Schannel: Reject certificate not signed by a configured CA certificate

Not entirely clear why, but when building the certificate chain for a
peer the system certificate store is searched for root certificates.
General expectation is that after calling
`sslConfiguration.setCaCertificates()` the system certificates will
not be taken into consideration.

To work around this behavior, we do a manual check that the root of the
chain is part of the configured CA certificates.

Pick-to: 6.5 6.2 5.15
Change-Id: I03666a4d9b0eac39ae97e150b4743120611a11b3
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>

src/plugins/tls/schannel/qtls_schannel.cpp

@@ -2066,6 +2066,27 @@ bool TlsCryptographSchannel::verifyCertContext(CERT_CONTEXT *certContext)
         verifyDepth = DWORD(q->peerVerifyDepth());
 
     const auto &caCertificates = q->sslConfiguration().caCertificates();
+
+    if (!rootCertOnDemandLoadingAllowed()
+            && !(chain->TrustStatus.dwErrorStatus & CERT_TRUST_IS_PARTIAL_CHAIN)
+            && (q->peerVerifyMode() == QSslSocket::VerifyPeer
+                    || (isClient && q->peerVerifyMode() == QSslSocket::AutoVerifyPeer))) {
+        // When verifying a peer Windows "helpfully" builds a chain that
+        // may include roots from the system store. But we don't want that if
+        // the user has set their own CA certificates.
+        // Since Windows claims this is not a partial chain the root is included
+        // and we have to check that it is one of our configured CAs.
+        CERT_CHAIN_ELEMENT *element = chain->rgpElement[chain->cElement - 1];
+        QSslCertificate certificate = getCertificateFromChainElement(element);
+        if (!caCertificates.contains(certificate)) {
+            auto error = QSslError(QSslError::CertificateUntrusted, certificate);
+            sslErrors += error;
+            emit q->peerVerifyError(error);
+            if (q->state() != QAbstractSocket::ConnectedState)
+                return false;
+        }
+    }
+
     QList<QSslCertificate> peerCertificateChain;
     for (DWORD i = 0; i < verifyDepth; i++) {
         CERT_CHAIN_ELEMENT *element = chain->rgpElement[i];

tests/manual/network/ssl/client-auth/CMakeLists.txt

@@ -0,0 +1,24 @@
+# Copyright (C) 2023 The Qt Company Ltd.
+# SPDX-License-Identifier: BSD-3-Clause
+
+qt_internal_add_manual_test(tst_manual_ssl_client_auth
+    SOURCES
+        tst_manual_ssl_client_auth.cpp
+    LIBRARIES
+        Qt::Network
+)
+
+qt_internal_add_resource(tst_manual_ssl_client_auth "tst_manual_ssl_client_auth"
+    PREFIX
+        "/"
+    FILES
+        "certs/127.0.0.1.pem"
+        "certs/127.0.0.1-key.pem"
+        "certs/127.0.0.1-client.pem"
+        "certs/127.0.0.1-client-key.pem"
+        "certs/accepted-client.pem"
+        "certs/accepted-client-key.pem"
+        "certs/rootCA.pem"
+    BASE
+        "certs"
+)

tests/manual/network/ssl/client-auth/certs/.gitignore

@@ -0,0 +1,4 @@
+*
+!/.gitignore
+!/generate.sh
+!/accepted-client.conf

tests/manual/network/ssl/client-auth/certs/accepted-client.conf

@@ -0,0 +1,14 @@
+[req]
+default_md = sha512
+basicConstraints = CA:FALSE
+extendedKeyUsage = clientAuth
+[req]
+distinguished_name = client_distinguished_name
+prompt = no
+[client_distinguished_name]
+C   = NO
+ST  = Oslo
+L   = Oslo
+O   = The Qt Project
+OU  = The Qt Project
+CN  = Fake Qt Project Client Certificate

tests/manual/network/ssl/client-auth/certs/generate.sh

@@ -0,0 +1,33 @@
+#!/bin/bash
+# Copyright (C) 2023 The Qt Company Ltd.
+# SPDX-License-Identifier: LicenseRef-Qt-Commercial OR GPL-3.0-only WITH Qt-GPL-exception-1.0
+
+# Requires mkcert and openssl
+
+warn () { echo "$@" >&2; }
+die () { warn "$@"; exit 1; }
+
+
+command -v mkcert 1>/dev/null 2>&1 || die "Failed to find mkcert"
+command -v openssl 1>/dev/null 2>&1 || die "Failed to find openssl"
+
+SCRIPT=$(realpath "$0")
+SCRIPTPATH=$(dirname "$SCRIPT")
+
+pushd "$SCRIPTPATH" || die "Unable to pushd to $SCRIPTPATH"
+mkcert 127.0.0.1
+mkcert -client 127.0.0.1
+warn "Remember to run mkcert -install if you haven't already"
+
+# Generate CA
+openssl genrsa -out ca-key.pem 2048
+openssl req -new -x509 -noenc -days 365 -key ca-key.pem -out rootCA.pem
+
+# Generate accepted client certificate
+openssl genrsa -out accepted-client-key.pem 2048
+openssl req -new -sha512 -nodes -key accepted-client-key.pem -out accepted-client.csr -config accepted-client.conf
+openssl x509 -req -sha512 -days 45 -in accepted-client.csr -CA rootCA.pem -CAkey ca-key.pem -CAcreateserial -out accepted-client.pem
+rm accepted-client.csr
+rm rootCA.srl
+
+popd || die "Unable to popd"

