protostack.hpp

//    OpenVPN -- An application to securely tunnel IP networks
//               over a single port, with support for SSL/TLS-based
//               session authentication and key exchange,
//               packet encryption, packet authentication, and
//               packet compression.
//
//    Copyright (C) 2012-2022 OpenVPN Inc.
//
//    This program is free software: you can redistribute it and/or modify
//    it under the terms of the GNU Affero General Public License Version 3
//    as published by the Free Software Foundation.
//
//    This program is distributed in the hope that it will be useful,
//    but WITHOUT ANY WARRANTY; without even the implied warranty of
//    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
//    GNU Affero General Public License for more details.
//
//    You should have received a copy of the GNU Affero General Public License
//    along with this program in the COPYING file.
//    If not, see <http://www.gnu.org/licenses/>.

#ifndef OPENVPN_SSL_PROTOSTACK_H
#define OPENVPN_SSL_PROTOSTACK_H

#include <deque>
#include <utility>

#include <openvpn/common/exception.hpp>
#include <openvpn/common/size.hpp>
#include <openvpn/common/usecount.hpp>
#include <openvpn/buffer/buffer.hpp>
#include <openvpn/time/time.hpp>
#include <openvpn/log/sessionstats.hpp>
#include <openvpn/reliable/relrecv.hpp>
#include <openvpn/reliable/relsend.hpp>
#include <openvpn/reliable/relack.hpp>
#include <openvpn/frame/frame.hpp>
#include <openvpn/error/excode.hpp>
#include <openvpn/ssl/sslconsts.hpp>
#include <openvpn/ssl/sslapi.hpp>

// ProtoStackBase is designed to allow general-purpose protocols (including
// but not limited to OpenVPN) to run over SSL, where the underlying transport
// layer is unreliable, such as UDP.  The OpenVPN protocol implementation in
// proto.hpp (ProtoContext) layers on top of ProtoStackBase.
// ProtoStackBase is independent of any particular SSL implementation, and
// accepts the SSL object type as a template parameter.

namespace openvpn {

// PACKET type must define the following methods:
//
// Default constructor:
//   PACKET()
//
// Constructor for BufferPtr:
//   explicit PACKET(const BufferPtr& buf)
//
// Return cloned packet, including cloned buffer content:
//   PACKET clone() const
//
// Test if defined:
//   operator bool() const
//
// Return false if packet is raw, or true if packet is SSL ciphertext:
//   bool contains_tls_ciphertext() const
//
// Reset back to post-default-constructor state:
//   void reset()
//
// Return internal BufferPtr:
//   const BufferPtr& buffer_ptr() const
//
// Call frame.prepare on internal buffer:
//   void frame_prepare(const Frame& frame, const unsigned int context)

template <typename PACKET, typename PARENT>
class ProtoStackBase
{
  public:
    static constexpr size_t ovpn_sending_window = 6;
    static constexpr size_t ovpn_receiving_window = ReliableAck::maximum_acks_ack_v1;

    typedef reliable::id_t id_t;
    typedef ReliableSendTemplate<PACKET> ReliableSend;
    typedef ReliableRecvTemplate<PACKET> ReliableRecv;

    OPENVPN_SIMPLE_EXCEPTION(proto_stack_invalidated);
    OPENVPN_SIMPLE_EXCEPTION(unknown_status_from_ssl_layer);

    enum NetSendType
    {
        NET_SEND_SSL,
        NET_SEND_RAW,
        NET_SEND_ACK,
        NET_SEND_RETRANSMIT,
    };

    ProtoStackBase(SSLFactoryAPI &ssl_factory,            // SSL factory object that can be used to generate new SSL sessions
                   TimePtr now_arg,                       // pointer to current time
                   const Time::Duration &tls_timeout_arg, // packet retransmit timeout
                   const Frame::Ptr &frame,               // contains info on how to allocate and align buffers
                   const SessionStats::Ptr &stats_arg)    // error statistics
        : tls_timeout(tls_timeout_arg),
          ssl_(ssl_factory.ssl()),
          frame_(frame),
          stats(stats_arg),
          now(now_arg)
    {
    }

