void)
+gather_time_entropy(void)
 {
-  unsigned int seed = time(NULL) % UINT_MAX;
-  s<tr><td><span title='2015-01-14 14:42:10 +0100'>2015-01-14</span></td><td><a href='/guix/commit/tests/builders.scm?id=f220a8384890b2a50f30c62fba56e507333f1a92'>packages: Convert source derivations to monadic style.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
timeofday_res == 0);
+
+  /* Microseconds time is <20 bits entropy */
+  return tv.tv_usec;
+#endif
+}
+
+static unsigned long
+generate_hash_secret_salt(XML_Parser parser)
+{
+  /* Process ID is 0 bits entropy if attacker has local access
+   * XML_Parser address is few bits of entropy if attacker has local access */
+  const unsigned long entropy =
+      gather_time_entropy() ^ getpid() ^ (unsigned long)parser;
+
+  /* Factors are 2^31-1 and 2^61-1 (Mersenne primes M31 and M61) */
+  if (sizeof(unsigned long) == 4) {
+    return entropy * 2147483647;
+  } else {
+    return entropy * 2305843009213693951;
+  }
 }
 
 static XML_Bool  /* only valid for root parser */
@@ -703,7 +737,7 @@ startParsing(XML_Parser parser)
 {
     /* hash functions must be initialized before setContext() is called */
     if (hash_secret_salt == 0)
-      hash_secret_salt = generate_hash_secret_salt();
+      hash_secret_salt = generate_hash_secret_salt(parser);
     if (ns) {
       /* implicit context only set for root parser, since child
          parsers (i.e. external entity parsers) will inherit it
-- 
2.8.2