fw: Protect zeroisation against compiler optimisation.

The memset() responsible for the zeroisation of the secure_ctx under
the compute_cdi() function in FW's main.c, was optimised away by the
compiler. Instead of using memset(), secure_wipe() is introduced
which uses a volatile keyword to prevent the compiler to try to
optimise it. Secure_wipe() is now used on all locations handling
removal of sensitive data.

hw/application_fpga/fw/tk1/lib.c

@@ -155,3 +155,10 @@ int memeq(void *dest, const void *src, size_t n)
 
 	return res;
 }
+
+void secure_wipe(void *v, size_t n)
+{
+	volatile uint8_t *p = (volatile uint8_t *)v;
+	while (n--)
+		*p++ = 0;
+}

hw/application_fpga/fw/tk1/lib.h

@@ -28,5 +28,5 @@ void *memset(void *dest, int c, unsigned n);
 void memcpy_s(void *dest, size_t destsize, const void *src, size_t n);
 void wordcpy_s(void *dest, size_t destsize, const void *src, size_t n);
 int memeq(void *dest, const void *src, size_t n);
-
+void secure_wipe(void *v, size_t n);
 #endif

hw/application_fpga/fw/tk1/main.c

@@ -115,7 +115,7 @@ static void compute_cdi(const uint8_t *digest, const uint8_t use_uss,
 	// while on the firmware stack which is in the special fw_ram.
 	wordcpy_s(local_uds, 8, (void *)uds, 8);
 	blake2s_update(&secure_ctx, (const void *)local_uds, 32);
-	(void)memset(local_uds, 0, 32);
+	(void)secure_wipe(local_uds, sizeof(local_uds));
 
 	// Update with TKey program digest
 	blake2s_update(&secure_ctx, digest, 32);
@@ -130,7 +130,7 @@ static void compute_cdi(const uint8_t *digest, const uint8_t use_uss,
 
 	// Clear secure_ctx of any residue of UDS. Don't want to keep
 	// that for long even though fw_ram is cleared later.
-	(void)memset(&secure_ctx, 0, sizeof(secure_ctx));
+	(void)secure_wipe(&secure_ctx, sizeof(secure_ctx));
 
 	// CDI only word writable
 	wordcpy_s((void *)cdi, 8, &local_cdi, 8);