/*

 * Dropbear - a SSH2 server

 *

 * Copyright (c) 2002,2003 Matt Johnston

 * All rights reserved.

 *

 * Permission is hereby granted, free of charge, to any person obtaining a copy

 * of this software and associated documentation files (the "Software"), to deal

 * in the Software without restriction, including without limitation the rights

 * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell

 * copies of the Software, and to permit persons to whom the Software is

 * furnished to do so, subject to the following conditions:

 *

 * The above copyright notice and this permission notice shall be included in

 * all copies or substantial portions of the Software.

 *

 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR

 * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,

 * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE

 * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER

 * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,

 * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE

 * SOFTWARE. */

/*

 * This file incorporates work covered by the following copyright and

 * permission notice:

 *

 *  Copyright (c) 2000 Markus Friedl.  All rights reserved.

 *

 *  Redistribution and use in source and binary forms, with or without

 *  modification, are permitted provided that the following conditions

 *  are met:

 *  1. Redistributions of source code must retain the above copyright

 *     notice, this list of conditions and the following disclaimer.

 *  2. Redistributions in binary form must reproduce the above copyright

 *     notice, this list of conditions and the following disclaimer in the

 *     documentation and/or other materials provided with the distribution.

 *

 *  THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR

 *  IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES

 *  OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.

 *  IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,

 *  INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT

 *  NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,

 *  DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY

 *  THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT

 *  (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF

 *  THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.

 *

 * This copyright and permission notice applies to the code parsing public keys

 * options string which can also be found in OpenSSH auth2-pubkey.c file

 * (user_key_allowed2). It has been adapted to work with buffers.

 *

 */

/* Process a pubkey auth request */

#include "includes.h"

#include "session.h"

#include "dbutil.h"

#include "buffer.h"

#include "signkey.h"

#include "auth.h"

#include "ssh.h"

#include "packet.h"

#include "algo.h"

#include "runopts.h"

#if DROPBEAR_SVR_PUBKEY_AUTH

#define MIN_AUTHKEYS_LINE 10 /* "ssh-rsa AB" - short but doesn't matter */

#define MAX_AUTHKEYS_LINE 4200 /* max length of a line in authkeys */

static char * authorized_keys_filepath(void);

static int checkpubkey(const char* keyalgo, unsigned int keyalgolen,

    const unsigned char* keyblob, unsigned int keybloblen);

static int checkpubkeyperms(void);

static void send_msg_userauth_pk_ok(const char* sigalgo, unsigned int sigalgolen,

    const unsigned char* keyblob, unsigned int keybloblen);

static int checkfileperm(char * filename);

/* process a pubkey auth request, sending success or failure message as

 * appropriate */

void svr_auth_pubkey(int valid_user) {

  unsigned char testkey; /* whether we're just checking if a key is usable */

  char* sigalgo = NULL;

  unsigned int sigalgolen;

  const char* keyalgo;

  unsigned int keyalgolen;

  unsigned char* keyblob = NULL;

  unsigned int keybloblen;

  unsigned int sign_payload_length;

  buffer * signbuf = NULL;

  sign_key * key = NULL;

  char* fp = NULL;

  enum signature_type sigtype;

  enum signkey_type keytype;

    int auth_failure = 1;

  TRACE(("enter pubkeyauth"))

  /* 0 indicates user just wants to check if key can be used, 1 is an

   * actual attempt*/

  testkey = (buf_getbool(ses.payload) == 0);

  sigalgo = buf_getstring(ses.payload, &sigalgolen);

  keybloblen = buf_getint(ses.payload);

  keyblob = buf_getptr(ses.payload, keybloblen);

  if (!valid_user) {

    /* Return failure once we have read the contents of the packet

    required to validate a public key.

