Windows Pageant: turn 'has_security' into a global function.

Now it can be called from places other than Pageant's WinMain(). In
particular, the attempt to make a security descriptor in
lock_interprocess_mutex() is gated on it.

In return, however, I've tightened up the semantics. In normal PuTTY
builds that aren't trying to support pre-NT systems, the function
*unconditionally* returns true, on the grounds that we don't expect to
target any system that doesn't support the security APIs, and if
someone manages to contrive one anyway - or, more likely, if we some
day introduce a bug in our loading of the security API functions -
then this safety catch should make Pageant less likely to accidentally
fall back to 'never mind, just run in insecure mode'.

windows/pageant.c

@@ -91,8 +91,6 @@ void modalfatalbox(const char *fmt, ...)
     exit(1);
 }
 
-static bool has_security;
-
 struct PassphraseProcStruct {
     bool modal;
     const char *help_topic;
@@ -999,7 +997,7 @@ static char *answer_filemapping_message(const char *mapname)
     debug("maphandle = %p\n", maphandle);
 #endif
 
-    if (has_security) {
+    if (should_have_security()) {
         DWORD retd;
 
         if ((expectedsid = get_user_sid()) == NULL) {
@@ -1405,14 +1403,7 @@ int WINAPI WinMain(HINSTANCE inst, HINSTANCE prev, LPSTR cmdline, int show)
 
     hinst = inst;
 
-    /*
+    if (should_have_security()) {
-     * Determine whether we're an NT system (should have security
-     * APIs) or a non-NT system (don't do security).
-     */
-    init_winver();
-    has_security = (osPlatformId == VER_PLATFORM_WIN32_NT);
-
-    if (has_security) {
         /*
          * Attempt to get the security API we need.
          */
@@ -1543,9 +1534,10 @@ int WINAPI WinMain(HINSTANCE inst, HINSTANCE prev, LPSTR cmdline, int show)
         /*
          * Set up a named-pipe listener.
          */
-        Plug *pl_plug;
         wpc->plc.vt = &winpgnt_vtable;
         wpc->plc.suppress_logging = true;
+        if (should_have_security()) {
+            Plug *pl_plug;
             struct pageant_listen_state *pl =
                 pageant_listener_new(&pl_plug, &wpc->plc);
             char *pipename = agent_named_pipe_name();
@@ -1566,9 +1558,10 @@ int WINAPI WinMain(HINSTANCE inst, HINSTANCE prev, LPSTR cmdline, int show)
             if (openssh_config_file) {
                 FILE *fp = fopen(openssh_config_file, "w");
                 if (!fp) {
-                char *err = dupprintf("Unable to write OpenSSH config file "
+                    char *err = dupprintf("Unable to write OpenSSH config "
-                                      "to %s", openssh_config_file);
+                                          "file to %s", openssh_config_file);
-                MessageBox(NULL, err, "Pageant Error", MB_ICONERROR | MB_OK);
+                    MessageBox(NULL, err, "Pageant Error",
+                               MB_ICONERROR | MB_OK);
                     return 1;
                 }
                 fprintf(fp, "IdentityAgent %s\n", pipename);
@@ -1576,6 +1569,7 @@ int WINAPI WinMain(HINSTANCE inst, HINSTANCE prev, LPSTR cmdline, int show)
             }
 
             sfree(pipename);
+        }
 
         /*
          * Set up the window class for the hidden window that receives

windows/platform.h

@@ -574,6 +574,7 @@ void dll_hijacking_protection(void);
 const char *get_system_dir(void);
 HMODULE load_system32_dll(const char *libname);
 const char *win_strerror(int error);
+bool should_have_security(void);
 void restrict_process_acl(void);
 bool restricted_acl(void);
 void escape_registry_key(const char *in, strbuf *out);

windows/utils/interprocess_mutex.c

@@ -13,8 +13,8 @@ HANDLE lock_interprocess_mutex(const char *mutexname, char **error)
     PACL acl = NULL;
     HANDLE mutex = NULL;
 
-    if (!make_private_security_descriptor(MUTEX_ALL_ACCESS,
+    if (should_have_security() && !make_private_security_descriptor(
-                                          &psd, &acl, error))
+            MUTEX_ALL_ACCESS, &psd, &acl, error))
         goto out;
 
     SECURITY_ATTRIBUTES sa;

windows/utils/security.c

@@ -20,6 +20,20 @@ DEF_WINDOWS_FUNCTION(GetSecurityInfo);
 DEF_WINDOWS_FUNCTION(SetSecurityInfo);
 DEF_WINDOWS_FUNCTION(SetEntriesInAclA);
 
+bool should_have_security(void)
+{
+#ifdef LEGACY_WINDOWS
+    /* Legacy pre-NT platforms are not expected to have any of these APIs */
+    init_winver();
+    return (osPlatformId == VER_PLATFORM_WIN32_NT);
+#else
+    /* In the up-to-date PuTTY builds which do not support those
+     * platforms, unconditionally return true, to minimise the risk of
+     * compiling out security checks. */
+    return true;
+#endif
+}
+
 bool got_advapi(void)
 {
     static bool attempted = false;