Provide access to the pinsets during the TLS verification callback

We do this by associating a getdns_upstream object with the SSL object
handled by that upstream.

This allows us to collapse the verification callback code to a single
function.

Note that if we've agreed that fallback is ok, we are now willing to
accept *any* cert verification error, not just HOSTNAME_MISMATCH.
This is fine, because the alternative is falling back to cleartext,
which would be worse.

We also always set SSL_VERIFY_PEER, since we might as well try to do
so; we'll drop the verification error ourselves if we know we're OK
with falling back.

src/context.h

@@ -149,6 +149,7 @@ typedef struct getdns_upstream {
 	unsigned has_prev_client_cookie : 1;
 	unsigned has_server_cookie : 1;
 	unsigned server_cookie_len : 5;
+	unsigned tls_fallback_ok : 1;
 
 	/* TSIG */
 	uint8_t          tsig_dname[256];

src/pubkey-pinning.c

@@ -308,5 +308,54 @@ _getdns_get_pubkey_pinset_list(getdns_context *ctx,
 	return r;
 }
 
+/* this should only happen once ever in the life of the library. it's
+   used to associate a getdns_context_t with an SSL_CTX, to be able to
+   do custom verification.
+   
+   see doc/HOWTO/proxy_certificates.txt as an example
+*/
+static int
+_get_ssl_getdns_upstream_idx()
+{
+	static volatile int idx = -1;
+	if (idx < 0) {
+		CRYPTO_w_lock(CRYPTO_LOCK_X509_STORE);
+		if (idx < 0)
+			idx = SSL_get_ex_new_index(0, "associated getdns upstream",
+						   NULL,NULL,NULL);
+		CRYPTO_w_unlock(CRYPTO_LOCK_X509_STORE);
+	}
+	return idx;
+}
+
+getdns_upstream*
+_getdns_upstream_from_x509_store(X509_STORE_CTX *store)
+{
+	int uidx = _get_ssl_getdns_upstream_idx();
+	int sslidx = SSL_get_ex_data_X509_STORE_CTX_idx();
+	const SSL *ssl;
+
+	/* all *_get_ex_data() should return NULL on failure anyway */
+	ssl = X509_STORE_CTX_get_ex_data(store, sslidx);
+	if (ssl)
+		return (getdns_upstream*) SSL_get_ex_data(ssl, uidx);
+	else
+		return NULL;
+	/* TODO: if we want more details about errors somehow, we
+	 * might call ERR_get_error (see CRYPTO_set_ex_data(3ssl))*/
+}
+
+getdns_return_t
+_getdns_associate_upstream_with_SSL(SSL *ssl,
+				    getdns_upstream *upstream)
+{
+	int uidx = _get_ssl_getdns_upstream_idx();
+	if (SSL_set_ex_data(ssl, uidx, upstream))
+		return GETDNS_RETURN_GOOD;
+	else
+		return GETDNS_RETURN_GENERIC_ERROR;
+	/* TODO: if we want more details about errors somehow, we
+	 * might call ERR_get_error (see CRYPTO_set_ex_data(3ssl))*/
+}
 
 /* pubkey-pinning.c */

src/pubkey-pinning.h

@@ -48,6 +48,18 @@ _getdns_get_pubkey_pinset_list(getdns_context *ctx,
 			       const sha256_pin_t *pinset_in,
 			       getdns_list **pinset_list);
 
+
+/* internal functions for associating X.509 verification processes in
+ * OpenSSL with getdns_upstream objects. */
+
+getdns_upstream*
+_getdns_upstream_from_x509_store(X509_STORE_CTX *store);
+
+
+getdns_return_t
+_getdns_associate_upstream_with_SSL(SSL *ssl,
+				    getdns_upstream *upstream);
+
 	
 #endif
 /* pubkey-pinning.h */

src/stub.c

@@ -47,6 +47,7 @@
 #include "context.h"
 #include "util-internal.h"
 #include "general.h"
+#include "pubkey-pinning.h"
 
 #define STUB_OUT_OF_OPTIONS -5 /* upstream options exceeded MAXIMUM_UPSTREAM_OPTION_SPACE */
 #define STUB_TLS_SETUP_ERROR -4
@@ -841,40 +842,20 @@ tls_auth_status_ok(getdns_upstream *upstream, getdns_network_req *netreq) {
 int
 tls_verify_callback(int preverify_ok, X509_STORE_CTX *ctx)
 {
-#if defined(STUB_DEBUG) && STUB_DEBUG
 	int     err;
-	const char * err_str;
-
-	err = X509_STORE_CTX_get_error(ctx);
-	err_str = X509_verify_cert_error_string(err);
-	DEBUG_STUB("--- %s, VERIFY RESULT: %s\n", __FUNCTION__, err_str);
-#endif
-	/*Always proceed without changing result*/
-	return preverify_ok;
-}
-
-int
-tls_verify_callback_with_fallback(int preverify_ok, X509_STORE_CTX *ctx)
-{
-#ifdef X509_V_ERR_HOSTNAME_MISMATCH
-	int     err;
-# if defined(STUB_DEBUG) && STUB_DEBUG
-	const char * err_str;
-# endif
+	getdns_upstream *upstream;
 	