    Avoids blind user enumeration though it isn't possible to prevent

    testing for user existence if the public key is known */

    send_msg_userauth_failure(0, 0);

    goto out;

  }

  sigtype = signature_type_from_name(sigalgo, sigalgolen);

  if (sigtype == DROPBEAR_SIGNATURE_NONE) {

    send_msg_userauth_failure(0, 0);

    goto out;

  }

  keytype = signkey_type_from_signature(sigtype);

  keyalgo = signkey_name_from_type(keytype, &keyalgolen);

#if DROPBEAR_PLUGIN

    if (svr_ses.plugin_instance != NULL) {

        char *options_buf;

        if (svr_ses.plugin_instance->checkpubkey(

                    svr_ses.plugin_instance,

                    &ses.plugin_session,

                    keyalgo,

                    keyalgolen,

                    keyblob,

                    keybloblen,

                    ses.authstate.username) == DROPBEAR_SUCCESS) {

            /* Success */

            auth_failure = 0;

            /* Options provided? */

            options_buf = ses.plugin_session->get_options(ses.plugin_session);

            if (options_buf) {

                struct buf temp_buf = {

                    .data = (unsigned char *)options_buf,

                    .len = strlen(options_buf),

                    .pos = 0,

                    .size = 0

                };

                int ret = svr_add_pubkey_options(&temp_buf, 0, "N/A");

                if (ret == DROPBEAR_FAILURE) {

                    /* Fail immediately as the plugin provided wrong options */

                    send_msg_userauth_failure(0, 0);

                    goto out;

                }

            }

        }

    }

#endif

  /* check if the key is valid */

  if (auth_failure) {

      auth_failure = checkpubkey(keyalgo, keyalgolen, keyblob, keybloblen) == DROPBEAR_FAILURE;

  }

  if (auth_failure) {

    /* MAX_PUBKEY_QUERIES allows a greater limit of pubkey queries

     * than the standard maxauthtries.

     * Start counting failures (incrfail) only when it's reaching

     * the limit.

     */

    unsigned int free_query_limit = 0;

      MAX(0, (int)svr_opts.maxauthtries - MAX_PUBKEY_QUERIES);

    int incrfail = ses.authstate.serv_pubkey_query_count > free_query_limit;

    send_msg_userauth_failure(0, incrfail);

    ses.authstate.serv_pubkey_query_count++;

    goto out;

  }

  /* let them know that the key is ok to use */

  if (testkey) {

    send_msg_userauth_pk_ok(sigalgo, sigalgolen, keyblob, keybloblen);

    goto out;

  }

  /* now we can actually verify the signature */

  /* get the key */

  key = new_sign_key();

  if (buf_get_pub_key(ses.payload, key, &keytype) == DROPBEAR_FAILURE) {

    send_msg_userauth_failure(0, 1);

    goto out;

  }

#if DROPBEAR_SK_ECDSA || DROPBEAR_SK_ED25519

  key->sk_flags_mask = SSH_SK_USER_PRESENCE_REQD;

#if DROPBEAR_SVR_PUBKEY_OPTIONS_BUILT

  if (ses.authstate.pubkey_options && ses.authstate.pubkey_options->no_touch_required_flag) {

    key->sk_flags_mask &= ~SSH_SK_USER_PRESENCE_REQD;

  }

  if (ses.authstate.pubkey_options && ses.authstate.pubkey_options->verify_required_flag) {

    key->sk_flags_mask |= SSH_SK_USER_VERIFICATION_REQD;

  }

#endif /* DROPBEAR_SVR_PUBKEY_OPTIONS */

#endif

  /* create the data which has been signed - this a string containing

   * session_id, concatenated with the payload packet up to the signature */

  assert(ses.payload_beginning <= ses.payload->pos);

  sign_payload_length = ses.payload->pos - ses.payload_beginning;

  signbuf = buf_new(ses.payload->pos + 4 + ses.session_id->len);

  buf_putbufstring(signbuf, ses.session_id);

  /* The entire contents of the payload prior. */

  buf_setpos(ses.payload, ses.payload_beginning);

  buf_putbytes(signbuf,

    buf_getptr(ses.payload, sign_payload_length),

    sign_payload_length);

  buf_incrpos(ses.payload, sign_payload_length);

  buf_setpos(signbuf, 0);

  /* ... and finally verify the signature */

  fp = sign_key_fingerprint(keyblob, keybloblen);

  if (buf_verify(ses.payload, key, sigtype, signbuf) == DROPBEAR_SUCCESS) {

    if (svr_opts.multiauthmethod && (ses.authstate.authtypes & ~AUTH_TYPE_PUBKEY)) {

      /* successful pubkey authentication, but extra auth required */

      dropbear_log(LOG_NOTICE,

          "Pubkey auth succeeded for '%s' with %s key %s from %s, extra auth required",

          ses.authstate.pw_name,

          signkey_name_from_type(keytype, NULL), fp,

          svr_ses.addrstring);

      ses.authstate.authtypes &= ~AUTH_TYPE_PUBKEY; /* pubkey auth ok, delete the method flag */

      send_msg_userauth_failure(1, 0); /* Send partial success */

    } else {

      /* successful authentication */

      dropbear_log(LOG_NOTICE,

          "Pubkey auth succeeded for '%s' with %s key %s from %s",

          ses.authstate.pw_name,

          signkey_name_from_type(keytype, NULL), fp,

          svr_ses.addrstring);

      send_msg_userauth_success();

