Add "protocols" options to the "tls" clause

This commit adds the ability to specify allowed TLS protocols versions
within the "tls" clause. If an unsupported TLS protocol version is
specified in a file, the configuration file will not pass
verification.

Also, this commit adds strict checks for "tls" clauses verification,
in particular:

- it ensures that loading configuration files containing duplicated
"tls" clauses is not allowed;

- it ensures that loading configuration files containing "tls" clauses
missing "cert-file" or "key-file" is not allowed;

- it ensures that loading configuration files containing "tls" clauses
named as "ephemeral" or "none" is not allowed.

lib/isc/tls.c

@@ -10,6 +10,7 @@
  */
 
 #include <inttypes.h>
+#include <string.h>
 #if HAVE_LIBNGHTTP2
 #include <nghttp2/nghttp2.h>
 #endif /* HAVE_LIBNGHTTP2 */
@@ -364,6 +365,101 @@ ssl_error:
 	return (ISC_R_TLSERROR);
 }
 
+static long
+get_tls_version_disable_bit(const isc_tls_protocol_version_t tls_ver) {
+	long bit = 0;
+
+	switch (tls_ver) {
+	case ISC_TLS_PROTO_VER_1_2:
+#ifdef SSL_OP_NO_TLSv1_2
+		bit = SSL_OP_NO_TLSv1_2;
+#else
+		bit = 0;
+#endif
+		break;
+	case ISC_TLS_PROTO_VER_1_3:
+#ifdef SSL_OP_NO_TLSv1_3
+		bit = SSL_OP_NO_TLSv1_3;
+#else
+		bit = 0;
+#endif
+		break;
+	default:
+		INSIST(0);
+		ISC_UNREACHABLE();
+		break;
+	};
+
+	return (bit);
+}
+
+bool
+isc_tls_protocol_supported(const isc_tls_protocol_version_t tls_ver) {
+	return (get_tls_version_disable_bit(tls_ver) != 0);
+}
+
+isc_tls_protocol_version_t
+isc_tls_protocol_name_to_version(const char *name) {
+	REQUIRE(name != NULL);
+
+	if (strcasecmp(name, "TLSv1.2") == 0) {
+		return (ISC_TLS_PROTO_VER_1_2);
+	} else if (strcasecmp(name, "TLSv1.3") == 0) {
+		return (ISC_TLS_PROTO_VER_1_3);
+	}
+
+	return (ISC_TLS_PROTO_VER_UNDEFINED);
+}
+
+void
+isc_tlsctx_set_protocols(isc_tlsctx_t *ctx, const uint32_t tls_versions) {
+	REQUIRE(ctx != NULL);
+	REQUIRE(tls_versions != 0);
+	long set_options = 0;
+	long clear_options = 0;
+	uint32_t versions = tls_versions;
+
+	/*
+	 * The code below might be initially hard to follow because of the
+	 * double negation that OpenSSL enforces.
+	 *
+	 * Taking into account that OpenSSL provides bits to *disable*
+	 * specific protocol versions, like SSL_OP_NO_TLSv1_2,
+	 * SSL_OP_NO_TLSv1_3, etc., the code has the following logic:
+	 *
+	 * If a protocol version is not specified in the bitmask, get the
+	 * bit that disables it and add it to the set of TLS options to
+	 * set ('set_options'). Otherwise, if a protocol version is set,
+	 * add the bit to the set of options to clear ('clear_options').
+	 */
+
+	/* TLS protocol versions are defined as powers of two. */
+	for (uint32_t tls_ver = ISC_TLS_PROTO_VER_1_2;
+	     tls_ver < ISC_TLS_PROTO_VER_UNDEFINED; tls_ver <<= 1)
+	{
+		/* Only supported versions should ever be passed to the
+		 * function. The configuration file was not verified
+		 * properly, if we are trying to enable an unsupported
+		 * TLS version */
+		INSIST(isc_tls_protocol_supported(tls_ver));
+		if ((tls_versions & tls_ver) == 0) {
+			set_options |= get_tls_version_disable_bit(tls_ver);
+		} else {
+			clear_options |= get_tls_version_disable_bit(tls_ver);
+		}
+		versions &= ~(tls_ver);
+	}
+
+	/* All versions should be processed at this point, thus the value
+	 * must equal zero. If it is not, then some garbage has been
+	 * passed to the function; this situation is worth
+	 * investigation. */
+	INSIST(versions == 0);
+
+	(void)SSL_CTX_set_options(ctx, set_options);
+	(void)SSL_CTX_clear_options(ctx, clear_options);
+}
+
 isc_tls_t *
 isc_tls_create(isc_tlsctx_t *ctx) {
 	isc_tls_t *newctx = NULL;