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

CVE: CVE-2012-6702
CVE: CVE-2016-5300
Upstream-Status: Backport

Removed changes from CMakeLists.txt from original patch, since that code is
not part of fix for these CVEs.
Reference to the commit for CMakeLists.txt changes:
https://sourceforge.net/p/expat/code_git/ci/37f7efb878660d55ff5fd67ad2cda1c103297df6

Signed-off-by: Sona Sarmadi <sona.sarmadi@enea.com>
---
diff -Nurp a/lib/xmlparse.c b/lib/xmlparse.c
--- a/lib/xmlparse.c	2017-01-13 10:16:35.570784710 +0100
+++ b/lib/xmlparse.c	2017-01-13 11:22:20.522433486 +0100
@@ -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)
+{
+#ifdef COMPILED_FROM_DSP
+  FILETIME ft;
+  GetSystemTimeAsFileTime(&ft); /* never fails */
+  return ft.dwHighDateTime ^ ft.dwLowDateTime;
+#else
+  struct timeval tv;
+  int gettimeofday_res;
+
+  gettimeofday_res = gettimeofday(&tv, NULL);
+  assert (gettimeofday_res == 0);
+
+  /* Microseconds time is <20 bits entropy */
+  return tv.tv_usec;
+#endif
+}
+
+static unsigned long
+generate_hash_secret_salt(XML_Parser parser)
 {
-  unsigned int seed = time(NULL) % UINT_MAX;
-  srand(seed);
-  return rand();
+  /* 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