[PATCH libICE v2] Use getentropy() if arc4random_buf() is not available

[Benjamin Tissoires]

This allows to fix CVE-2017-2626 on Linux platforms without pulling in
libbsd.
The libc getentropy() is available since glibc 2.25 but also on OpenBSD.
For Linux, we need at least a v3.17 kernel. If the recommended
arc4random_buf() function is not available, emulate it by first trying
to use getentropy() on a supported glibc and kernel. If the call fails,
fall back to the current (partly vulnerable) code.

Signed-off-by: Benjamin Tissoires <benjamin.tissoires at gmail.com>

---

changes in v2:
- use the getentropy() from glibc, not the plain syscall
- make it clear that arc4random_buf() should be preferred and that we
  are only adding band-aids on top of the missing function
---
 configure.ac  |  2 +-
 src/iceauth.c | 65 ++++++++++++++++++++++++++++++++++++++++++-----------------
 2 files changed, 47 insertions(+), 20 deletions(-)

diff --git a/configure.ac b/configure.ac
index 458882a..c971ab6 100644
--- a/configure.ac
+++ b/configure.ac
@@ -38,7 +38,7 @@ AC_DEFINE(ICE_t, 1, [Xtrans transport type])
 
 # Checks for library functions.
 AC_CHECK_LIB([bsd], [arc4random_buf])
-AC_CHECK_FUNCS([asprintf arc4random_buf])
+AC_CHECK_FUNCS([asprintf arc4random_buf getentropy])
 
 # Allow checking code with lint, sparse, etc.
 XORG_WITH_LINT
diff --git a/src/iceauth.c b/src/iceauth.c
index ed31683..de4785b 100644
--- a/src/iceauth.c
+++ b/src/iceauth.c
@@ -44,31 +44,19 @@ Author: Ralph Mor, X Consortium
 
 static int was_called_state;
 
-/*
- * MIT-MAGIC-COOKIE-1 is a sample authentication method implemented by
- * the SI.  It is not part of standard ICElib.
- */
+#ifndef HAVE_ARC4RANDOM_BUF
 
-
-char *
-IceGenerateMagicCookie (
+static void
+emulate_getrandom_buf (
+	char *auth,
 	int len
 )
 {
-    char    *auth;
-#ifndef HAVE_ARC4RANDOM_BUF
     long    ldata[2];
     int	    seed;
     int	    value;
     int	    i;
-#endif
 
-    if ((auth = malloc (len + 1)) == NULL)
-	return (NULL);
-
-#ifdef HAVE_ARC4RANDOM_BUF
-    arc4random_buf(auth, len);
-#else
 #ifdef ITIMER_REAL
     {
 	struct timeval  now;
@@ -76,13 +64,13 @@ IceGenerateMagicCookie (
 	ldata[0] = now.tv_sec;
 	ldata[1] = now.tv_usec;
     }
-#else
+#else /* ITIMER_REAL */
     {
 	long    time ();
 	ldata[0] = time ((long *) 0);
 	ldata[1] = getpid ();
     }
-#endif
+#endif /* ITIMER_REAL */
     seed = (ldata[0]) + (ldata[1] << 16);
     srand (seed);
     for (i = 0; i < len; i++)
@@ -90,7 +78,46 @@ IceGenerateMagicCookie (
 	value = rand ();
 	auth[i] = value & 0xff;
     }
-#endif
+}
+
+static void
+arc4random_buf (
+	char *auth,
+	int len
+)
+{
+    int	    ret;
+
+#if HAVE_GETENTROPY
+    /* weak emulation of arc4random through the entropy libc */
+    ret = getentropy (auth, len);
+    if (ret == 0)
+	return;
+#endif /* HAVE_GETENTROPY */
+
+    emulate_getrandom_buf (auth, len);
+}
+
+#endif /* !defined(HAVE_ARC4RANDOM_BUF) */
+
+/*
+ * MIT-MAGIC-COOKIE-1 is a sample authentication method implemented by
+ * the SI.  It is not part of standard ICElib.
+ */
+
+
+char *
+IceGenerateMagicCookie (
+	int len
+)
+{
+    char    *auth;
+
+    if ((auth = malloc (len + 1)) == NULL)
+	return (NULL);
+
+    arc4random_buf (auth, len);
+
     auth[len] = '\0';
     return (auth);
 }
-- 
2.9.3