Subject: Re: bug#22202: 24.5; SECURITY ISSUE -- Emacs Server vulnerable to random number generator attack on Windows systems

From: Eli Zaretskii <[email-masked]>
Date: Thu, 31 Dec 2015 16:14:00 +0200

> Date: Wed, 30 Dec 2015 21:15:13 +0000
> From: Richard Copley <[email-masked]>
> Cc: David Engster <[email-masked]>, [email-masked], 
> 	Demetri Obenour <[email-masked]>
> 
> > That'd be the worst of both worlds, IMO: a not-so-good PRNG with no
> > way whatsoever to get a repeatable sequence.  Am I right?
> 
> Oh dear. Yes. Worse than that, repeatability is part of the contract for
> the lisp "random" function, so seed_random() and get_random() are
> constrained to be deterministic. Right?
> 
> Seems as though init_random() is the only place that could use
> CryptGenRandom, which is a pity if you're trying to confine the
> changes to w32.c.

How about the following, then?

(Not sure it's a good idea to read from /dev/random, as that could
block; perhaps we should use /dev/urandom instead.)

--- src/sysdep.c~2	2015-11-11 07:57:41.000000000 +0200
+++ src/sysdep.c	2015-12-31 16:09:46.987229300 +0200
@@ -2095,8 +2095,35 @@ seed_random (void *seed, ptrdiff_t seed_
 void
 init_random (void)
 {
-  struct timespec t = current_timespec ();
-  uintmax_t v = getpid () ^ t.tv_sec ^ t.tv_nsec;
+  uintmax_t v;
+  struct timespec t;
+  bool success = false;
+
+#if HAVE_DEV_RANDOM
+  FILE *fp = fopen ("/dev/random", "rb");
+
+  if (fp)
+    {
+      int i;
+
+      for (i = 0, v = 0; i < sizeof (uintmax_t); i++)
+	{
+	  v <<= 8;
+	  v |= fgetc (fp);
+	}
+      fclose (fp);
+      success = true;
+    }
+#elif defined WINDOWSNT
+  if (w32_init_random (&v, sizeof v) == 0)
+    success = true;
+#endif	/* HAVE_DEV_RANDOM || WINDOWSNT */
+  if (!success)
+    {
+      /* Fall back to current time value + PID.  */
+      t = current_timespec ();
+      v = getpid () ^ t.tv_sec ^ t.tv_nsec;
+    }
   seed_random (&v, sizeof v);
 }

--- src/w32.c~0	2015-11-29 06:48:07.000000000 +0200
+++ src/w32.c	2015-12-31 16:05:06.775707600 +0200
@@ -224,6 +224,8 @@ typedef struct _REPARSE_DATA_BUFFER {
 
 #include <iphlpapi.h>	/* should be after winsock2.h */
 
+#include <wincrypt.h>
+
 #include <c-strcase.h>
 
 #include "w32.h"
@@ -2093,6 +2095,34 @@ init_user_info (void)
     CloseHandle (token);
 }
 
+static HCRYPTPROV w32_crypto_hprov;
+static int
+w32_init_crypt_random (void)
+{
+  if (!CryptAcquireContext (&w32_crypto_hprov, NULL, NULL, PROV_RSA_FULL,
+			    CRYPT_VERIFYCONTEXT | CRYPT_SILENT))
+    {
+      DebPrint (("CryptAcquireContext failed with error %x\n",
+		 GetLastError ()));
+      w32_crypto_hprov = 0;
+      return -1;
+    }
+  return 0;
+}
+
+int
+w32_init_random (void *buf, ptrdiff_t buflen)
+{
+  if (w32_crypto_hprov)
+    w32_init_crypt_random ();
+  if (w32_crypto_hprov)
+    {
+      if (CryptGenRandom (w32_crypto_hprov, buflen, (BYTE *)buf))
+	return 0;
+    }
+  return -1;
+}
+
 int
 random (void)
 {
@@ -9386,6 +9416,8 @@ globals_of_w32 (void)
   extern void dynlib_reset_last_error (void);
   dynlib_reset_last_error ();
 #endif
+
+  w32_crypto_hprov = (HCRYPTPROV)0;
 }
 
 /* For make-serial-process  */

--- src/w32.h~2	2015-11-29 06:48:07.000000000 +0200
+++ src/w32.h	2015-12-31 16:09:21.960654500 +0200
@@ -222,6 +222,9 @@ extern int w32_memory_info (unsigned lon
 /* Compare 2 UTF-8 strings in locale-dependent fashion.  */
 extern int w32_compare_strings (const char *, const char *, char *, int);
 
+/* Return a cryptographically secure seed for PRNG.  */
+extern int w32_init_random (void *, ptrdiff_t);
+
 #ifdef HAVE_GNUTLS
 #include <gnutls/gnutls.h>

--- src/fns.c~	2015-11-22 07:57:20.000000000 +0200
+++ src/fns.c	2015-12-31 16:57:43.607286800 +0200
@@ -50,7 +50,8 @@ All integers representable in Lisp, i.e.
 and `most-positive-fixnum', inclusive, are equally likely.
 
 With positive integer LIMIT, return random number in interval [0,LIMIT).
-With argument t, set the random number seed from the current time and pid.
+With argument t, set the random number seed from the system's entropy
+pool, or from the current time and pid if entropy is unavailable.
 With a string argument, set the seed based on the string's contents.
 Other values of LIMIT are ignored.
 
--- configure.ac~2	2015-12-20 06:45:33.000000000 +0200
+++ configure.ac	2015-12-31 16:06:48.959511800 +0200
@@ -4145,6 +4145,22 @@
 
 AC_TYPE_MBSTATE_T
 
+AC_MSG_CHECKING([whether "/dev/random" is available])
+dev_random=no
+dnl MSYS, being a Cygwin fork, thinks "/dev/random" does exist, so
+dnl don't check this for the MinGW builds.
+if test "${opsys}" != "mingw32"; then
+   if test -r "/dev/random"; then
+      AC_DEFINE(HAVE_DEV_RANDOM, 1, [Define if the system supports the "/dev/random" device.])
+      dev_random=yes
+   fi
+fi
+if test $dev_random = yes; then
+   AC_MSG_RESULT(yes)
+else
+   AC_MSG_RESULT(no)
+fi
+
 dnl Fixme: AC_SYS_POSIX_TERMIOS should probably be used, but it's not clear
 dnl how the tty code is related to POSIX and/or other versions of termios.
 dnl The following looks like a useful start.