/*
 *  OpenVPN -- An application to securely tunnel IP networks
 *             over a single TCP/UDP port, with support for SSL/TLS-based
 *             session authentication and key exchange,
 *             packet encryption, packet authentication, and
 *             packet compression.
 *
 *  Copyright (C) 2002-2026 OpenVPN Inc <sales@openvpn.net>
 *
 *  This program is free software; you can redistribute it and/or modify
 *  it under the terms of the GNU General Public License version 2
 *  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 General Public License for more details.
 *
 *  You should have received a copy of the GNU General Public License along
 *  with this program; if not, see <https://www.gnu.org/licenses/>.
 */

#ifdef HAVE_CONFIG_H
#include "config.h"
#endif

#include "syshead.h"

#include "multi.h"
#include <inttypes.h>
#include "forward.h"

#include "memdbg.h"
#include "ssl_pkt.h"

#ifdef HAVE_SYS_INOTIFY_H
#include <sys/inotify.h>
#endif

static void
send_hmac_reset_packet(struct multi_context *m, struct tls_pre_decrypt_state *state,
                       struct tls_auth_standalone *tas, struct session_id *sid,
                       bool request_resend_wkc, struct link_socket *sock)
{
    reset_packet_id_send(&state->tls_wrap_tmp.opt.packet_id.send);
    state->tls_wrap_tmp.opt.packet_id.rec.initialized = true;
    uint8_t header = 0 | (P_CONTROL_HARD_RESET_SERVER_V2 << P_OPCODE_SHIFT);
    struct buffer buf = tls_reset_standalone(&state->tls_wrap_tmp, tas, sid,
                                             &state->peer_session_id, header, request_resend_wkc);

    struct context *c = &m->top;

    /* dco-win server requires prepend with sockaddr, so preserve offset */
    ASSERT(buf_init(&c->c2.buffers->aux_buf, buf.offset));

    buf_copy(&c->c2.buffers->aux_buf, &buf);

    /*
     * We do not want to keep any state here, so we send the reply to the
     * initial packet synchronously without queueing anything.
     *
     * If we hit EAGAIN on a busy socket, the packet will be lost and the
     * client will have to retransmit its HARD_RESET. This is considered an
     * acceptable compromise to avoid consuming server resources under attack.
     */
    msg_set_prefix("Connection Attempt");
    c->c2.to_link = c->c2.buffers->aux_buf;
    c->c2.to_link_addr = &c->c2.from;
    msg(D_MULTI_DEBUG, "Reset packet from client, sending HMAC based reset challenge");
    process_outgoing_link(c, sock);
    c->c2.to_link.len = 0;
    c->c2.to_link_addr = NULL;
    msg_set_prefix(NULL);
}


/* Returns true if this packet should create a new session */
static bool
do_pre_decrypt_check(struct multi_context *m, struct tls_pre_decrypt_state *state,
                     struct mroute_addr addr, struct link_socket *sock)
{
    ASSERT(m->top.c2.tls_auth_standalone);

    enum first_packet_verdict verdict;

    struct tls_auth_standalone *tas = m->top.c2.tls_auth_standalone;

    verdict = tls_pre_decrypt_lite(tas, state, &m->top.c2.from, &m->top.c2.buf);

    hmac_ctx_t *hmac = m->top.c2.session_id_hmac;
    struct openvpn_sockaddr *from = &m->top.c2.from.dest;
    int handwindow = m->top.options.handshake_window;

    if (verdict == VERDICT_VALID_RESET_V3 || verdict == VERDICT_VALID_RESET_V2)
    {
        /* Check if we are still below our limit for sending out
         * responses */
        if (!reflect_filter_rate_limit_check(m->initial_rate_limiter))
        {
            return false;
        }
    }

    if (verdict == VERDICT_VALID_RESET_V3)
    {
        /* Extract the packet id to check if it has the special format that
         * indicates early negotiation support */
        struct packet_id_net pin;
        struct buffer tmp = m->top.c2.buf;
        ASSERT(buf_advance(&tmp, 1 + SID_SIZE));
        ASSERT(packet_id_read(&pin, &tmp, true));

        /* The most significant byte is 0x0f if early negotiation is supported */
        bool early_neg_support = ((pin.id & EARLY_NEG_MASK) & EARLY_NEG_START) == EARLY_NEG_START;

