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x11fwd.c

/*
 * Platform-independent bits of X11 forwarding.
 */

#include <stdio.h>
#include <stdlib.h>
#include <assert.h>
#include <time.h>

#include "putty.h"
#include "ssh.h"
#include "tree234.h"

#define GET_16BIT(endian, cp) \
  (endian=='B' ? GET_16BIT_MSB_FIRST(cp) : GET_16BIT_LSB_FIRST(cp))

#define PUT_16BIT(endian, cp, val) \
  (endian=='B' ? PUT_16BIT_MSB_FIRST(cp, val) : PUT_16BIT_LSB_FIRST(cp, val))

const char *const x11_authnames[] = {
    "", "MIT-MAGIC-COOKIE-1", "XDM-AUTHORIZATION-1"
};

struct XDMSeen {
    unsigned int time;
    unsigned char clientid[6];
};

struct X11Auth {
    unsigned char fakedata[64], realdata[64];
    int fakeproto, realproto;
    int fakelen, reallen;
    tree234 *xdmseen;
};

struct X11Private {
    const struct plug_function_table *fn;
    /* the above variable absolutely *must* be the first in this structure */
    unsigned char firstpkt[12];            /* first X data packet */
    struct X11Auth *auth;
    char *auth_protocol;
    unsigned char *auth_data;
    int data_read, auth_plen, auth_psize, auth_dlen, auth_dsize;
    int verified;
    int throttled, throttle_override;
    unsigned long peer_ip;
    int peer_port;
    void *c;                         /* data used by ssh.c */
    Socket s;
};

static int xdmseen_cmp(void *a, void *b)
{
    struct XDMSeen *sa = a, *sb = b;
    return sa->time > sb->time ? 1 :
         sa->time < sb->time ? -1 :
           memcmp(sa->clientid, sb->clientid, sizeof(sa->clientid));
}

void *x11_invent_auth(char *proto, int protomaxlen,
                  char *data, int datamaxlen, int proto_id)
{
    struct X11Auth *auth = snew(struct X11Auth);
    char ourdata[64];
    int i;

    if (proto_id == X11_MIT) {
      auth->fakeproto = X11_MIT;

      /* MIT-MAGIC-COOKIE-1. Cookie size is 128 bits (16 bytes). */
      auth->fakelen = 16;
      for (i = 0; i < 16; i++)
          auth->fakedata[i] = random_byte();
      auth->xdmseen = NULL;
    } else {
      assert(proto_id == X11_XDM);
      auth->fakeproto = X11_XDM;

      /* XDM-AUTHORIZATION-1. Cookie size is 16 bytes; byte 8 is zero. */
      auth->fakelen = 16;
      for (i = 0; i < 16; i++)
          auth->fakedata[i] = (i == 8 ? 0 : random_byte());
      auth->xdmseen = newtree234(xdmseen_cmp);
    }

    /* Now format for the recipient. */
    strncpy(proto, x11_authnames[auth->fakeproto], protomaxlen);
    ourdata[0] = '\0';
    for (i = 0; i < auth->fakelen; i++)
      sprintf(ourdata + strlen(ourdata), "%02x", auth->fakedata[i]);
    strncpy(data, ourdata, datamaxlen);

    return auth;
}

void x11_free_auth(void *authv)
{
    struct X11Auth *auth = (struct X11Auth *)authv;
    struct XDMSeen *seen;

    if (auth->xdmseen != NULL) {
      while ((seen = delpos234(auth->xdmseen, 0)) != NULL)
          sfree(seen);
      freetree234(auth->xdmseen);
    }
    sfree(auth);
}

/*
 * Fetch the real auth data for a given display string, and store
 * it in an X11Auth structure. Returns NULL on success, or an error
 * string.
 */
void x11_get_real_auth(void *authv, char *display)
{
    struct X11Auth *auth = (struct X11Auth *)authv;

    auth->realproto = X11_NO_AUTH;     /* in case next call does nothing */

    auth->reallen = sizeof(auth->realdata);
    platform_get_x11_auth(display, &auth->realproto,
                          auth->realdata, &auth->reallen);
}

