Prevent dead-store elimination when clearing secrets in examples

Makefile.am

@@ -69,7 +69,7 @@ noinst_HEADERS += contrib/lax_der_parsing.h
 noinst_HEADERS += contrib/lax_der_parsing.c
 noinst_HEADERS += contrib/lax_der_privatekey_parsing.h
 noinst_HEADERS += contrib/lax_der_privatekey_parsing.c
-noinst_HEADERS += examples/random.h
+noinst_HEADERS += examples/examples_util.h
 
 PRECOMPUTED_LIB = libsecp256k1_precomputed.la
 noinst_LTLIBRARIES = $(PRECOMPUTED_LIB)

examples/ecdh.c

@@ -14,8 +14,7 @@
 #include <secp256k1.h>
 #include <secp256k1_ecdh.h>
 
-#include "random.h"
-
+#include "examples_util.h"
 
 int main(void) {
     unsigned char seckey1[32];
@@ -112,12 +111,12 @@ int main(void) {
      * example through "out of bounds" array access (see Heartbleed), Or the OS
      * swapping them to disk. Hence, we overwrite the secret key buffer with zeros.
      *
-     * TODO: Prevent these writes from being optimized out, as any good compiler
+     * Here we are preventing these writes from being optimized out, as any good compiler
      * will remove any writes that aren't used. */
-    memset(seckey1, 0, sizeof(seckey1));
-    memset(seckey2, 0, sizeof(seckey2));
-    memset(shared_secret1, 0, sizeof(shared_secret1));
-    memset(shared_secret2, 0, sizeof(shared_secret2));
+    secure_erase(seckey1, sizeof(seckey1));
+    secure_erase(seckey2, sizeof(seckey2));
+    secure_erase(shared_secret1, sizeof(shared_secret1));
+    secure_erase(shared_secret2, sizeof(shared_secret2));
 
     return 0;
 }

examples/ecdsa.c

@@ -13,9 +13,7 @@
 
 #include <secp256k1.h>
 
-#include "random.h"
-
-
+#include "examples_util.h"
 
 int main(void) {
     /* Instead of signing the message directly, we must sign a 32-byte hash.
@@ -125,9 +123,9 @@ int main(void) {
      * example through "out of bounds" array access (see Heartbleed), Or the OS
      * swapping them to disk. Hence, we overwrite the secret key buffer with zeros.
      *
-     * TODO: Prevent these writes from being optimized out, as any good compiler
+     * Here we are preventing these writes from being optimized out, as any good compiler
      * will remove any writes that aren't used. */
-    memset(seckey, 0, sizeof(seckey));
+    secure_erase(seckey, sizeof(seckey));
 
     return 0;
 }

examples/random.h → examples/examples_util.h

@@ -71,3 +71,32 @@ static void print_hex(unsigned char* data, size_t size) {
     }
     printf("\n");
 }
+
+#if defined(_MSC_VER)
+// For SecureZeroMemory
+#include <Windows.h>
+#endif
+/* Cleanses memory to prevent leaking sensitive info. Won't be optimized out. */
+static SECP256K1_INLINE void secure_erase(void *ptr, size_t len) {
+#if defined(_MSC_VER)
+    /* SecureZeroMemory is guaranteed not to be optimized out by MSVC. */
+    SecureZeroMemory(ptr, len);
+#elif defined(__GNUC__)
+    /* We use a memory barrier that scares the compiler away from optimizing out the memset.
+     *
+     * Quoting Adam Langley <[email protected]> in commit ad1907fe73334d6c696c8539646c21b11178f20f
+     * in BoringSSL (ISC License):
+     *    As best as we can tell, this is sufficient to break any optimisations that
+     *    might try to eliminate "superfluous" memsets.
+     * This method used in memzero_explicit() the Linux kernel, too. Its advantage is that it is
+     * pretty efficient, because the compiler can still implement the memset() efficently,
+     * just not remove it entirely. See "Dead Store Elimination (Still) Considered Harmful" by
+     * Yang et al. (USENIX Security 2017) for more background.
+     */
+    memset(ptr, 0, len);
+    __asm__ __volatile__("" : : "r"(ptr) : "memory");
+#else
+    void *(*volatile const volatile_memset)(void *, int, size_t) = memset;
+    volatile_memset(ptr, 0, len);
+#endif
+}

examples/schnorr.c

@@ -15,7 +15,7 @@
 #include <secp256k1_extrakeys.h>
 #include <secp256k1_schnorrsig.h>
 
-#include "random.h"
+#include "examples_util.h"
 
 int main(void) {
     unsigned char msg[12] = "Hello World!";
@@ -140,9 +140,8 @@ int main(void) {
      * example through "out of bounds" array access (see Heartbleed), Or the OS
      * swapping them to disk. Hence, we overwrite the secret key buffer with zeros.
      *
-     * TODO: Prevent these writes from being optimized out, as any good compiler
+     * Here we are preventing these writes from being optimized out, as any good compiler
      * will remove any writes that aren't used. */
-    memset(seckey, 0, sizeof(seckey));
-
+    secure_erase(seckey, sizeof(seckey));
     return 0;
 }