with --allow-insecure-crypto, additionally attempt to disable insecure...

with --allow-insecure-crypto, additionally attempt to disable insecure systemwide minimum crypto settings

Because openconnect_set_allow_insecure_crypto() now does more than just attempt to reenable 3DES and ARC4,
its failure to enable those ciphers should not be treated as fatal, but merely a warning.

Setting the appropriate environment variable (GNUTLS_SYSTEM_PRIORITY_FILE or OPENSSL_CONF) to `/dev/null`
*before* crypto library initialization should ensure that a systemwide crypto configuration file doesn't
set a minimum crypto requirement which would override the user choice.

See https://gitlab.com/openconnect/openconnect/-/issues/211#note_482161646 for discussion of GnuTLS
settings, and https://www.openssl.org/docs/man1.1.1/man5/config.html for OpenSSL.

FIXME: OpenSSL implementation needs library reinitialization.
Signed-off-by: Daniel Lenski <dlenski@gmail.com>

gnutls.c

@@ -78,12 +78,23 @@ const char *openconnect_get_tls_library_version()
 
 int can_enable_insecure_crypto()
 {
+	int ret = 0;
+
+	if (setenv("GNUTLS_SYSTEM_PRIORITY_FILE", DEVNULL, 1) < 0)
+		return -errno;
+
+	gnutls_global_deinit();
+	ret = openconnect_init_ssl();
+	if (ret)
+		return ret;
+
 	/* XX: As of GnuTLS 3.6.13, no released version has (yet) removed 3DES/RC4 from default builds,
 	 * but like OpenSSL (removed in 1.1.0) it may happen. */
 	if (gnutls_cipher_get_id("3DES-CBC") == GNUTLS_CIPHER_UNKNOWN ||
 	    gnutls_cipher_get_id("ARCFOUR-128") == GNUTLS_CIPHER_UNKNOWN)
-		return -ENOENT;
-	return 0;
+		ret = -ENOENT;
+
+	return ret;
 }
 
 /* Helper functions for reading/writing lines over TLS/DTLS. */

library.c

@@ -757,10 +757,8 @@ void openconnect_set_pfs(struct openconnect_info *vpninfo, unsigned val)
 int openconnect_set_allow_insecure_crypto(struct openconnect_info *vpninfo, unsigned val)
 {
 	int ret = can_enable_insecure_crypto();
-	if (ret)
-		return ret;
 	vpninfo->allow_insecure_crypto = val;
-	return 0;
+	return ret;
 }
 
 void openconnect_set_cancel_fd(struct openconnect_info *vpninfo, int fd)

main.c

@@ -924,6 +924,7 @@ static void usage(void)
 	printf("      --no-http-keepalive         %s\n", _("Disable HTTP connection re-use"));
 	printf("      --no-xmlpost                %s\n", _("Do not attempt XML POST authentication"));
 	printf("      --allow-insecure-crypto     %s\n", _("Allow use of the ancient, insecure 3DES and RC4 ciphers"));
+	printf("  				  %s\n", _("(and attempt to override OS crypto policies)"));
 
 	printf("\n");
 
@@ -1639,9 +1640,12 @@ int main(int argc, char **argv)
 			openconnect_set_pfs(vpninfo, 1);
 			break;
 		case OPT_ALLOW_INSECURE_CRYPTO:
-			if (openconnect_set_allow_insecure_crypto(vpninfo, 1)) {
-				fprintf(stderr, _("Cannot enable insecure 3DES or RC4 ciphers, because the library\n"
+			ret = openconnect_set_allow_insecure_crypto(vpninfo, 1);
+			if (ret == -ENOENT)
+				fprintf(stderr, _("WARNING: cannot enable insecure 3DES and/or RC4 ciphers, because the library\n"
 						  "%s no longer supports them.\n"), openconnect_get_tls_library_version());
+			else if (ret < 0) {
+				fprintf(stderr, _("Unknown error while enabling insecure crypto.\n"));
 				exit(1);
 			}
 			break;

openconnect-internal.h

@@ -41,6 +41,15 @@
 
 #include "openconnect.h"
 
+/* Equivalent of "/dev/null" on Windows.
+ * See https://stackoverflow.com/a/44163934
+ */
+#ifdef _WIN32
+#define DEVNULL "NUL"
+#else
+#define DEVNULL "/dev/null"
+#endif
+
 #if defined(OPENCONNECT_OPENSSL)
 #include <openssl/ssl.h>
 #include <openssl/err.h>
@@ -1069,7 +1078,7 @@ int do_gen_hotp_code(struct openconnect_info *vpninfo,
 		     struct oc_auth_form *form,
 		     struct oc_form_opt *opt);
 
-int set_oidc_token(struct openconnect_info *vpninfo, 
+int set_oidc_token(struct openconnect_info *vpninfo,
 		     const char *token_str);
 
 /* stoken.c */

openconnect.8.in

@@ -469,8 +469,11 @@ The ancient, broken 3DES and RC4 ciphers are insecure; we explicitly
 disable them by default. However, some still-in-use VPN servers can't do
 any better.
 
-This option enables use of these insecure ciphers, as well as the use
-of SHA1 for server certificate validation.
+This option
+.B attempts
+to enable use of these insecure ciphers, as well as
+the use of SHA1 for server certificate validation, and to override any
+other system policies regarding minimum crypto requirements.
 .TP
 .B \-\-non\-inter
 Do not expect user input; exit if it is required.

openssl.c

@@ -63,10 +63,23 @@ const char *openconnect_get_tls_library_version()
 
 int can_enable_insecure_crypto()
 {
+	int ret = 0;
+
+	if (setenv("OPENSSL_CONF", DEVNULL, 1) < 0)
+		return -errno;
+
+	/* FIXME: deinitialize and reinitialize library, as is done for GnuTLS,
+	 * to ensure that updated value is used.
+	 *
+	 * Cleaning up and reinitalizing OpenSSL appears to be complex:
+	 *   https://wiki.openssl.org/index.php/Library_Initialization#Cleanup
+	 */
+
 	if (EVP_des_ede3_cbc() == NULL ||
 	    EVP_rc4() == NULL)
-		return -ENOENT;
-	return 0;
+		ret = -ENOENT;
+
+	return ret;
 }
 
 int openconnect_sha1(unsigned char *result, void *data, int len)