src: enable clear memory on all platforms

- convert `_libssh2_explicit_zero()` to macro. This allows inlining
  where supported (e.g. `SecureZeroMemory()`).

- replace `SecureZeroMemory()` (in `wincng.c`) and
  `LIBSSH2_CLEAR_MEMORY`-guarded `memset()` (in `os400qc3.c`) with
  `_libssh2_explicit_zero()` macro.

- delete `LIBSSH2_CLEAR_MEMORY` guards, which enables secure-zeroing
  universally.

- add `LIBSSH2_NO_CLEAR_MEMORY` option to disable secure-zeroing.

- while here, delete double/triple inclusion of `misc.h`.
  `libssh2_priv.h` included it already.

Closes #810

acinclude.m4

@@ -456,7 +456,6 @@ m4_case([$1],
     AC_DEFINE(LIBSSH2_MBEDTLS, 1, [Use $1])
     LIBS="$LIBS -lmbedcrypto"
     found_crypto="$1"
-    support_clear_memory=yes
   ])
 ],
 

configure.ac

@@ -83,7 +83,6 @@ AC_SYS_LARGEFILE
 
 found_crypto=none
 found_crypto_str=""
-support_clear_memory=no
 crypto_errors=""
 
 m4_set_add([crypto_backends], [openssl])
@@ -176,25 +175,11 @@ fi
 AC_ARG_ENABLE(clear-memory,
   AC_HELP_STRING([--disable-clear-memory],[Disable clearing of memory before being freed]),
   [CLEAR_MEMORY=$enableval])
-if test "$CLEAR_MEMORY" != "no"; then
-  if test "$support_clear_memory" = "yes"; then
-    AC_DEFINE(LIBSSH2_CLEAR_MEMORY, 1, [Enable clearing of memory before being freed])
-    enable_clear_memory=yes
-  else
-    if test "$CLEAR_MEMORY" = "yes"; then
-      AC_MSG_ERROR([secure clearing/zeroing of memory is not supported by the selected crypto backend])
-    else
-      AC_MSG_WARN([secure clearing/zeroing of memory is not supported by the selected crypto backend])
-    fi
-    enable_clear_memory=unsupported
-  fi
-else
-  if test "$support_clear_memory" = "yes"; then
+if test "$CLEAR_MEMORY" = "no"; then
+  AC_DEFINE(LIBSSH2_NO_CLEAR_MEMORY, 1, [Disable clearing of memory before being freed])
   enable_clear_memory=no
-  else
-    AC_MSG_WARN([secure clearing/zeroing of memory is not supported by the selected crypto backend])
-    enable_clear_memory=unsupported
-  fi
+else
+  enable_clear_memory=yes
 fi
 
 dnl ************************************************************

os400/libssh2_config.h

@@ -185,9 +185,6 @@
 /* to make a symbol visible */
 #undef LIBSSH2_API
 
-/* Enable clearing of memory before being freed */
-#define LIBSSH2_CLEAR_MEMORY 1
-
 /* Enable "none" cipher -- NOT RECOMMENDED */
 #undef LIBSSH2_CRYPT_NONE
 

src/CMakeLists.txt

@@ -236,9 +236,9 @@ target_include_directories(libssh2
 ## Options
 
 option(CLEAR_MEMORY "Enable clearing of memory before being freed" ON)
-if(CLEAR_MEMORY)
-    add_definitions(-DLIBSSH2_CLEAR_MEMORY)
-endif(CLEAR_MEMORY)
+if(NOT CLEAR_MEMORY)
+    add_definitions(-DLIBSSH2_NO_CLEAR_MEMORY)
+endif()
 
 add_feature_info("Shared library" BUILD_SHARED_LIBS
   "creating libssh2 as a shared library (.so/.dll)")

src/agent.c

@@ -39,7 +39,6 @@
 
 #include "libssh2_priv.h"
 #include "agent.h"
-#include "misc.h"
 #include <errno.h>
 #ifdef HAVE_SYS_UN_H
 #include <sys/un.h>