    // Start SSL handshake on underlying SSL connection object.
    void start_handshake()
    {
        if (!invalidated())
        {
            ssl_->start_handshake();
            ssl_started_ = true;
            up_sequenced();
        }
    }

    uint32_t get_tls_warnings() const
    {
        return ssl_->get_tls_warnings();
    }

    // Incoming ciphertext packet arriving from network,
    // we will take ownership of pkt.
    bool net_recv(PACKET &&pkt)
    {
        if (!invalidated())
            return up_stack(pkt);
        return false;
    }

    // Outgoing application-level cleartext packet ready to send
    // (will be encrypted via SSL), we will take ownership
    // of buf.
    void app_send(BufferPtr &&buf)
    {
        if (!invalidated())
            app_write_queue.push_back(std::move(buf));
    }

    // Outgoing raw packet ready to send (will NOT be encrypted
    // via SSL, but will still be encapsulated, sequentialized,
    // and tracked via reliability layer).
    void raw_send(PACKET &&pkt)
    {
        if (!invalidated())
            raw_write_queue.push_back(std::move(pkt));
    }

    // Write any pending data to network and update retransmit
    // timer.  Should be called as a final step after one or more
    // net_recv, app_send, raw_send, or start_handshake calls.
    void flush()
    {
        if (!invalidated() && !up_stack_reentry_level)
        {
            down_stack_raw();
            down_stack_app();
            update_retransmit();
        }
    }

    // Send pending ACKs back to sender for packets already received
    void send_pending_acks()
    {
        if (!invalidated())
        {
            while (!xmit_acks.empty())
            {
                ack_send_buf.frame_prepare(*frame_, Frame::WRITE_ACK_STANDALONE);

                // encapsulate standalone ACK
                parent().generate_ack(ack_send_buf);

                // transmit it
                parent().net_send(ack_send_buf, NET_SEND_ACK);
            }
        }
    }

    // Send any pending retransmissions
    void retransmit()
    {
        if (!invalidated() && *now >= next_retransmit_)
        {
            for (id_t i = rel_send.head_id(); i < rel_send.tail_id(); ++i)
            {
                typename ReliableSend::Message &m = rel_send.ref_by_id(i);
                if (m.ready_retransmit(*now))
                {
                    // preserve original packet non-encapsulated
                    PACKET pkt = m.packet.clone();

                    // encapsulate packet
                    try
                    {
                        parent().encapsulate(m.id(), pkt);
                    }
                    catch (...)
                    {
                        error(Error::ENCAPSULATION_ERROR);
                        throw;
                    }
                    parent().net_send(pkt, NET_SEND_RETRANSMIT);
                    m.reset_retransmit(*now, tls_timeout);
                }
            }
            update_retransmit();
        }
    }

    // When should we next call retransmit()
    Time next_retransmit() const
    {
        if (!invalidated())
            return next_retransmit_;
        else
            return Time::infinite();
    }

    // Has SSL handshake been started yet?
    bool ssl_started() const
    {
        return ssl_started_;
    }

    // Was session invalidated by an exception?
    bool invalidated() const
    {
        return invalidated_;
    }

    // Reason for invalidation
    Error::Type invalidation_reason() const
    {
        return invalidation_reason_;
    }