    }

#if DROPBEAR_PLUGIN

    if ((ses.plugin_session != NULL) && (svr_ses.plugin_instance->auth_success != NULL)) {

        /* Was authenticated through the external plugin. tell plugin that signature verification was ok */

        svr_ses.plugin_instance->auth_success(ses.plugin_session);

    }

#endif

  } else {

    dropbear_log(LOG_WARNING,

        "Pubkey auth bad signature for '%s' with key %s from %s",

        ses.authstate.pw_name, fp, svr_ses.addrstring);

    send_msg_userauth_failure(0, 1);

  }

  m_free(fp);

out:

  /* cleanup stuff */

  if (signbuf) {

    buf_free(signbuf);

  }

  if (sigalgo) {

    m_free(sigalgo);

  }

  if (key) {

    sign_key_free(key);

    key = NULL;

  }

  /* Retain pubkey options only if auth succeeded */

  if (!ses.authstate.authdone) {

    svr_pubkey_options_cleanup();

  }

  TRACE(("leave pubkeyauth"))

}

/* Reply that the key is valid for auth, this is sent when the user sends

 * a straight copy of their pubkey to test, to avoid having to perform

 * expensive signing operations with a worthless key */

static void send_msg_userauth_pk_ok(const char* sigalgo, unsigned int sigalgolen,

    const unsigned char* keyblob, unsigned int keybloblen) {

  TRACE(("enter send_msg_userauth_pk_ok"))

  CHECKCLEARTOWRITE();

  buf_putbyte(ses.writepayload, SSH_MSG_USERAUTH_PK_OK);

  buf_putstring(ses.writepayload, sigalgo, sigalgolen);

  buf_putstring(ses.writepayload, (const char*)keyblob, keybloblen);

  encrypt_packet();

  TRACE(("leave send_msg_userauth_pk_ok"))

}

/* Content for SSH_PUBKEYINFO is optionally returned malloced in ret_info (will be

   freed if already set */

static int checkpubkey_line(buffer* line, int line_num, const char* filename,

    const char* algo, unsigned int algolen,

    const unsigned char* keyblob, unsigned int keybloblen,

    char ** ret_info) {

  buffer *options_buf = NULL;

  char *info_str = NULL;

  unsigned int pos, len, infopos, infolen;

  int ret = DROPBEAR_FAILURE;

  if (line->len < MIN_AUTHKEYS_LINE || line->len > MAX_AUTHKEYS_LINE) {

    TRACE(("checkpubkey_line: bad line length %d", line->len))

    goto out;

  }

  if (memchr(line->data, 0x0, line->len) != NULL) {

    TRACE(("checkpubkey_line: bad line has null char"))

    goto out;

  }

  /* compare the algorithm. +3 so we have enough bytes to read a space and some base64 characters too. */

  if (line->pos + algolen+3 > line->len) {

    goto out;

  }

  /* check the key type */

  if (strncmp((const char *) buf_getptr(line, algolen), algo, algolen) != 0) {

    int is_comment = 0;

    unsigned char *options_start = NULL;

    int options_len = 0;

    int escape, quoted;

    /* skip over any comments or leading whitespace */

    while (line->pos < line->len) {

      const char c = buf_getbyte(line);

      if (c == ' ' || c == '\t') {

        continue;

      } else if (c == '#') {

        is_comment = 1;

        break;

      }

      buf_decrpos(line, 1);

      break;

    }

    if (is_comment) {

      /* next line */

      goto out;

    }

    /* remember start of options */

    options_start = buf_getptr(line, 1);

    quoted = 0;

    escape = 0;

    options_len = 0;

    /* figure out where the options are */

    while (line->pos < line->len) {

      const char c = buf_getbyte(line);

      if (!quoted && (c == ' ' || c == '\t')) {

        break;