        /* All clients that support early negotiation and tls-crypt are assumed
         * to also support resending the WKc in the 2nd packet */
        if (early_neg_support)
        {
            /* Calculate the session ID HMAC for our reply and create reset packet */
            struct session_id sid =
                calculate_session_id_hmac(state->peer_session_id, from, hmac, handwindow, 0);
            send_hmac_reset_packet(m, state, tas, &sid, true, sock);

            return false;
        }
        else
        {
            /* For tls-crypt-v2 we need to keep the state of the first packet
             * to store the unwrapped key if the client doesn't support resending
             * the wrapped key. Unless the user specifically disallowed
             * compatibility with such clients to avoid state exhaustion */
            if (tas->tls_wrap.opt.flags & CO_FORCE_TLSCRYPTV2_COOKIE)
            {
                struct gc_arena gc = gc_new();
                const char *peer = print_link_socket_actual(&m->top.c2.from, &gc);
                msg(D_MULTI_DEBUG,
                    "tls-crypt-v2 force-cookie is enabled, "
                    "ignoring connection attempt from old client (%s)",
                    peer);
                gc_free(&gc);
                return false;
            }
            else
            {
                return true;
            }
        }
    }
    else if (verdict == VERDICT_VALID_RESET_V2)
    {
        /* Calculate the session ID HMAC for our reply and create reset packet */
        struct session_id sid =
            calculate_session_id_hmac(state->peer_session_id, from, hmac, handwindow, 0);

        send_hmac_reset_packet(m, state, tas, &sid, false, sock);

        /* We have a reply do not create a new session */
        return false;
    }
    else if (verdict == VERDICT_VALID_CONTROL_V1 || verdict == VERDICT_VALID_ACK_V1
             || verdict == VERDICT_VALID_WKC_V1)
    {
        /* ACK_V1 contains the peer id (our id) while CONTROL_V1 can but does not
         * need to contain the peer id */
        struct gc_arena gc = gc_new();

        bool pkt_is_ack = (verdict == VERDICT_VALID_ACK_V1);
        bool ret = check_session_hmac_and_pkt_id(state, from, hmac, handwindow, pkt_is_ack);

        const char *peer = print_link_socket_actual(&m->top.c2.from, &gc);
        uint8_t pkt_firstbyte = *BPTR(&m->top.c2.buf);
        int op = pkt_firstbyte >> P_OPCODE_SHIFT;

        if (!ret)
        {
            msg(D_MULTI_MEDIUM, "Packet (%s) with invalid or missing SID from"
                                " %s or wrong packet id",
                packet_opcode_name(op), peer);
        }
        else
        {
            msg(D_MULTI_DEBUG,
                "Valid packet (%s) with HMAC challenge from peer (%s), "
                "accepting new connection.",
                packet_opcode_name(op), peer);
        }
        gc_free(&gc);

        return ret;
    }

    /* VERDICT_INVALID */
    return false;
}

/**
 * Handles a packet if no existing session exists for this incoming packet.
 *
 * It will either send a reply with a hmac cookie if this is the first
 * packet of a three-way handshake or create a multi_instance if it is a
 * packet that completes the three-way handshake.
 */
static struct multi_instance *
handle_connection_attempt(struct multi_context *m,
                          struct link_socket *sock,
                          struct mroute_addr *real,
                          const uint64_t hv,
                          struct hash_bucket *bucket)
{
    struct hash *hash = m->hash;
    struct tls_pre_decrypt_state state = { 0 };
    struct multi_instance *mi = NULL;
    struct gc_arena gc = gc_new();

    if (m->deferred_shutdown_signal.signal_received)
    {
        msg(D_MULTI_ERRORS,
            "MULTI: Connection attempt from %s ignored while server is "
            "shutting down",
            mroute_addr_print(real, &gc));
    }
    else if (do_pre_decrypt_check(m, &state, *real, sock))
    {
        /* This is an unknown session but with valid tls-auth/tls-crypt
         * (or no auth at all).  If this is the initial packet of a
         * session, we just send a reply with a HMAC session id and
         * do not generate a session slot */

        if (frequency_limit_event_allowed(m->new_connection_limiter))
        {
            /* a successful three-way handshake only counts against
             * connect-freq but not against connect-freq-initial */
            reflect_filter_rate_limit_decrease(m->initial_rate_limiter);

            mi = multi_create_instance(m, real, sock);
            if (mi)
            {
                hash_add_fast(hash, bucket, &mi->real, hv, mi);
                mi->did_real_hash = true;
                multi_assign_peer_id(m, mi);

