Turn off Windows process ACL restriction by default.

As documented in bug 'win-process-acl-finesse', we've had enough
assorted complaints about it breaking various non-malicious pieces of
Windows process interaction (ranging from git->plink integration to
screen readers for the vision-impaired) that I think it's more
sensible to set the process back to its default level of protection.

This precaution was never a fully effective protection anyway, due to
the race condition at process startup; the only properly effective
defence would have been to prevent malware running under the same user
ID as PuTTY in the first place, so in that sense, nothing has changed.
But people who want the arguable defence-in-depth advantage of the ACL
restriction can now turn it on with the '-restrict-acl' command-line
option, and it's up to them whether they can live with the assorted
inconveniences that come with it.

In the course of this change, I've centralised a bit more of the
restriction code into winsecur.c, to avoid repeating the error
handling in multiple places.

Recipe

@@ -54,8 +54,7 @@
 #         security grounds (although it will run fine on Win95-series
 #         OSes where there is no access control anyway).
 #       - SSH connection sharing is disabled.
-#       - There is no restriction of the process ACLs (on all versions
+#       - There is no support for restriction of the process ACLs.
-#         of Windows, without warning), as if UNPROTECT below were set.
 #
 #  - COMPAT=/DNO_MULTIMON (Windows only)
 #      Disables PuTTY's use of <multimon.h>, which is not available
@@ -108,10 +107,6 @@
 #  - XFLAGS=/DDEBUG
 #      Causes PuTTY to enable internal debugging.
 #
-#  - XFLAGS=/DUNPROTECT
-#      Disable tightened ACL on PuTTY process so that e.g. debuggers
-#      can attach to it.
-#
 #  - XFLAGS=/DMALLOC_LOG
 #      Causes PuTTY to emit a file called putty_mem.log, logging every
 #      memory allocation and free, so you can track memory leaks.

cmdline.c

@@ -609,6 +609,17 @@ int cmdline_process_param(const char *p, char *value,
 	conf_set_str(conf, CONF_proxy_telnet_command, value);
     }
 
+#ifdef _WINDOWS
+    /*
+     * Cross-tool options only available on Windows.
+     */
+    if (!strcmp(p, "-restrict-acl") || !strcmp(p, "-restrict_acl") ||
+        !strcmp(p, "-restrictacl")) {
+	RETURN(1);
+        restrict_process_acl();
+    }
+#endif
+
     return ret;			       /* unrecognised */
 }
 

doc/using.but

@@ -1010,3 +1010,24 @@ connection. It expects a shell command string as an argument.
 
 See \k{config-proxy-type} for more information on this, and on other
 proxy settings.
+
+\S2{using-cmdline-restrict-acl} \i\c{-restrict-acl}: restrict the
+Windows process ACL
+
+This option (on Windows only) causes PuTTY to try to lock down the
+operating system's access control on its own process. If this
+succeeds, it should present an extra obstacle to malware that has
+managed to run under the same user id as the PuTTY process, by
+preventing it from attaching to PuTTY using the same interfaces
+debuggers use and either reading sensitive information out of its
+memory or hijacking its network session.
+
+This option is not enabled by default, because this form of
+interaction between Windows programs has many legitimate uses,
+including accessibility software such as screen readers. Also, it
+cannot provide full security against this class of attack in any case,
+because PuTTY can only lock down its own ACL \e{after} it has started
+up, and malware could still get in if it attacks the process between
+startup and lockdown. So it trades away noticeable convenience, and
+delivers less real security than you might want. However, if you do
+want to make that tradeoff anyway, the option is available.

windows/window.c

@@ -403,21 +403,6 @@ int WINAPI WinMain(HINSTANCE inst, HINSTANCE prev, LPSTR cmdline, int show)
 	return 1;
     }
 
-    /*
-     * Protect our process
-     */
-    {
-#if !defined UNPROTECT && !defined NO_SECURITY
-        char *error = NULL;
-        if (! setprocessacl(error)) {
-            char *message = dupprintf("Could not restrict process ACL: %s",
-                                      error);
-	    logevent(NULL, message);
-            sfree(message);
-	    sfree(error);
-	}
-#endif
-    }
     /*
      * Process the command line.
      */

windows/winpgen.c

@@ -1517,7 +1517,7 @@ void cleanup_exit(int code)
 
 int WINAPI WinMain(HINSTANCE inst, HINSTANCE prev, LPSTR cmdline, int show)
 {
-    int argc;
+    int argc, i;
     char **argv;
     int ret;
 
@@ -1534,36 +1534,24 @@ int WINAPI WinMain(HINSTANCE inst, HINSTANCE prev, LPSTR cmdline, int show)
 
     split_into_argv(cmdline, &argc, &argv, NULL);
 
-    if (argc > 0) {
+    for (i = 0; i < argc; i++) {
-	if (!strcmp(argv[0], "-pgpfp")) {
+	if (!strcmp(argv[i], "-pgpfp")) {
 	    pgp_fingerprints();
-	    exit(1);
+	    return 1;
+        } else if (!strcmp(argv[i], "-restrict-acl") ||
+                   !strcmp(argv[i], "-restrict_acl") ||
+                   !strcmp(argv[i], "-restrictacl")) {
+            restrict_process_acl();
 	} else {
 	    /*
 	     * Assume the first argument to be a private key file, and
 	     * attempt to load it.
 	     */
-	    cmdline_keyfile = argv[0];
+	    cmdline_keyfile = argv[i];
+            break;
 	}
     }
 
