eap-authenticator: Enforce failure if MSK generation fails

Without this, the authentication succeeded if the server sent an early
EAP-Success message for mutual, key-generating EAP methods like EAP-TLS,
which may be used in EAP-only scenarios but would complete without server
or client authentication.  For clients configured for such EAP-only
scenarios, a rogue server could capture traffic after the tunnel is
established or even access hosts behind the client.  For non-mutual EAP
methods, public key server authentication has been enforced for a while.

A server previously could also crash a client by sending an EAP-Success
immediately without initiating an actual EAP method.

Fixes: 0706c39cda ("added support for EAP methods not establishing an MSK")
Fixes: CVE-2021-45079

src/libcharon/plugins/eap_gtc/eap_gtc.c

@@ -195,7 +195,7 @@ METHOD(eap_method_t, get_type, eap_type_t,
 METHOD(eap_method_t, get_msk, status_t,
 	private_eap_gtc_t *this, chunk_t *msk)
 {
-	return FAILED;
+	return NOT_SUPPORTED;
 }
 
 METHOD(eap_method_t, get_identifier, uint8_t,

src/libcharon/plugins/eap_md5/eap_md5.c

@@ -213,7 +213,7 @@ METHOD(eap_method_t, get_type, eap_type_t,
 METHOD(eap_method_t, get_msk, status_t,
 	private_eap_md5_t *this, chunk_t *msk)
 {
-	return FAILED;
+	return NOT_SUPPORTED;
 }
 
 METHOD(eap_method_t, is_mutual, bool,

src/libcharon/plugins/eap_radius/eap_radius.c

@@ -733,7 +733,9 @@ METHOD(eap_method_t, get_msk, status_t,
 		*out = msk;
 		return SUCCESS;
 	}
-	return FAILED;
+	/* we assume the selected method did not establish an MSK, if it failed
+	 * to establish one, process() would have failed */
+	return NOT_SUPPORTED;
 }
 
 METHOD(eap_method_t, get_identifier, uint8_t,