Shard the SSL session cache by version fallback.

This addresses two issues:

- NSS clamps client_version to the session version. This means that a
  successful fallback connection is effectively cached, despite our fallback
  being stateless. This causing our metrics to be under-reported and, more
  problematic, makes spurious fallbacks stick.

- BoringSSL does not clamp, but many versions of OpenSSL on the server will
  happily resume older sessions at newer protocol versions, rather than doing a
  full handshake at the newer protocol version. This means a successful
  spurious fallback causes us later resume with a weaker cipher than we should.
  Moreover, this mismatch is forbidden by every other client implementation.
  The metrics are reporting 0.06% of connections on beta channel hit this case.
  I expect it to go down after this change.

Note: this will also increase traffic to version-intolerant servers on NSS
ports. But that's only Linux/CrOS/iOS now and the BoringSSL switch did the same
thing by losing the version clamp.

BUG=459690,441456

Review URL: https://codereview.chromium.org/947603002

Cr-Commit-Position: refs/heads/master@{#317605}

1 files changed, 21 insertions, 0 deletions

diff --git a/net/socket/ssl_client_socket_openssl.cc b/net/socket/ssl_client_socket_openssl.cc
index f166b69..0389375 100644
--- a/net/socket/ssl_client_socket_openssl.cc
+++ b/net/socket/ssl_client_socket_openssl.cc
@@ -402,6 +402,27 @@ std::string SSLClientSocketOpenSSL::GetSessionCacheKey() const {
   std::string result = host_and_port_.ToString();
   result.append("/");
   result.append(ssl_session_cache_shard_);
+
+  // Shard the session cache based on maximum protocol version. This causes
+  // fallback connections to use a separate session cache.
+  result.append("/");
+  switch (ssl_config_.version_max) {
+    case SSL_PROTOCOL_VERSION_SSL3:
+      result.append("ssl3");
+      break;
+    case SSL_PROTOCOL_VERSION_TLS1:
+      result.append("tls1");
+      break;
+    case SSL_PROTOCOL_VERSION_TLS1_1:
+      result.append("tls1.1");
+      break;
+    case SSL_PROTOCOL_VERSION_TLS1_2:
+      result.append("tls1.2");
+      break;
+    default:
+      NOTREACHED();
+  }
+
   return result;
 }