Add secp256k1_memclear() for clearing secret data

We rely on memset() and an __asm__ memory barrier where it's available or
on SecureZeroMemory() on Windows. The fallback implementation uses a
volatile function pointer to memset which the compiler is not clever
enough to optimize.

src/util.h

@@ -8,11 +8,17 @@
 #define SECP256K1_UTIL_H
 
 #include "../include/secp256k1.h"
+#include "checkmem.h"
 
+#include <string.h>
 #include <stdlib.h>
 #include <stdint.h>
 #include <stdio.h>
 #include <limits.h>
+#if defined(_MSC_VER)
+/* For SecureZeroMemory */
+#include <Windows.h>
+#endif
 
 #define STR_(x) #x
 #define STR(x) STR_(x)
@@ -221,6 +227,34 @@ static SECP256K1_INLINE void secp256k1_memczero(void *s, size_t len, int flag) {
     }
 }
 
+/* Cleanses memory to prevent leaking sensitive info. Won't be optimized out. */
+static SECP256K1_INLINE void secp256k1_memclear(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 <agl@google.com> 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 is 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
+#ifdef VERIFY
+    SECP256K1_CHECKMEM_UNDEFINE(ptr, len);
+#endif
+}
+
 /** Semantics like memcmp. Variable-time.
  *
  * We use this to avoid possible compiler bugs with memcmp, e.g.