tests/manual/network/ssl/client-auth/tst_manual_ssl_client_auth.cpp

@@ -0,0 +1,118 @@
+// Copyright (C) 2023 The Qt Company Ltd.
+// SPDX-License-Identifier: LicenseRef-Qt-Commercial OR GPL-3.0-only WITH Qt-GPL-exception-1.0
+
+#include <QtCore/qcoreapplication.h>
+
+#include <QtCore/qthread.h>
+#include <QtCore/qfile.h>
+#include <QtCore/qdir.h>
+
+#include <QtNetwork/qsslsocket.h>
+#include <QtNetwork/qsslserver.h>
+#include <QtNetwork/qsslconfiguration.h>
+#include <QtNetwork/qsslkey.h>
+
+// Client and/or server presents a certificate signed by a system-trusted CA
+// but the other side presents a certificate signed by a different CA.
+constexpr bool TestServerPresentsIncorrectCa = false;
+constexpr bool TestClientPresentsIncorrectCa = true;
+
+class ServerThread : public QThread
+{
+    Q_OBJECT
+public:
+    void run() override
+    {
+        QSslServer server;
+
+        QSslConfiguration config = server.sslConfiguration();
+        QList<QSslCertificate> certs = QSslCertificate::fromPath(QStringLiteral(":/rootCA.pem"));
+        config.setCaCertificates(certs);
+        config.setLocalCertificate(QSslCertificate::fromPath(QStringLiteral(":/127.0.0.1.pem"))
+                                           .first());
+        QFile keyFile(QStringLiteral(":/127.0.0.1-key.pem"));
+        if (!keyFile.open(QIODevice::ReadOnly))
+            qFatal("Failed to open key file");
+        config.setPrivateKey(QSslKey(&keyFile, QSsl::Rsa));
+        config.setPeerVerifyMode(QSslSocket::VerifyPeer);
+        server.setSslConfiguration(config);
+
+        connect(&server, &QSslServer::pendingConnectionAvailable, [&server]() {
+            QSslSocket *socket = static_cast<QSslSocket *>(server.nextPendingConnection());
+            qDebug() << "[s] newConnection" << socket->peerAddress() << socket->peerPort();
+            socket->disconnectFromHost();
+            qApp->quit();
+        });
+        connect(&server, &QSslServer::startedEncryptionHandshake, [](QSslSocket *socket) {
+            qDebug() << "[s] new handshake" << socket->peerAddress() << socket->peerPort();
+        });
+        connect(&server, &QSslServer::errorOccurred,
+                [](QSslSocket *socket, QAbstractSocket::SocketError error) {
+                    qDebug() << "[s] errorOccurred" << socket->peerAddress() << socket->peerPort()
+                             << error << socket->errorString();
+                });
+        connect(&server, &QSslServer::peerVerifyError,
+                [](QSslSocket *socket, const QSslError &error) {
+                    qDebug() << "[s] peerVerifyError" << socket->peerAddress() << socket->peerPort()
+                             << error;
+                });
+        server.listen(QHostAddress::LocalHost, 24242);
+
+        exec();
+
+        server.close();
+    }
+};
+
+int main(int argc, char **argv)
+{
+    QCoreApplication app(argc, argv);
+
+    using namespace Qt::StringLiterals;
+
+    if (!QFileInfo(u":/rootCA.pem"_s).exists())
+        qFatal("rootCA.pem not found. Did you run generate.sh in the certs directory?");
+
+    ServerThread serverThread;
+    serverThread.start();
+
+    QSslSocket socket;
+    QSslConfiguration config = socket.sslConfiguration();
+    QString certificatePath;
+    QString keyFileName;
+    if constexpr (TestClientPresentsIncorrectCa) { // true: Present cert signed with incorrect CA: should fail
+        certificatePath = u":/127.0.0.1-client.pem"_s;
+        keyFileName = u":/127.0.0.1-client-key.pem"_s;
+    } else { // false: Use correct CA: should succeed
+        certificatePath = u":/accepted-client.pem"_s;
+        keyFileName = u":/accepted-client-key.pem"_s;
+    }
+    config.setLocalCertificate(QSslCertificate::fromPath(certificatePath).first());
+    if (TestServerPresentsIncorrectCa) // true: Verify server using incorrect CA: should fail
+        config.setCaCertificates(QSslCertificate::fromPath(u":/rootCA.pem"_s));
+    QFile keyFile(keyFileName);
+    if (!keyFile.open(QIODevice::ReadOnly))
+        qFatal("Failed to open key file");
+    config.setPrivateKey(QSslKey(&keyFile, QSsl::Rsa));
+    socket.setSslConfiguration(config);
+
+    QObject::connect(&socket, &QSslSocket::encrypted, []() { qDebug() << "[c] encrypted"; });
+    QObject::connect(&socket, &QSslSocket::errorOccurred,
+            [&socket](QAbstractSocket::SocketError error) {
+                qDebug() << "[c] errorOccurred" << error << socket.errorString();
+                qApp->quit();
+            });
+    QObject::connect(&socket, &QSslSocket::sslErrors, [](const QList<QSslError> &errors) {
+        qDebug() << "[c] sslErrors" << errors;
+    });
+    QObject::connect(&socket, &QSslSocket::connected, []() { qDebug() << "[c] connected"; });
+
+    socket.connectToHostEncrypted(QStringLiteral("127.0.0.1"), 24242);
+
+    const int res = app.exec();
+    serverThread.quit();
+    serverThread.wait();
+    return res;
+}
+
+#include "tst_manual_ssl_client_auth.moc"