TLS server support SNI based certificate selection (#596)

* tls: support multiple TLS server certificates

Multiple certificates combined with private key and host name for the host name
verification can be added with a new function `tls_add_certf()`.

For incoming SIP requests a certificate is selected by SNI in the TLS client
hello before `SSL_accept()` is called.

CMakeLists.txt

@@ -471,6 +471,7 @@ if(USE_OPENSSL)
     src/tls/openssl/tls_tcp.c
     src/tls/openssl/tls_udp.c
     src/tls/openssl/tls.c
+    src/tls/openssl/sni.c
     src/hmac/openssl/hmac.c
   )
 endif()

include/re_tls.h

@@ -120,3 +120,4 @@ struct evp_pkey_st;
 
 int tls_set_certificate_openssl(struct tls *tls, struct x509_st *cert,
 				struct evp_pkey_st *pkey, bool up_ref);
+int tls_add_certf(struct tls *tls, const char *certf, const char *host);

src/tls/openssl/sni.c

@@ -0,0 +1,229 @@
+/**
+ * @file openssl/sni.c Server Name Indication API
+ *
+ * Copyright (C) 2022 Commend.com - [email protected]
+ */
+#include <openssl/tls1.h>
+#include <string.h>
+#include <openssl/err.h>
+#include <openssl/ssl.h>
+#include <openssl/x509.h>
+#include <openssl/x509v3.h>
+#include <re_types.h>
+#include <re_fmt.h>
+#include <re_mem.h>
+#include <re_sa.h>
+#include <re_list.h>
+#include <re_srtp.h>
+#include <re_tls.h>
+#include "tls.h"
+
+
+#define DEBUG_MODULE "tls"
+#define DEBUG_LEVEL 5
+#include <re_dbg.h>
+
+
+struct tls_conn;
+
+
+static int x509_match_alt_name(X509 *x509, const char *sni, bool *match)
+{
+	GENERAL_NAMES *gs = NULL;
+	ASN1_STRING *astr = NULL;
+	ASN1_OCTET_STRING *octet = NULL;
+	int err = 0;
+
+	*match = false;
+	gs = X509_get_ext_d2i(x509, NID_subject_alt_name, NULL, NULL);
+	if (!gs)
+		return 0;
+
+	for (int i = 0; i < sk_GENERAL_NAME_num(gs); i++) {
+		GENERAL_NAME *g = sk_GENERAL_NAME_value(gs, i);
+
+		if (g->type == GEN_DNS) {
+			astr = ASN1_IA5STRING_new();
+			if (!astr) {
+				err = ENOMEM;
+				goto out;
+			}
+
+			if (!ASN1_STRING_set(astr, sni, -1)) {
+				err = ENOMEM;
+				goto out;
+			}
+
+			if (!ASN1_STRING_cmp(astr, g->d.dNSName)) {
+				*match = true;
+				break;
+			}
+		}
+		else if (g->type == GEN_IPADD) {
+			octet = a2i_IPADDRESS(sni);
+			if (!ASN1_OCTET_STRING_cmp(octet, g->d.iPAddress)) {
+				*match = true;
+				break;
+			}
+		}
+	}
+
+out:
+	ASN1_IA5STRING_free(astr);
+	ASN1_OCTET_STRING_free(octet);
+	return err;
+}
+
+
+/**
+ * Finds a TLS certificate that matches a given Server Name Indication (SNI)
+ *
+ * @param tls TLS Context
+ * @param sni Server Name Indication
+ *
+ * @return TLS certificate or NULL if not found
+ */
+struct tls_cert *tls_cert_for_sni(const struct tls *tls, const char *sni)
+{
+	struct tls_cert *tls_cert = NULL;