#define XDM_MAXSKEW 20*60      /* 20 minute clock skew should be OK */

static char *x11_verify(unsigned long peer_ip, int peer_port,
                  struct X11Auth *auth, char *proto,
                  unsigned char *data, int dlen)
{
    if (strcmp(proto, x11_authnames[auth->fakeproto]) != 0)
      return "wrong authentication protocol attempted";
    if (auth->fakeproto == X11_MIT) {
        if (dlen != auth->fakelen)
            return "MIT-MAGIC-COOKIE-1 data was wrong length";
        if (memcmp(auth->fakedata, data, dlen) != 0)
            return "MIT-MAGIC-COOKIE-1 data did not match";
    }
    if (auth->fakeproto == X11_XDM) {
      unsigned long t;
      time_t tim;
      int i;
      struct XDMSeen *seen, *ret;

        if (dlen != 24)
            return "XDM-AUTHORIZATION-1 data was wrong length";
      if (peer_port == -1)
            return "cannot do XDM-AUTHORIZATION-1 without remote address data";
      des_decrypt_xdmauth(auth->fakedata+9, data, 24);
        if (memcmp(auth->fakedata, data, 8) != 0)
            return "XDM-AUTHORIZATION-1 data failed check"; /* cookie wrong */
      if (GET_32BIT_MSB_FIRST(data+8) != peer_ip)
            return "XDM-AUTHORIZATION-1 data failed check";   /* IP wrong */
      if ((int)GET_16BIT_MSB_FIRST(data+12) != peer_port)
            return "XDM-AUTHORIZATION-1 data failed check";   /* port wrong */
      t = GET_32BIT_MSB_FIRST(data+14);
      for (i = 18; i < 24; i++)
          if (data[i] != 0)          /* zero padding wrong */
            return "XDM-AUTHORIZATION-1 data failed check";
      tim = time(NULL);
      if (abs(t - tim) > XDM_MAXSKEW)
          return "XDM-AUTHORIZATION-1 time stamp was too far out";
      seen = snew(struct XDMSeen);
      seen->time = t;
      memcpy(seen->clientid, data+8, 6);
      assert(auth->xdmseen != NULL);
      ret = add234(auth->xdmseen, seen);
      if (ret != seen) {
          sfree(seen);
          return "XDM-AUTHORIZATION-1 data replayed";
      }
      /* While we're here, purge entries too old to be replayed. */
      for (;;) {
          seen = index234(auth->xdmseen, 0);
          assert(seen != NULL);
          if (t - seen->time <= XDM_MAXSKEW)
            break;
          sfree(delpos234(auth->xdmseen, 0));
      }
    }
    /* implement other protocols here if ever required */
    return NULL;
}

static void x11_log(Plug p, int type, SockAddr addr, int port,
                const char *error_msg, int error_code)
{
    /* We have no interface to the logging module here, so we drop these. */
}

static int x11_closing(Plug plug, const char *error_msg, int error_code,
                   int calling_back)
{
    struct X11Private *pr = (struct X11Private *) plug;

    /*
     * We have no way to communicate down the forwarded connection,
     * so if an error occurred on the socket, we just ignore it
     * and treat it like a proper close.
     */
    sshfwd_close(pr->c);
    x11_close(pr->s);
    return 1;
}

static int x11_receive(Plug plug, int urgent, char *data, int len)
{
    struct X11Private *pr = (struct X11Private *) plug;

    if (sshfwd_write(pr->c, data, len) > 0) {
      pr->throttled = 1;
      sk_set_frozen(pr->s, 1);
    }

    return 1;
}

static void x11_sent(Plug plug, int bufsize)
{
    struct X11Private *pr = (struct X11Private *) plug;

    sshfwd_unthrottle(pr->c, bufsize);
}

/*
 * When setting up X forwarding, we should send the screen number
 * from the specified local display. This function extracts it from
 * the display string.
 */
int x11_get_screen_number(char *display)
{
    int n;

    n = strcspn(display, ":");
    if (!display[n])
      return 0;
    n = strcspn(display, ".");
    if (!display[n])
      return 0;
    return atoi(display + n + 1);
}

/* Find the right display, returns an allocated string */
char *x11_display(const char *display) {
    char *ret;
    if(!display || !*display) {
      /* try to find platform-specific local display */
      if((ret = platform_get_x_display())==0 || !*ret)
          /* plausible default for all platforms */
          ret = dupstr(":0");
    } else
      ret = dupstr(display);
    if(ret[0] == ':') {
      /* no transport specified, use whatever we think is best */
      char *s = dupcat(platform_x11_best_transport, ret, (char *)0);
      sfree(ret);
      return s;
    } else
      return ret;
}