    // Invalidate session
    void invalidate(const Error::Type reason)
    {
        if (!invalidated_)
        {
            invalidated_ = true;
            invalidation_reason_ = reason;
            parent().invalidate_callback();
        }
    }

    std::string ssl_handshake_details() const
    {
        return ssl_->ssl_handshake_details();
    }

    void export_key_material(OpenVPNStaticKey &key, const std::string &label) const
    {
        if (!ssl_->export_keying_material(label, key.raw_alloc(), OpenVPNStaticKey::KEY_SIZE))
            throw ErrorCode(Error::KEY_EXPANSION_ERROR, true, "TLS Keying material export error");
    }

    const AuthCert::Ptr &auth_cert() const
    {
        return ssl_->auth_cert();
    }

  private:
    // Parent methods -- derived class must define these methods

    // Encapsulate packet, use id as sequence number.  If xmit_acks is non-empty,
    // try to piggy-back ACK replies from xmit_acks to sender in encapsulated
    // packet. Any exceptions thrown will invalidate session, i.e. this object
    // can no longer be used.
    //
    // void encapsulate(id_t id, PACKET& pkt) = 0;

    // Perform integrity check on packet.  If packet is good, unencapsulate it and
    // pass it into the rel_recv object.  Any ACKs received for messages previously
    // sent should be marked in rel_send.  Message sequence number should be recorded
    // in xmit_acks.  Exceptions may be thrown here and they will be passed up to
    // caller of net_recv and will not invalidate the session.
    // Method should return true if packet was placed into rel_recv.
    //
    // bool decapsulate(PACKET& pkt) = 0;

    // Generate a standalone ACK message in buf based on ACKs in xmit_acks
    // (PACKET will be already be initialized by frame_prepare()).
    //
    // void generate_ack(PACKET& pkt) = 0;

    // Transmit encapsulated ciphertext packet to peer.  Method may not modify
    // or take ownership of net_pkt or underlying data unless it copies it.
    //
    // void net_send(const PACKET& net_pkt, const NetSendType nstype) = 0;

    // Pass cleartext data up to application, which make take
    // ownership of to_app_buf via std::move.
    //
    // void app_recv(BufferPtr&& to_app_buf) = 0;

    // Pass raw data up to application.  A packet is considered to be raw
    // if is_raw() method returns true.  Method may take ownership
    // of raw_pkt via std::move.
    //
    // void raw_recv(PACKET&& raw_pkt) = 0;

    // called if session is invalidated by an error (optional)
    //
    // void invalidate_callback() {}

    // END of parent methods

    // get reference to parent for CRTP
    PARENT &parent()
    {
        return *static_cast<PARENT *>(this);
    }

    // app data -> SSL -> protocol encapsulation -> reliability layer -> network
    void down_stack_app()
    {
        if (ssl_started_)
        {
            // push app-layer cleartext through SSL object
            while (!app_write_queue.empty())
            {
                BufferPtr &buf = app_write_queue.front();
                ssize_t size;
                try
                {
                    size = ssl_->write_cleartext_unbuffered(buf->data(), buf->size());
                }
                catch (...)
                {
                    error(Error::SSL_ERROR);
                    throw;
                }
                if (size == static_cast<ssize_t>(buf->size()))
                    app_write_queue.pop_front();
                else if (size == SSLConst::SHOULD_RETRY)
                    break;
                else if (size >= 0)
                {
                    // partial write
                    app_write_queue.front()->advance(size);
                    break;
                }
                else
                {
                    error(Error::SSL_ERROR);
                    throw unknown_status_from_ssl_layer();
                }
            }

            // encapsulate SSL ciphertext packets
            while (ssl_->read_ciphertext_ready() && rel_send.ready())
            {
                typename ReliableSend::Message &m = rel_send.send(*now, tls_timeout);
                m.packet = PACKET(ssl_->read_ciphertext());

                // encapsulate and send cloned packet, preserve original one for retransmit
                PACKET pkt = m.packet.clone();

                // encapsulate packet
                try
                {
                    parent().encapsulate(m.id(), pkt);
                }
                catch (...)
                {
                    error(Error::ENCAPSULATION_ERROR);
                    throw;
                }

                // transmit it
                parent().net_send(pkt, NET_SEND_SSL);
            }
        }
    }

