coap_prng.c: Remove support for rand() (CVE-2021-34430)

Replace it using random().

CMakeLists.txt

@@ -169,6 +169,7 @@ check_function_exists(getaddrinfo HAVE_GETADDRINFO)
 check_function_exists(strnlen HAVE_STRNLEN)
 check_function_exists(strrchr HAVE_STRRCHR)
 check_function_exists(getrandom HAVE_GETRANDOM)
+check_function_exists(random HAVE_RANDOM)
 check_function_exists(if_nametoindex HAVE_IF_NAMETOINDEX)
 
 # check for symbols

cmake_coap_config.h.in

@@ -125,6 +125,12 @@
 /* Define to 1 if you have the `strrchr' function. */
 #cmakedefine HAVE_STRRCHR @HAVE_STRRCHR@
 
+/* Define to 1 if you have the `getrandom' function. */
+#cmakedefine HAVE_GETRANDOM @HAVE_GETRANDOM@
+
+/* Define to 1 if you have the `randon' function. */
+#cmakedefine HAVE_RANDOM @HAVE_RANDOM@
+
 /* Define to 1 if the system has the type `struct cmsghdr'. */
 #cmakedefine HAVE_STRUCT_CMSGHDR @HAVE_STRUCT_CMSGHDR@
 

configure.ac

@@ -891,7 +891,7 @@ AC_TYPE_SSIZE_T
 
 # Checks for library functions.
 AC_CHECK_FUNCS([memset select socket strcasecmp strrchr getaddrinfo \
-                strnlen malloc pthread_mutex_lock getrandom if_nametoindex])
+                strnlen malloc pthread_mutex_lock getrandom random if_nametoindex])
 
 # Check if -lsocket -lnsl is required (specifically Solaris)
 AC_SEARCH_LIBS([socket], [socket])

src/coap_prng.c

@@ -36,18 +36,19 @@ errno_t __cdecl rand_s( _Out_ unsigned int* _RandomValue );
  */
 COAP_STATIC_INLINE int
 coap_prng_impl( unsigned char *buf, size_t len ) {
-        while ( len != 0 ) {
-                uint32_t r = 0;
-                size_t i;
-                if ( rand_s( &r ) != 0 )
-                        return 0;
-                for ( i = 0; i < len && i < 4; i++ ) {
-                        *buf++ = (uint8_t)r;
-                        r >>= 8;
-                }
-                len -= i;
-        }
-        return 1;
+  while ( len != 0 ) {
+    uint32_t r = 0;
+    size_t i;
+
+    if ( rand_s( &r ) != 0 )
+      return 0;
+    for ( i = 0; i < len && i < 4; i++ ) {
+      *buf++ = (uint8_t)r;
+      r >>= 8;
+    }
+    len -= i;
+  }
+  return 1;
 }
 
 #endif /* _WIN32 */
@@ -61,20 +62,46 @@ coap_prng_default(void *buf, size_t len) {
 #if defined(MBEDTLS_ENTROPY_HARDWARE_ALT)
   /* mbedtls_hardware_poll() returns 0 on success */
   return (mbedtls_hardware_poll(NULL, buf, len, NULL) ? 0 : 1);
-#else /* !MBEDTLS_ENTROPY_HARDWARE_ALT */
-#ifdef HAVE_GETRANDOM
+
+#elif defined(HAVE_GETRANDOM)
   return (getrandom(buf, len, 0) > 0) ? 1 : 0;
-#else /* !HAVE_GETRANDOM */
-#if defined(_WIN32)
+
+#elif defined(HAVE_RANDOM)
+#define RAND_BYTES (RAND_MAX >= 0xffffff ? 3 : (RAND_MAX >= 0xffff ? 2 : 1))
+  unsigned char *dst = (unsigned char *)buf;
+
+  if (len) {
+    uint8_t byte_counter = RAND_BYTES;
+    uint32_t r_v = random();
+
+    while (1) {
+      *dst++ = r_v & 0xFF;
+      if (!--len) {
+        break;
+      }
+      if (--byte_counter) {
+        r_v >>= 8;
+      } else {
+        r_v = random();
+        byte_counter = RAND_BYTES;
+      }
+    }
+  }
+  return 1;
+
+#elif defined(_WIN32)
   return coap_prng_impl(buf,len);
-#else /* !_WIN32 */
+
+#else /* !MBEDTLS_ENTROPY_HARDWARE_ALT && !HAVE_GETRANDOM &&
+         !HAVE_RANDOM && !_WIN32 */
+  #error "CVE-2021-34430: using rand() for crypto randoms is not secure!"
+  #error "Please update you C-library and rerun the auto-configuration."
   unsigned char *dst = (unsigned char *)buf;
   while (len--)
     *dst++ = rand() & 0xFF;
   return 1;
-#endif /* !_WIN32 */
-#endif /* !HAVE_GETRANDOM */
-#endif /* !MBEDTLS_ENTROPY_HARDWARE_ALT */
+#endif /* !MBEDTLS_ENTROPY_HARDWARE_ALT && !HAVE_GETRANDOM &&
+          !HAVE_RANDOM && !_WIN32 */
 }
 
 static coap_rand_func_t rand_func = coap_prng_default;
@@ -93,10 +120,11 @@ coap_set_prng(coap_rand_func_t rng) {
 void
 coap_prng_init(unsigned int seed) {
 #ifdef HAVE_GETRANDOM
-  /* No seed to seed the random source if getrandom() is used,
-   * see dtls_prng(). */
+  /* No seed to seed the random source if getrandom() is used */
   (void)seed;
-#else /* !HAVE_GETRANDOM */
+#elif defined(HAVE_RANDOM)
+  srandom(seed);
+#else /* !HAVE_GETRANDOM  && !HAVE_RANDOM */
   srand(seed);
 #endif /* !HAVE_GETRANDOM */
 }