/*
 * Called to set up the raw connection.
 * 
 * Returns an error message, or NULL on success.
 * also, fills the SocketsStructure
 */
const char *x11_init(Socket * s, char *display, void *c, void *auth,
                 const char *peeraddr, int peerport, const Config *cfg)
{
    static const struct plug_function_table fn_table = {
      x11_log,
      x11_closing,
      x11_receive,
      x11_sent,
      NULL
    };

    SockAddr addr;
    int port;
    const char *err;
    char *dummy_realhost;
    char host[128];
    int n, displaynum;
    struct X11Private *pr;

    /* default display */
    display = x11_display(display);
    /*
     * Split up display name into host and display-number parts.
     */
    n = strcspn(display, ":");
    assert(n != 0);           /* x11_display() promises this */
    if (display[n])
      displaynum = atoi(display + n + 1);
    else
      displaynum = 0;                /* sensible default */
    if (n > sizeof(host) - 1)
      n = sizeof(host) - 1;
    strncpy(host, display, n);
    host[n] = '\0';
    sfree(display);
    
    if(!strcmp(host, "unix")) {
      /* use AF_UNIX sockets (doesn't make sense on all platforms) */
      addr = platform_get_x11_unix_address(displaynum,
                                   &dummy_realhost);
      port = 0;         /* to show we are not confused */
    } else {
      port = 6000 + displaynum;
      
      /*
       * Try to find host.
       */
      addr = name_lookup(host, port, &dummy_realhost, cfg, ADDRTYPE_UNSPEC);
      if ((err = sk_addr_error(addr)) != NULL) {
          sk_addr_free(addr);
          return err;
      }
    }

    /*
     * Open socket.
     */
    pr = snew(struct X11Private);
    pr->fn = &fn_table;
    pr->auth_protocol = NULL;
    pr->auth = (struct X11Auth *)auth;
    pr->verified = 0;
    pr->data_read = 0;
    pr->throttled = pr->throttle_override = 0;
    pr->c = c;

    pr->s = *s = new_connection(addr, dummy_realhost, port,
                        0, 1, 0, 0, (Plug) pr, cfg);
    if ((err = sk_socket_error(*s)) != NULL) {
      sfree(pr);
      return err;
    }

    /*
     * See if we can make sense of the peer address we were given.
     */
    {
      int i[4];
      if (peeraddr &&
          4 == sscanf(peeraddr, "%d.%d.%d.%d", i+0, i+1, i+2, i+3)) {
          pr->peer_ip = (i[0] << 24) | (i[1] << 16) | (i[2] << 8) | i[3];
          pr->peer_port = peerport;
      } else {
          pr->peer_ip = 0;
          pr->peer_port = -1;
      }
    }

    sk_set_private_ptr(*s, pr);
    return NULL;
}

void x11_close(Socket s)
{
    struct X11Private *pr;
    if (!s)
      return;
    pr = (struct X11Private *) sk_get_private_ptr(s);
    if (pr->auth_protocol) {
      sfree(pr->auth_protocol);
      sfree(pr->auth_data);
    }

    sfree(pr);

    sk_close(s);
}

void x11_unthrottle(Socket s)
{
    struct X11Private *pr;
    if (!s)
      return;
    pr = (struct X11Private *) sk_get_private_ptr(s);

    pr->throttled = 0;
    sk_set_frozen(s, pr->throttled || pr->throttle_override);
}

void x11_override_throttle(Socket s, int enable)
{
    struct X11Private *pr;
    if (!s)
      return;
    pr = (struct X11Private *) sk_get_private_ptr(s);

    pr->throttle_override = enable;
    sk_set_frozen(s, pr->throttled || pr->throttle_override);
}

/*
 * Called to send data down the raw connection.
 */
int x11_send(Socket s, char *data, int len)
{
    struct X11Private *pr;
    if (!s)
      return 0;
    pr = (struct X11Private *) sk_get_private_ptr(s);

    /*
     * Read the first packet.
     */
    while (len > 0 && pr->data_read < 12)
      pr->firstpkt[pr->data_read++] = (unsigned char) (len--, *data++);
    if (pr->data_read < 12)
      return 0;

    /*
     * If we have not allocated the auth_protocol and auth_data
     * strings, do so now.
     */
    if (!pr->auth_protocol) {
      pr->auth_plen = GET_16BIT(pr->firstpkt[0], pr->firstpkt + 6);
      pr->auth_dlen = GET_16BIT(pr->firstpkt[0], pr->firstpkt + 8);
      pr->auth_psize = (pr->auth_plen + 3) & ~3;
      pr->auth_dsize = (pr->auth_dlen + 3) & ~3;
      /* Leave room for a terminating zero, to make our lives easier. */
      pr->auth_protocol = snewn(pr->auth_psize + 1, char);
      pr->auth_data = snewn(pr->auth_dsize, unsigned char);
    }