      }

      escape = (!escape && c == '\\');

      if (!escape && c == '"') {

        quoted = !quoted;

      }

      options_len++;

    }

    options_buf = buf_new(options_len);

    buf_putbytes(options_buf, options_start, options_len);

    /* compare the algorithm. +3 so we have enough bytes to read a space and some base64 characters too. */

    if (line->pos + algolen+3 > line->len) {

      goto out;

    }

    if (strncmp((const char *) buf_getptr(line, algolen), algo, algolen) != 0) {

      goto out;

    }

  }

  buf_incrpos(line, algolen);

  /* check for space (' ') character */

  if (buf_getbyte(line) != ' ') {

    TRACE(("checkpubkey_line: space character expected, isn't there"))

    goto out;

  }

  /* find the length of base64 data */

  pos = line->pos;

  for (len = 0; line->pos < line->len; len++) {

    if (buf_getbyte(line) == ' ') {

      break;

    }

  }

  /* find out the length of the public key info, stop at the first space */

  infopos = line->pos;

  for (infolen = 0; line->pos < line->len; infolen++) {

    const char c = buf_getbyte(line);

    if (c == ' ') {

      break;

    }

    /* We have an allowlist - authorized_keys lines can't be fully trusted,

    some shell scripts may do unsafe things with env var values */

    if (!(isalnum(c) || strchr(".,_-+@", c))) {

      TRACE(("Not setting SSH_PUBKEYINFO, special characters"))

      infolen = 0;

      break;

    }

  }

  if (infolen > 0) {

    info_str = m_malloc(infolen + 1);

    buf_setpos(line, infopos);

  strncpy(info_str, buf_getptr(line, infolen), infolen);

  }

  /* truncate to base64 data length */

  buf_setpos(line, pos);

  buf_setlen(line, line->pos + len);

  TRACE(("checkpubkey_line: line pos = %d len = %d", line->pos, line->len))

  ret = cmp_base64_key(keyblob, keybloblen, (const unsigned char *) algo, algolen, line, NULL);

  /* free pubkey_info if it is filled */

  if (ret_info && *ret_info) {

    m_free(*ret_info);

    *ret_info = NULL;

  }

  if (ret == DROPBEAR_SUCCESS) {

    if (options_buf) {

      ret = svr_add_pubkey_options(options_buf, line_num, filename);

    }

    if (ret_info) {

      /* take the (optional) public key information */

      *ret_info = info_str;

      info_str = NULL;

    }

  }

out:

  if (options_buf) {

    buf_free(options_buf);

  }

  if (info_str) {

    m_free(info_str);

  }

  return ret;

}

/* Returns the full path to the user's authorized_keys file in an

 * allocated string which caller must free. */

static char *authorized_keys_filepath() {

  size_t len = 0;

  char *pathname = NULL, *dir = NULL;

  const char *filename = "authorized_keys";

  dir = expand_homedir_path_home(svr_opts.authorized_keys_dir,

               ses.authstate.pw_dir);

  /* allocate max required pathname storage,

   * = dir + "/" + "authorized_keys" + '\0' */;

  len = strlen(dir) + strlen(filename) + 2;

  pathname = m_malloc(len);

  snprintf(pathname, len, "%s/%s", dir, filename);

  m_free(dir);

  return pathname;

}

/* Checks whether a specified publickey (and associated algorithm) is an

 * acceptable key for authentication */

/* Returns DROPBEAR_SUCCESS if key is ok for auth, DROPBEAR_FAILURE otherwise */

static int checkpubkey(const char* keyalgo, unsigned int keyalgolen,

    const unsigned char* keyblob, unsigned int keybloblen) {

  FILE * authfile = NULL;

  char * filename = NULL;

  int ret = DROPBEAR_FAILURE;

  buffer * line = NULL;

  int line_num;

  uid_t origuid;

  gid_t origgid;

  TRACE(("enter checkpubkey"))

#if DROPBEAR_SVR_MULTIUSER

  /* access the file as the authenticating user. */

  origuid = getuid();

  origgid = getgid();

  if ((setegid(ses.authstate.pw_gid)) < 0 ||

    (seteuid(ses.authstate.pw_uid)) < 0) {

    dropbear_exit("Failed to set euid");

  }

#endif

  /* check file permissions, also whether file exists */

  if (checkpubkeyperms() == DROPBEAR_FAILURE) {

    TRACE(("bad authorized_keys permissions, or file doesn't exist"))