src/agent.h

@@ -40,7 +40,6 @@
  */
 
 #include "libssh2_priv.h"
-#include "misc.h"
 #include "session.h"
 #ifdef WIN32
 #include <stdlib.h>

src/agent_win.c

@@ -39,7 +39,6 @@
 
 #include "libssh2_priv.h"
 #include "agent.h"
-#include "misc.h"
 #include <errno.h>
 #ifdef HAVE_SYS_UN_H
 #include <sys/un.h>

src/hostkey.c

@@ -37,7 +37,6 @@
  */
 
 #include "libssh2_priv.h"
-#include "misc.h"
 
 /* Needed for struct iovec on some platforms */
 #ifdef HAVE_SYS_UIO_H

src/knownhost.c

@@ -37,7 +37,6 @@
  */
 
 #include "libssh2_priv.h"
-#include "misc.h"
 
 struct known_host {
     struct list_node node;

src/libssh2_priv.h

@@ -59,17 +59,6 @@
 #    define LIBSSH2_WINDOWS_APP
 #  endif
 # endif
-
-/* TODO: Enable this unconditionally for all platforms.
-         Also delete autotools logic that enables it only for mbedTLS.
-         And CMake logic which already enabled it unconditionally.
-         The actual memory clearing logic uses SecureZeroMemory(),
-         memset_s() or plain memset(), whichever is available, and
-         does not depend on any crypto backend function. */
-#ifndef LIBSSH2_CLEAR_MEMORY
-#define LIBSSH2_CLEAR_MEMORY
-#endif
-
 #endif
 
 #ifdef HAVE_WS2TCPIP_H
@@ -117,7 +106,7 @@
 #include "libssh2.h"
 #include "libssh2_publickey.h"
 #include "libssh2_sftp.h"
-#include "misc.h" /* for the linked list stuff */
+#include "misc.h"
 
 #ifndef FALSE
 #define FALSE 0

src/mbedtls.c

@@ -96,12 +96,8 @@ _libssh2_mbedtls_safe_free(void *buf, int len)
     if(!buf)
         return;
 
-#ifdef LIBSSH2_CLEAR_MEMORY
     if(len > 0)
         _libssh2_explicit_zero(buf, len);
-#else
-    (void)len;
-#endif
 
     mbedtls_free(buf);
 }

src/misc.c

@@ -739,20 +739,14 @@ void _libssh2_aes_ctr_increment(unsigned char *ctr,
     }
 }
 
-#if !defined(WIN32) && !defined(HAVE_MEMSET_S)
+#ifdef LIBSSH2_MEMZERO
 static void * (* const volatile memset_libssh)(void *, int, size_t) = memset;
-#endif
 
-void _libssh2_explicit_zero(void *buf, size_t size)
+void _libssh2_memzero(void *buf, size_t size)
 {
-#ifdef WIN32
-    SecureZeroMemory(buf, size);
-#elif defined(HAVE_MEMSET_S)
-    (void)memset_s(buf, size, 0, size);
-#else
     memset_libssh(buf, 0, size);
-#endif
 }
+#endif
 
 /* String buffer */
 

src/misc.h

@@ -38,6 +38,23 @@
  * OF SUCH DAMAGE.
  */
 
+#ifdef LIBSSH2_NO_CLEAR_MEMORY
+#define _libssh2_explicit_zero(buf, size) do { \
+                                              (void)buf; \
+                                              (void)size; \
+                                          } while(0)
+#else
+#ifdef WIN32
+#define _libssh2_explicit_zero(buf, size) SecureZeroMemory(buf, size)
+#elif defined(HAVE_MEMSET_S)
+#define _libssh2_explicit_zero(buf, size) (void)memset_s(buf, size, 0, size)
+#else
+#define LIBSSH2_MEMZERO
+void _libssh2_memzero(void *buf, size_t size);
+#define _libssh2_explicit_zero(buf, size) _libssh2_memzero(buf, size)
+#endif
+#endif
+
 struct list_head {
     struct list_node *last;
     struct list_node *first;
@@ -89,7 +106,6 @@ void _libssh2_store_bignum2_bytes(unsigned char **buf,
                                   const unsigned char *bytes,
                                   size_t len);
 void *_libssh2_calloc(LIBSSH2_SESSION *session, size_t size);
-void _libssh2_explicit_zero(void *buf, size_t size);
 
