Implement client identity verification

This commit fixes two security vulnerabilities: CVE-2021-42072 and
CVE-2021-42073.

The issues have been reported by Matthias Gerstner <mgerstner@suse.de>.

src/lib/barrier/ArgParser.cpp

@@ -65,7 +65,9 @@ ArgParser::parseServerArgs(ServerArgs& args, int argc, const char* const* argv)
             // save screen change script path
             args.m_screenChangeScript = argv[++i];
         }
-        else {
+        else if (isArg(i, argc, argv, nullptr, "--disable-client-cert-checking")) {
+            args.check_client_certificates = false;
+        } else {
             LOG((CLOG_PRINT "%s: unrecognized option `%s'" BYE, args.m_exename.c_str(), argv[i], args.m_exename.c_str()));
             return false;
         }

src/lib/barrier/ServerApp.cpp

@@ -148,7 +148,10 @@ ServerApp::help()
            << "Options:\n"
            << "  -a, --address <address>  listen for clients on the given address.\n"
            << "  -c, --config <pathname>  use the named configuration file instead.\n"
-           << HELP_COMMON_INFO_1 << WINAPI_INFO << HELP_SYS_INFO << HELP_COMMON_INFO_2 << "\n"
+           << HELP_COMMON_INFO_1
+           << "      --disable-client-cert-checking disable client SSL certificate \n"
+              "                                     checking (deprecated)\n"
+           << WINAPI_INFO << HELP_SYS_INFO << HELP_COMMON_INFO_2 << "\n"
            << "Default options are marked with a *\n"
            << "\n"
            << "The argument for --address is of the form: [<hostname>][:<port>].  The\n"
@@ -658,6 +661,9 @@ ServerApp::openClientListener(const NetworkAddress& address)
     auto security_level = ConnectionSecurityLevel::PLAINTEXT;
     if (args().m_enableCrypto) {
         security_level = ConnectionSecurityLevel::ENCRYPTED;
+        if (args().check_client_certificates) {
+            security_level = ConnectionSecurityLevel::ENCRYPTED_AUTHENTICATED;
+        }
     }
 
     ClientListener* listen = new ClientListener(

src/lib/barrier/ServerArgs.h

@@ -30,4 +30,5 @@ public:
     String                m_configFile;
     Config*                m_config;
     String                m_screenChangeScript;
+    bool check_client_certificates = true;
 };

src/lib/client/Client.cpp

@@ -129,7 +129,8 @@ Client::connect()
 
     auto security_level = ConnectionSecurityLevel::PLAINTEXT;
     if (m_useSecureNetwork) {
-        security_level = ConnectionSecurityLevel::ENCRYPTED;
+        // client always authenticates server
+        security_level = ConnectionSecurityLevel::ENCRYPTED_AUTHENTICATED;
     }
 
     try {

src/lib/net/ConnectionSecurityLevel.h

@@ -21,6 +21,7 @@
 enum class ConnectionSecurityLevel {
     PLAINTEXT,
     ENCRYPTED,
+    ENCRYPTED_AUTHENTICATED
 };
 
 #endif // BARRIER_LIB_NET_CONNECTION_SECURITY_LEVEL_H

src/lib/net/SecureSocket.cpp

@@ -361,6 +361,11 @@ bool SecureSocket::load_certificates(const barrier::fs::path& path)
     return true;
 }
 
+static int cert_verify_ignore_callback(X509_STORE_CTX*, void*)
+{
+    return 1;
+}
+
 void
 SecureSocket::initContext(bool server)
 {
@@ -396,6 +401,14 @@ SecureSocket::initContext(bool server)
     if (m_ssl->m_context == NULL) {
         showError("");
     }
+
+    if (security_level_ == ConnectionSecurityLevel::ENCRYPTED_AUTHENTICATED) {
+        // We want to ask for peer certificate, but not verify it. If we don't ask for peer
+        // certificate, e.g. client won't send it.
+        SSL_CTX_set_verify(m_ssl->m_context, SSL_VERIFY_PEER | SSL_VERIFY_FAIL_IF_NO_PEER_CERT,
+                           nullptr);
+        SSL_CTX_set_cert_verify_callback(m_ssl->m_context, cert_verify_ignore_callback, nullptr);
+    }
 }
 
 void
@@ -436,6 +449,24 @@ SecureSocket::secureAccept(int socket)
 
     // If not fatal and no retry, state is good
     if (retry == 0) {
+        if (security_level_ == ConnectionSecurityLevel::ENCRYPTED_AUTHENTICATED) {
+            if (verify_cert_fingerprint(
+                        barrier::DataDirectories::trusted_clients_ssl_fingerprints_path())) {
+                LOG((CLOG_INFO "accepted secure socket"));
+                if (!ensure_peer_certificate()) {
+                    retry = 0;
+                    disconnect();
+                    return -1;// Cert fail, error
+                }
+            }
+            else {
+                LOG((CLOG_ERR "failed to verify server certificate fingerprint"));
+                retry = 0;
+                disconnect();
+                return -1; // Fingerprint failed, error
+            }
+        }
+
         m_secureReady = true;
         LOG((CLOG_INFO "accepted secure socket"));
         if (CLOG->getFilter() >= kDEBUG1) {
@@ -670,7 +701,7 @@ bool SecureSocket::verify_cert_fingerprint(const barrier::fs::path& fingerprint_
     }
 
     // note: the GUI parses the following two lines of logs, don't change unnecessarily
-    LOG((CLOG_NOTE "server fingerprint (SHA1): %s (SHA256): %s",
+    LOG((CLOG_NOTE "peer fingerprint (SHA1): %s (SHA256): %s",
          barrier::format_ssl_fingerprint(fingerprint_sha1.data).c_str(),
          barrier::format_ssl_fingerprint(fingerprint_sha256.data).c_str()));