 	err = X509_STORE_CTX_get_error(ctx);
-# if defined(STUB_DEBUG) && STUB_DEBUG
-	err_str = X509_verify_cert_error_string(err);
-	DEBUG_STUB("--- %s, VERIFY RESULT: (%d) \"%s\"\n", __FUNCTION__, err, err_str);
-# endif
+	upstream = _getdns_upstream_from_x509_store(ctx);
+	DEBUG_STUB("--- %s, VERIFY RESULT: (%d) \"%s\"\n", __FUNCTION__,
+		   err, X509_verify_cert_error_string(err));
+
+#ifdef X509_V_ERR_HOSTNAME_MISMATCH
 	/*Proceed if error is hostname mismatch*/
-	if (err == X509_V_ERR_HOSTNAME_MISMATCH) {
+	if (upstream && upstream->tls_fallback_ok && err == X509_V_ERR_HOSTNAME_MISMATCH)
 		DEBUG_STUB("--- %s, PROCEEDING WITHOUT HOSTNAME VALIDATION!!\n", __FUNCTION__);
-		return 1;
-	}
-	else
 #endif
-		return preverify_ok;
+	return (upstream && upstream->tls_fallback_ok) ? 1 : preverify_ok;
 }
 
 static SSL*
@@ -892,11 +873,17 @@ tls_create_object(getdns_dns_req *dnsreq, int fd, getdns_upstream *upstream)
 		SSL_free(ssl);
 		return NULL;
 	}
+	/* make sure we'll be able to find the context again when we need it */
+	if (_getdns_associate_upstream_with_SSL(ssl, upstream) != GETDNS_RETURN_GOOD) {
+		SSL_free(ssl);
+		return NULL;
+	}
 
 	/* NOTE: this code will fallback on a given upstream, without trying
 	   authentication on other upstreams first. This is non-optimal and but avoids
 	   multiple TLS handshakes before getting a usable connection. */
 
+	upstream->tls_fallback_ok = 0;
 	/* If we have a hostname, always use it */
 	if (upstream->tls_auth_name[0] != '\0') {
 		/*Request certificate for the auth_name*/
@@ -920,12 +907,8 @@ tls_create_object(getdns_dns_req *dnsreq, int fd, getdns_upstream *upstream)
 		}
 #endif
 		/* Allow fallback to opportunistic if settings permit it*/
-		if (dnsreq->netreqs[0]->tls_auth_min == GETDNS_AUTHENTICATION_HOSTNAME)
-			SSL_set_verify(ssl, SSL_VERIFY_PEER, tls_verify_callback);
-		else {
-			SSL_set_verify(ssl, SSL_VERIFY_NONE, tls_verify_callback_with_fallback);
-			SSL_set_cipher_list(ssl, "DEFAULT");
-		}
+		if (dnsreq->netreqs[0]->tls_auth_min != GETDNS_AUTHENTICATION_HOSTNAME)
+			upstream->tls_fallback_ok = 1;
 	} else {
 		/* Lack of host name is OK unless only authenticated TLS is specified*/
 		if (dnsreq->netreqs[0]->tls_auth_min == GETDNS_AUTHENTICATION_HOSTNAME) {
@@ -937,10 +920,12 @@ tls_create_object(getdns_dns_req *dnsreq, int fd, getdns_upstream *upstream)
                   /* no hostname verification, so we will make opportunistic connections */
 			DEBUG_STUB("--- %s, PROCEEDING WITHOUT HOSTNAME VALIDATION!!\n", __FUNCTION__);
 			upstream->tls_auth_failed = 1;
-			SSL_set_verify(ssl, SSL_VERIFY_NONE, tls_verify_callback_with_fallback);
-			SSL_set_cipher_list(ssl, "DEFAULT");
+			upstream->tls_fallback_ok = 1;
 		}
 	}
+	if (upstream->tls_fallback_ok)
+		SSL_set_cipher_list(ssl, "DEFAULT");
+	SSL_set_verify(ssl, SSL_VERIFY_PEER, tls_verify_callback);
 	
 	SSL_set_connect_state(ssl);
 	(void) SSL_set_mode(ssl, SSL_MODE_AUTO_RETRY);