Disable SSLv3 by default.

As a precursor to removing the code entirely later, disable the protocol
by default. Callers must use SSL_CTX_set_min_version to enable it.

This change also makes SSLv3_method *not* enable SSL 3.0. Normally
version-specific methods set the minimum and maximum version to their
version. SSLv3_method leaves the minimum at the default, so we will
treat it as all versions disabled. To help debugging, the error code is
switched from WRONG_SSL_VERSION to a new NO_SUPPORTED_VERSIONS_ENABLED.

This also defines OPENSSL_NO_SSL3 and OPENSSL_NO_SSL3_METHOD to kick in
any no-ssl3 build paths in consumers which should provide a convenient
hook for any upstreaming changes that may be needed. (OPENSSL_NO_SSL3
existed in older versions of OpenSSL, so in principle one may encounter
an OpenSSL with the same settings.)

Change-Id: I96a8f2f568eb77b2537b3a774b2f7108bd67dd0c
Reviewed-on: https://boringssl-review.googlesource.com/14031
Reviewed-by: David Benjamin <davidben@google.com>
Commit-Queue: David Benjamin <davidben@google.com>
CQ-Verified: CQ bot account: commit-bot@chromium.org <commit-bot@chromium.org>

ssl/ssl_lib.c

diff --git a/ssl/ssl_lib.c b/ssl/ssl_lib.c
index 7adf103..9f77c5a 100644
--- a/ssl/ssl_lib.c
+++ b/ssl/ssl_lib.c
@@ -296,9 +296,12 @@
   ret->mode = SSL_MODE_NO_AUTO_CHAIN;
 
   /* Lock the SSL_CTX to the specified version, for compatibility with legacy
-   * uses of SSL_METHOD. */
+   * uses of SSL_METHOD, but we do not set the minimum version for
+   * |SSLv3_method|. */
   if (!SSL_CTX_set_max_proto_version(ret, method->version) ||
-      !SSL_CTX_set_min_proto_version(ret, method->version)) {
+      !SSL_CTX_set_min_proto_version(ret, method->version == SSL3_VERSION
+                                              ? 0 /* default */
+                                              : method->version)) {
     OPENSSL_PUT_ERROR(SSL, ERR_R_INTERNAL_ERROR);
     goto err2;
   }
@@ -945,6 +948,10 @@
   /* Zero is interpreted as the default minimum version. */
   if (version == 0) {
     *out = method->min_version;
+    /* SSL 3.0 is disabled unless explicitly enabled. */
+    if (*out < TLS1_VERSION) {
+      *out = TLS1_VERSION;
+    }
     return 1;
   }
 
@@ -2398,7 +2405,7 @@
   }
 
   if (!any_enabled) {
-    OPENSSL_PUT_ERROR(SSL, SSL_R_WRONG_SSL_VERSION);
+    OPENSSL_PUT_ERROR(SSL, SSL_R_NO_SUPPORTED_VERSIONS_ENABLED);
     return 0;
   }