    /*
     * Read the auth_protocol and auth_data strings.
     */
    while (len > 0 && pr->data_read < 12 + pr->auth_psize)
      pr->auth_protocol[pr->data_read++ - 12] = (len--, *data++);
    while (len > 0 && pr->data_read < 12 + pr->auth_psize + pr->auth_dsize)
      pr->auth_data[pr->data_read++ - 12 -
                  pr->auth_psize] = (unsigned char) (len--, *data++);
    if (pr->data_read < 12 + pr->auth_psize + pr->auth_dsize)
      return 0;

    /*
     * If we haven't verified the authentication, do so now.
     */
    if (!pr->verified) {
      char *err;

      pr->auth_protocol[pr->auth_plen] = '\0';  /* ASCIZ */
      err = x11_verify(pr->peer_ip, pr->peer_port,
                   pr->auth, pr->auth_protocol,
                   pr->auth_data, pr->auth_dlen);

      /*
       * If authentication failed, construct and send an error
       * packet, then terminate the connection.
       */
      if (err) {
          char *message;
          int msglen, msgsize;
          unsigned char *reply;

          message = dupprintf("PuTTY X11 proxy: %s", err);
          msglen = strlen(message);
          reply = snewn(8 + msglen+1 + 4, unsigned char); /* include zero */
          msgsize = (msglen + 3) & ~3;
          reply[0] = 0;        /* failure */
          reply[1] = msglen;         /* length of reason string */
          memcpy(reply + 2, pr->firstpkt + 2, 4);     /* major/minor proto vsn */
          PUT_16BIT(pr->firstpkt[0], reply + 6, msgsize >> 2);/* data len */
          memset(reply + 8, 0, msgsize);
          memcpy(reply + 8, message, msglen);
          sshfwd_write(pr->c, (char *)reply, 8 + msgsize);
          sshfwd_close(pr->c);
          x11_close(s);
          sfree(reply);
          sfree(message);
          return 0;
      }

      /*
       * Now we know we're going to accept the connection. Strip
       * the fake auth data, and optionally put real auth data in
       * instead.
       */
        {
            char realauthdata[64];
            int realauthlen = 0;
            int authstrlen = strlen(x11_authnames[pr->auth->realproto]);
          int buflen = 0;            /* initialise to placate optimiser */
            static const char zeroes[4] = { 0,0,0,0 };
          void *buf;

            if (pr->auth->realproto == X11_MIT) {
                assert(pr->auth->reallen <= lenof(realauthdata));
                realauthlen = pr->auth->reallen;
                memcpy(realauthdata, pr->auth->realdata, realauthlen);
            } else if (pr->auth->realproto == X11_XDM &&
                   pr->auth->reallen == 16 &&
                   ((buf = sk_getxdmdata(s, &buflen))!=0)) {
            time_t t;
                realauthlen = (buflen+12+7) & ~7;
            assert(realauthlen <= lenof(realauthdata));
            memset(realauthdata, 0, realauthlen);
            memcpy(realauthdata, pr->auth->realdata, 8);
            memcpy(realauthdata+8, buf, buflen);
            t = time(NULL);
            PUT_32BIT_MSB_FIRST(realauthdata+8+buflen, t);
            des_encrypt_xdmauth(pr->auth->realdata+9,
                            (unsigned char *)realauthdata,
                            realauthlen);
            sfree(buf);
          }
            /* implement other auth methods here if required */

            PUT_16BIT(pr->firstpkt[0], pr->firstpkt + 6, authstrlen);
            PUT_16BIT(pr->firstpkt[0], pr->firstpkt + 8, realauthlen);
        
            sk_write(s, (char *)pr->firstpkt, 12);

            if (authstrlen) {
                sk_write(s, x11_authnames[pr->auth->realproto], authstrlen);
                sk_write(s, zeroes, 3 & (-authstrlen));
            }
            if (realauthlen) {
                sk_write(s, realauthdata, realauthlen);
                sk_write(s, zeroes, 3 & (-realauthlen));
            }
        }
      pr->verified = 1;
    }

    /*
     * After initialisation, just copy data simply.
     */

    return sk_write(s, data, len);
}

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