    // raw app data -> protocol encapsulation -> reliability layer -> network
    void down_stack_raw()
    {
        while (!raw_write_queue.empty() && rel_send.ready())
        {
            typename ReliableSend::Message &m = rel_send.send(*now, tls_timeout);
            m.packet = raw_write_queue.front();
            raw_write_queue.pop_front();

            PACKET pkt = m.packet.clone();

            // encapsulate packet
            try
            {
                parent().encapsulate(m.id(), pkt);
            }
            catch (...)
            {
                error(Error::ENCAPSULATION_ERROR);
                throw;
            }

            // transmit it
            parent().net_send(pkt, NET_SEND_RAW);
        }
    }

    // network -> reliability layer -> protocol decapsulation -> SSL -> app
    bool up_stack(PACKET &recv)
    {
        UseCount use_count(up_stack_reentry_level);
        if (parent().decapsulate(recv))
        {
            up_sequenced();
            return true;
        }
        else
            return false;
    }

    // if a sequenced packet is available from reliability layer,
    // move it up the stack
    void up_sequenced()
    {
        // is sequenced receive packet available?
        while (rel_recv.ready())
        {
            typename ReliableRecv::Message &m = rel_recv.next_sequenced();
            if (!m.packet.contains_tls_ciphertext())
                parent().raw_recv(std::move(m.packet));
            else // SSL packet
            {
                if (ssl_started_)
                    ssl_->write_ciphertext(m.packet.buffer_ptr());
                else
                    break;
            }
            rel_recv.advance();
        }

        // read cleartext data from SSL object
        if (ssl_started_)
            while (ssl_->read_cleartext_ready())
            {
                ssize_t size;
                to_app_buf.reset(new BufferAllocated());
                frame_->prepare(Frame::READ_SSL_CLEARTEXT, *to_app_buf);
                try
                {
                    size = ssl_->read_cleartext(to_app_buf->data(), to_app_buf->max_size());
                }
                catch (...)
                {
                    // SSL fatal errors will invalidate the session
                    error(Error::SSL_ERROR);
                    throw;
                }
                if (size >= 0)
                {
                    to_app_buf->set_size(size);

                    // pass cleartext data to app
                    parent().app_recv(std::move(to_app_buf));
                }
                else if (size == SSLConst::SHOULD_RETRY)
                    break;
                else if (size == SSLConst::PEER_CLOSE_NOTIFY)
                {
                    error(Error::SSL_ERROR);
                    throw ErrorCode(Error::CLIENT_HALT, true, "SSL Close Notify received");
                }
                else
                {
                    error(Error::SSL_ERROR);
                    throw unknown_status_from_ssl_layer();
                }
            }
    }

    void update_retransmit()
    {
        next_retransmit_ = *now + rel_send.until_retransmit(*now);
    }

    void error(const Error::Type reason)
    {
        if (stats)
            stats->error(reason);
        invalidate(reason);
    }

  private:
    const Time::Duration tls_timeout;
    typename SSLAPI::Ptr ssl_;
    Frame::Ptr frame_;
    int up_stack_reentry_level = 0;
    bool invalidated_ = false;
    Error::Type invalidation_reason_ = Error::SUCCESS;
    bool ssl_started_ = false;
    Time next_retransmit_ = Time::infinite();
    BufferPtr to_app_buf; // cleartext data decrypted by SSL that is to be passed to app via app_recv method
    PACKET ack_send_buf;  // only used for standalone ACKs to be sent to peer
    std::deque<BufferPtr> app_write_queue;
    std::deque<PACKET> raw_write_queue;
    SessionStats::Ptr stats;

  protected:
    TimePtr now;
    ReliableRecv rel_recv{ovpn_receiving_window};
    ReliableSend rel_send{ovpn_sending_window};
    ReliableAck xmit_acks{};
};

} // namespace openvpn

#endif // OPENVPN_SSL_PROTOSTACK_H