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

[Benjamin Tissoires]

This allows to fix CVE-2017-2625 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 (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
---
 Key.c        | 31 ++++++++++++++++++++++++++-----
 configure.ac |  2 +-
 2 files changed, 27 insertions(+), 6 deletions(-)

diff --git a/Key.c b/Key.c
index a09b316..70607d0 100644
--- a/Key.c
+++ b/Key.c
@@ -62,10 +62,11 @@ getbits (long data, unsigned char *dst)
 #define getpid(x) _getpid(x)
 #endif
 
-void
-XdmcpGenerateKey (XdmAuthKeyPtr key)
-{
 #ifndef HAVE_ARC4RANDOM_BUF
+
+static void
+emulate_getrandom_buf (char *auth, int len)
+{
     long    lowbits, highbits;
 
     srandom ((int)getpid() ^ time((Time_t *)0));
@@ -73,9 +74,29 @@ XdmcpGenerateKey (XdmAuthKeyPtr key)
     highbits = random ();
     getbits (lowbits, key->data);
     getbits (highbits, key->data + 4);
-#else
+}
+
+static void
+arc4random_buf (void *auth, int len)
+{
+    int	    ret;
+
+#if HAVE_GETENTROPY
+    /* weak emulation of arc4random through the getentropy libc call */
+    ret = getentropy (auth, len);
+    if (ret == 0)
+	return;
+#endif /* HAVE_GETENTROPY */
+
+    emulate_getrandom_buf (auth, len);
+}
+
+#endif /* !defined(HAVE_ARC4RANDOM_BUF) */
+
+void
+XdmcpGenerateKey (XdmAuthKeyPtr key)
+{
     arc4random_buf(key->data, 8);
-#endif
 }
 
 int
diff --git a/configure.ac b/configure.ac
index 2288502..d2b045d 100644
--- a/configure.ac
+++ b/configure.ac
@@ -65,7 +65,7 @@ esac
 
 # Checks for library functions.
 AC_CHECK_LIB([bsd], [arc4random_buf])
-AC_CHECK_FUNCS([srand48 lrand48 arc4random_buf])
+AC_CHECK_FUNCS([srand48 lrand48 arc4random_buf getentropy])
 
 # Obtain compiler/linker options for depedencies
 PKG_CHECK_MODULES(XDMCP, xproto)
-- 
2.9.3