Add random delay to enforce_volatile_reads

Add a random delay to mbedtls_platform_enforce_volatile_reads() as a
countermeasure to fault injection attacks.

include/mbedtls/platform_util.h

@@ -260,9 +260,12 @@ uint32_t mbedtls_platform_random_in_range( size_t num );
 int mbedtls_platform_random_delay( size_t num );
 
 /**
- * \brief       This function does nothing, but can be inserted between
- *              successive reads to a volatile local variable to prevent
- *              compilers from optimizing them away.
+ * \brief       This function can be inserted between successive reads to a
+ *              volatile local variable to prevent compilers from optimizing
+ *              them away. In addition, this function will spent a small random
+ *              time in a busy loop as a counter-measure to fault injection
+ *              attack.
+ *
  */
 void mbedtls_platform_enforce_volatile_reads( void );
 

library/platform_util.c

@@ -189,13 +189,10 @@ int mbedtls_platform_random_delay( size_t max_rand )
 #endif /* !MBEDTLS_ENTROPY_HARDWARE_ALT */
 }
 
-/* Some compilers (armcc 5 for example) optimize away successive reads from a
- * volatile local variable (which we use as a counter-measure to fault
- * injection attacks), unless there is a call to an external function between
- * them. This functions doesn't need to do anything, it just needs to be
- * in another compilation unit. So here's a function that does nothing. */
 void mbedtls_platform_enforce_volatile_reads( void )
 {
+    // Add a small random delay as a counter-measure to fault injection attack.
+    mbedtls_platform_random_delay( 50 );
 }
 
 #if defined(MBEDTLS_HAVE_TIME_DATE) && !defined(MBEDTLS_PLATFORM_GMTIME_R_ALT)