+	struct le *le;
+	size_t sz;
+	char *cn;
+	const struct list *certs = tls_certs(tls);
+
+	if (!certs)
+		return NULL;
+
+	if (!str_isset(sni))
+		return list_head(certs)->data;
+
+	sz = str_len(sni) + 1;
+	if (sz > TLSEXT_MAXLEN_host_name)
+		return NULL;
+
+	cn = mem_zalloc(sz, NULL);
+	LIST_FOREACH(certs, le) {
+		X509 *x509;
+		X509_NAME *nm;
+		bool match = false;
+		int err;
+
+		tls_cert = le->data;
+		x509 = tls_cert_x509(tls_cert);
+		if (!x509) {
+			tls_cert = NULL;
+			continue;
+		}
+
+		nm = X509_get_subject_name(x509);
+		X509_NAME_get_text_by_NID(nm, NID_commonName, cn, (int) sz);
+		if (!str_cmp(sni, cn))
+			break;
+
+		err = x509_match_alt_name(x509, sni, &match);
+		if (err) {
+			tls_cert = NULL;
+			break;
+		}
+
+		if (match)
+			break;
+	}
+
+	mem_deref(cn);
+	ERR_clear_error();
+	return tls_cert;
+}
+
+
+static int ssl_set_verify_client(SSL *ssl, const char *host)
+{
+#if OPENSSL_VERSION_NUMBER >= 0x10100000L && \
+	!defined(LIBRESSL_VERSION_NUMBER)
+	struct sa sa;
+
+	if (!ssl || !host)
+		return EINVAL;
+
+	if (sa_set_str(&sa, host, 0)) {
+		SSL_set_hostflags(ssl,
+				X509_CHECK_FLAG_NO_PARTIAL_WILDCARDS);
+
+		if (!SSL_set1_host(ssl, host)) {
+			DEBUG_WARNING("SSL_set1_host error\n");
+			ERR_clear_error();
+			return EPROTO;
+		}
+	}
+
+	SSL_set_verify(ssl, SSL_VERIFY_PEER, tls_verify_handler);
+
+	return 0;
+#else
+	(void)tc;
+	(void)host;
+
+	return ENOSYS;
+#endif
+}
+
+
+static int ssl_use_cert(SSL *ssl, struct tls_cert *uc)
+{
+	int err;
+	long r;
+
+#if !defined(LIBRESSL_VERSION_NUMBER)
+	SSL_certs_clear(ssl);
+#endif
+	r = SSL_clear_chain_certs(ssl);
+	if (r != 1)
+		return EINVAL;
+
+	r = SSL_use_cert_and_key(ssl, tls_cert_x509(uc), tls_cert_pkey(uc),
+				 tls_cert_chain(uc), 1);
+	if (r != 1) {
+		ERR_clear_error();
+		return EINVAL;
+	}
+
+	err = ssl_set_verify_client(ssl, tls_cert_host(uc));
+	return err;
+}
+
+
+static int ssl_servername_handler(SSL *ssl, int *al, void *arg)
+{
+	struct tls *tls = arg;
+	struct tls_cert *uc = NULL;
+	const char *sni;
+	(void)al;
+
+	sni = SSL_get_servername(ssl, TLSEXT_NAMETYPE_host_name);
+	if (!str_isset(sni))
+		goto out;
+
+	/* find and apply matching certificate */
+	uc = tls_cert_for_sni(tls, sni);
+	if (!uc)
+		goto out;
+
+	DEBUG_INFO("found cert for sni %s\n", sni);
+	(void)ssl_use_cert(ssl, uc);
+
+out:
+	return SSL_TLSEXT_ERR_OK;
+}
+
+
+/**
+ * Enables SNI handling on the given TLS context for incoming TLS connections
+ *
+ * @param tls TLS Context
+ */
+void tls_enable_sni(struct tls *tls)
+{
+	SSL_CTX_set_tlsext_servername_callback(tls_ssl_ctx(tls),
+					       ssl_servername_handler);
+	SSL_CTX_set_tlsext_servername_arg(tls_ssl_ctx(tls), tls);
+}