mem: Added mem_secclean function.

The function can be used to securely clear a block of memory without
compiler optimizing away this call.

include/re_mem.h

@@ -26,7 +26,7 @@ void    *mem_zalloc(size_t size, mem_destroy_h *dh);
 void    *mem_realloc(void *data, size_t size);
 void    *mem_reallocarray(void *ptr, size_t nmemb,
 			  size_t membsize, mem_destroy_h *dh);
-void 	mem_destructor(void *data, mem_destroy_h *dh);
+void     mem_destructor(void *data, mem_destroy_h *dh);
 void    *mem_ref(void *data);
 void    *mem_deref(void *data);
 uint32_t mem_nrefs(const void *data);
@@ -40,3 +40,4 @@ int      mem_get_stat(struct memstat *mstat);
 
 /* Secure memory functions */
 int mem_seccmp(const uint8_t *s1, const uint8_t *s2, size_t n);
+void mem_secclean(void *data, size_t size);

src/mem/secure.c

@@ -4,9 +4,19 @@
  * Copyright (C) 2010 Creytiv.com
  */
 
+#include <string.h>
 #include <re_types.h>
 #include <re_mem.h>
-
+#if !defined(__GNUC__)
+#if defined(USE_OPENSSL)
+#include <openssl/crypto.h>
+#elif defined(WIN32)
+#if !defined(WIN32_LEAN_AND_MEAN)
+#define WIN32_LEAN_AND_MEAN
+#endif
+#include <windows.h>
+#endif
+#endif /* !defined(__GNUC__) */
 
 /**
  * Compare two byte strings in constant time. This function can be used
@@ -35,3 +45,35 @@ int mem_seccmp(const uint8_t *s1, const uint8_t *s2, size_t n)
 
 	return val;
 }
+
+
+#if !defined(__GNUC__) && !defined(USE_OPENSSL) && !defined(WIN32)
+/* Use a volatile pointer to memset to force the compiler always
+ * call it and not optimize away. */
+typedef void *(memset_t)(void *, int, size_t);
+static memset_t *const volatile memset_ptr = &memset;
+#endif
+
+/**
+ * Securely clean memory. This function is guaranteed not to get optimized
+ * away by compiler.
+ *
+ * @param data Pointer to data buffer
+ * @param size Size of the buffer
+ */
+void mem_secclean(void *data, size_t size)
+{
+#if defined(__GNUC__)
+    memset(data, 0, size);
+    /* Insert an asm statement that may potentially depend
+     * on the memory contents that were affected by memset.
+     * This prevents optimizing away the memset. */
+    __asm__ __volatile__("" : : "r" (data), "r" (size) : "memory");
+#elif defined(USE_OPENSSL)
+    OPENSSL_cleanse(data, size);
+#elif defined(WIN32)
+    SecureZeroMemory(data, size);
+#else
+    (*memset_ptr)(data, 0, size);
+#endif
+}