-#if !defined UNPROTECT && !defined NO_SECURITY
-    /*
-     * Protect our process.
-     */
-    {
-        char *error = NULL;
-        if (!setprocessacl(error)) {
-            char *message = dupprintf("Could not restrict process ACL: %s",
-                                      error);
-            MessageBox(NULL, message, "PuTTYgen Warning",
-                       MB_ICONWARNING | MB_OK);
-            sfree(message);
-            sfree(error);
-        }
-    }
-#endif
-
     random_ref();
     ret = DialogBox(hinst, MAKEINTRESOURCE(201), NULL, MainDlgProc) != IDOK;
 

windows/winpgnt.c

@@ -1159,6 +1159,10 @@ int WINAPI WinMain(HINSTANCE inst, HINSTANCE prev, LPSTR cmdline, int show)
 	if (!strcmp(argv[i], "-pgpfp")) {
 	    pgp_fingerprints();
 	    return 1;
+        } else if (!strcmp(argv[i], "-restrict-acl") ||
+                   !strcmp(argv[i], "-restrict_acl") ||
+                   !strcmp(argv[i], "-restrictacl")) {
+            restrict_process_acl();
 	} else if (!strcmp(argv[i], "-c")) {
 	    /*
 	     * If we see `-c', then the rest of the
@@ -1178,23 +1182,6 @@ int WINAPI WinMain(HINSTANCE inst, HINSTANCE prev, LPSTR cmdline, int show)
 	}
     }
 
-#if !defined UNPROTECT && !defined NO_SECURITY
-    /*
-     * Protect our process.
-     */
-    {
-        char *error = NULL;
-        if (!setprocessacl(error)) {
-            char *message = dupprintf("Could not restrict process ACL: %s",
-                                      error);
-            MessageBox(NULL, message, "Pageant Warning",
-                       MB_ICONWARNING | MB_OK);
-            sfree(message);
-            sfree(error);
-        }
-    }
-#endif
-
     /*
      * Forget any passphrase that we retained while going over
      * command line keyfiles.

windows/winplink.c

@@ -502,22 +502,6 @@ int main(int argc, char **argv)
 	}
     }
 
-#if !defined UNPROTECT && !defined NO_SECURITY
-    /*
-     * Protect our process.
-     */
-    {
-        char *error = NULL;
-        if (!setprocessacl(error)) {
-            char *message = dupprintf("Could not restrict process ACL: %s",
-                                      error);
-            logevent(NULL, message);
-            sfree(message);
-            sfree(error);
-        }
-    }
-#endif
-
     if (errors)
 	return 1;
 

windows/winsecur.c

@@ -224,7 +224,7 @@ int make_private_security_descriptor(DWORD permissions,
     return ret;
 }
 
-int setprocessacl(char *error)
+static int really_restrict_process_acl(char *error)
 {
     EXPLICIT_ACCESS ea[2];
     int acl_err;
@@ -287,3 +287,35 @@ int setprocessacl(char *error)
     return ret;
 }  
 #endif /* !defined NO_SECURITY */
+
+/*
+ * Lock down our process's ACL, to present an obstacle to malware
+ * trying to write into its memory. This can't be a full defence,
+ * because well timed malware could attack us before this code runs -
+ * even if it was unconditionally run at the very start of main(),
+ * which we wouldn't want to do anyway because it turns out in practie
+ * that interfering with other processes in this way has significant
+ * non-infringing uses on Windows (e.g. screen reader software).
+ *
+ * If we've been requested to do this and are unsuccessful, bomb out
+ * via modalfatalbox rather than continue in a less protected mode.
+ *
+ * This function is intentionally outside the #ifndef NO_SECURITY that
+ * covers the rest of this file, because when PuTTY is compiled
+ * without the ability to restrict its ACL, we don't want it to
+ * silently pretend to honour the instruction to do so.
+ */
+void restrict_process_acl(void)
+{
+    char *error = NULL;
+    int ret;
+
+#if !defined NO_SECURITY
+    ret = really_restrict_process_acl(error);
+#else
+    ret = FALSE;
+    error = dupstr("ACL restrictions not compiled into this binary");
+#endif
+    if (!ret)
+        modalfatalbox("Could not restrict process ACL: %s", error);
+}

windows/winsecur.h

@@ -56,6 +56,4 @@ int make_private_security_descriptor(DWORD permissions,
                                      PACL *acl,
                                      char **error);
 
-int setprocessacl(char *error);
-
 #endif

windows/winsftp.c

@@ -749,21 +749,6 @@ char *ssh_sftp_get_cmdline(const char *prompt, int no_fds_ok)
 
 void platform_psftp_post_option_setup(void)
 {
-#if !defined UNPROTECT && !defined NO_SECURITY
-    /*
-     * Protect our process.
-     */
-    {
-        char *error = NULL;
-        if (!setprocessacl(error)) {
-            char *message = dupprintf("Could not restrict process ACL: %s",
-                                      error);
-            logevent(NULL, message);
-            sfree(message);
-            sfree(error);
-        }
-    }
-#endif
 }
 
 /* ----------------------------------------------------------------------

windows/winstuff.h

@@ -484,6 +484,7 @@ void dll_hijacking_protection(void);
 BOOL init_winver(void);
 HMODULE load_system32_dll(const char *libname);
 const char *win_strerror(int error);
+void restrict_process_acl(void);
 
 /*
  * Exports from sizetip.c.