 struct string_buf* _libssh2_string_buf_new(LIBSSH2_SESSION *session);
 void _libssh2_string_buf_free(LIBSSH2_SESSION *session,

src/openssl.c

@@ -44,7 +44,6 @@
 #if defined(LIBSSH2_OPENSSL) || defined(LIBSSH2_WOLFSSL)
 
 #include <string.h>
-#include "misc.h"
 
 #ifndef EVP_MAX_BLOCK_LENGTH
 #define EVP_MAX_BLOCK_LENGTH 32

src/os400qc3.c

@@ -757,10 +757,9 @@ _libssh2_bn_free(_libssh2_bn *bn)
 {
     if(bn) {
         if(bn->bignum) {
-#ifdef LIBSSH2_CLEAR_MEMORY
             if(bn->length)
-                memset((char *) bn->bignum, 0, bn->length);
-#endif
+                _libssh2_explicit_zero(bn->bignum, bn->length);
+
             free(bn->bignum);
         }
 
@@ -781,10 +780,9 @@ _libssh2_bn_resize(_libssh2_bn *bn, size_t newlen)
     if(!bn->bignum)
         bignum = (unsigned char *) malloc(newlen);
     else {
-#ifdef LIBSSH2_CLEAR_MEMORY
         if(newlen < bn->length)
-            memset((char *) bn->bignum + newlen, 0, bn->length - newlen);
-#endif
+            _libssh2_explicit_zero(bn->bignum + newlen, bn->length - newlen);
+
         if(!newlen) {
             free((char *) bn->bignum);
             bn->bignum = NULL;

src/session.c

@@ -56,7 +56,6 @@
 #include "session.h"
 #include "channel.h"
 #include "mac.h"
-#include "misc.h"
 
 /* libssh2_default_alloc
  */

src/wincng.c

@@ -59,7 +59,6 @@
 #include <windows.h>
 #include <bcrypt.h>
 #include <math.h>
-#include "misc.h"
 
 #ifdef HAVE_STDLIB_H
 #include <stdlib.h>
@@ -435,12 +434,8 @@ _libssh2_wincng_safe_free(void *buf, int len)
     if(!buf)
         return;
 
-#ifdef LIBSSH2_CLEAR_MEMORY
     if(len > 0)
-        SecureZeroMemory(buf, len);
-#else
-    (void)len;
-#endif
+        _libssh2_explicit_zero(buf, len);
 
     free(buf);
 }
@@ -2086,11 +2081,9 @@ _libssh2_wincng_bignum_resize(_libssh2_bn *bn, unsigned long length)
     if(length == bn->length)
         return 0;
 
-#ifdef LIBSSH2_CLEAR_MEMORY
     if(bn->bignum && bn->length > 0 && length < bn->length) {
-        SecureZeroMemory(bn->bignum + length, bn->length - length);
+        _libssh2_explicit_zero(bn->bignum + length, bn->length - length);
     }
-#endif
 
     bignum = realloc(bn->bignum, length);
     if(!bignum)
@@ -2289,9 +2282,7 @@ _libssh2_wincng_bignum_from_bin(_libssh2_bn *bn, unsigned long len,
     if(offset > 0) {
         memmove(bn->bignum, bn->bignum + offset, length);
 
-#ifdef LIBSSH2_CLEAR_MEMORY
-        SecureZeroMemory(bn->bignum + length, offset);
-#endif
+        _libssh2_explicit_zero(bn->bignum + length, offset);
 
         bignum = realloc(bn->bignum, length);
         if(bignum) {