                /* If we have a session id already, ensure that the
                 * state is using the same */
                if (session_id_defined(&state.server_session_id)
                    && session_id_defined((&state.peer_session_id)))
                {
                    mi->context.c2.tls_multi->n_sessions++;
                    struct tls_session *session =
                        &mi->context.c2.tls_multi->session[TM_INITIAL];
                    session_skip_to_pre_start(session, &state, &m->top.c2.from);
                }
            }
        }
        else
        {
            msg(D_MULTI_ERRORS,
                "MULTI: Connection from %s would exceed new connection frequency limit as controlled by --connect-freq",
                mroute_addr_print(real, &gc));
        }
    }
    free_tls_pre_decrypt_state(&state);
    gc_free(&gc);
    return mi;
}

/**
 * Get a client instance based on real address.  If
 * the instance doesn't exist, create it while
 * maintaining real address hash table atomicity.
 */
struct multi_instance *
multi_get_create_instance_udp(struct multi_context *m, bool *floated, struct link_socket *sock)
{
    struct gc_arena gc = gc_new();
    struct mroute_addr real = { 0 };
    struct multi_instance *mi = NULL;
    struct hash *hash = m->hash;
    real.proto = sock->info.proto;

    if (mroute_extract_openvpn_sockaddr(&real, &m->top.c2.from.dest, true) && m->top.c2.buf.len > 0)
    {
        struct hash_element *he;
        const uint64_t hv = hash_value(hash, &real);
        struct hash_bucket *bucket = hash_bucket(hash, hv);
        uint8_t *ptr = BPTR(&m->top.c2.buf);
        uint8_t op = ptr[0] >> P_OPCODE_SHIFT;
        bool v2 = (op == P_DATA_V2) && (m->top.c2.buf.len >= (1 + 3));
        bool peer_id_disabled = false;

        /* make sure buffer has enough length to read opcode (1 byte) and peer-id (3 bytes) */
        if (v2)
        {
            uint32_t peer_id = ((uint32_t)ptr[1] << 16) | ((uint32_t)ptr[2] << 8) | ((uint32_t)ptr[3]);
            peer_id_disabled = (peer_id == MAX_PEER_ID);

            if (!peer_id_disabled && (peer_id < m->max_clients) && m->instances[peer_id])
            {
                /* Floating on TCP will never be possible, so ensure we only process
                 * UDP clients */
                if (m->instances[peer_id]->context.c2.link_sockets[0]->info.proto
                    == sock->info.proto)
                {
                    mi = m->instances[peer_id];
                    *floated = !link_socket_actual_match(&mi->context.c2.from, &m->top.c2.from);

                    if (*floated)
                    {
                        /* reset prefix, since here we are not sure peer is the one it claims to be
                         */
                        ungenerate_prefix(mi);
                        msg(D_MULTI_MEDIUM, "Float requested for peer %" PRIu32 " to %s", peer_id,
                            mroute_addr_print(&real, &gc));
                    }
                }
            }
        }
        if (!v2 || peer_id_disabled)
        {
            he = hash_lookup_fast(hash, bucket, &real, hv);
            if (he)
            {
                mi = (struct multi_instance *)he->value;
            }
        }

        /* we have no existing multi instance for this connection */
        if (!mi)
        {
            mi = handle_connection_attempt(m, sock, &real, hv, bucket);
        }

#ifdef ENABLE_DEBUG
        if (check_debug_level(D_MULTI_DEBUG))
        {
            const char *status = mi ? "[ok]" : "[failed]";

            dmsg(D_MULTI_DEBUG, "GET INST BY REAL: %s %s", mroute_addr_print(&real, &gc), status);
        }
#endif
    }

    gc_free(&gc);
    ASSERT(!(mi && mi->halt));
    return mi;
}

/*
 * Send a packet to UDP socket.
 */
static inline void
multi_process_outgoing_link(struct multi_context *m, const unsigned int mpp_flags)
{
    struct multi_instance *mi = multi_process_outgoing_link_pre(m);
    if (mi)
    {
        multi_process_outgoing_link_dowork(m, mi, mpp_flags);
    }
}

/*
 * Process a UDP socket event.
 */
void
multi_process_io_udp(struct multi_context *m, struct link_socket *sock, unsigned int rwflags)
{
    const unsigned int mpp_flags = (MPP_PRE_SELECT | MPP_CLOSE_ON_SIGNAL);

    /* UDP port ready to accept write */
    if (rwflags & SOCKET_WRITE)
    {
        multi_process_outgoing_link(m, mpp_flags);
    }
    /* Incoming data on UDP port */
    else if (rwflags & SOCKET_READ)
    {
        read_incoming_link(&m->top, sock);
        if (!IS_SIG(&m->top))
        {
            multi_process_incoming_link(m, NULL, mpp_flags, sock);
        }
    }
}