  } else {

    /* we don't need to check pw and pw_dir for validity, since

     * its been done in checkpubkeyperms. */

    filename = authorized_keys_filepath();

    authfile = fopen(filename, "r");

    if (!authfile) {

      TRACE(("checkpubkey: failed opening %s: %s", filename, strerror(errno)))

    }

  }

#if DROPBEAR_SVR_MULTIUSER

  if ((seteuid(origuid)) < 0 ||

    (setegid(origgid)) < 0) {

    dropbear_exit("Failed to revert euid");

  }

#endif

  if (authfile == NULL) {

    goto out;

  }

  TRACE(("checkpubkey: opened authorized_keys OK"))

  line = buf_new(MAX_AUTHKEYS_LINE);

  line_num = 0;

  /* iterate through the lines */

  do {

    if (buf_getline(line, authfile) == DROPBEAR_FAILURE) {

      /* EOF reached */

      TRACE(("checkpubkey: authorized_keys EOF reached"))

      break;

    }

    line_num++;

    ret = checkpubkey_line(line, line_num, filename, keyalgo, keyalgolen,

      keyblob, keybloblen,

#if DROPBEAR_SVR_PUBKEY_OPTIONS_BUILT

      &ses.authstate.pubkey_info

#else

      NULL

#endif

    );

    if (ret == DROPBEAR_SUCCESS) {

      break;

    }

    /* We continue to the next line otherwise */

  } while (1);

out:

  if (authfile) {

    fclose(authfile);

  }

  if (line) {

    buf_free(line);

  }

  m_free(filename);

  TRACE(("leave checkpubkey: ret=%d", ret))

  return ret;

}

/* Returns DROPBEAR_SUCCESS if file permissions for pubkeys are ok,

 * DROPBEAR_FAILURE otherwise.

 * Checks that the authorized_keys path permissions are all owned by either

 * root or the user, and are g-w, o-w.

 * When this path is inside the user's home dir it checks up to and including

 * the home dir, otherwise it checks every path component. */

static int checkpubkeyperms() {

  char *path = authorized_keys_filepath(), *sep = NULL;

  int ret = DROPBEAR_SUCCESS;

  TRACE(("enter checkpubkeyperms"))

  /* Walk back up path checking permissions, stopping at either homedir,

   * or root if the path is outside of the homedir. */

  while ((sep = strrchr(path, '/')) != NULL) {

    if (sep == path) { /* root directory */

      sep++;

    }

    *sep = '\0';

    if (checkfileperm(path) != DROPBEAR_SUCCESS) {

      TRACE(("checkpubkeyperms: bad perm on %s", path))

      ret = DROPBEAR_FAILURE;

    }

    if (strcmp(path, ses.authstate.pw_dir) == 0 || strcmp(path, "/") == 0) {

      break;

    }

  }

  /* all looks ok, return success */

  m_free(path);

  TRACE(("leave checkpubkeyperms"))

  return ret;

}

/* Checks that a file is owned by the user or root, and isn't writable by

 * group or other */

/* returns DROPBEAR_SUCCESS or DROPBEAR_FAILURE */

static int checkfileperm(char * filename) {

  struct stat filestat;

  int badperm = 0;

  TRACE(("enter checkfileperm(%s)", filename))

  if (stat(filename, &filestat) != 0) {

    TRACE(("leave checkfileperm: stat() != 0"))

    return DROPBEAR_FAILURE;

  }

  /* check ownership - user or root only*/

  if (filestat.st_uid != ses.authstate.pw_uid

      && filestat.st_uid != 0) {

    badperm = 1;

    TRACE(("wrong ownership"))

  }

  /* check permissions - don't want group or others +w */

  if (filestat.st_mode & (S_IWGRP | S_IWOTH)) {

    badperm = 1;

    TRACE(("wrong perms"))

  }

  if (badperm) {

    if (!ses.authstate.perm_warn) {

      ses.authstate.perm_warn = 1;

      dropbear_log(LOG_INFO, "%s must be owned by user or root, and not writable by group or others", filename);

    }

    TRACE(("leave checkfileperm: failure perms/owner"))

    return DROPBEAR_FAILURE;

  }

  TRACE(("leave checkfileperm: success"))

  return DROPBEAR_SUCCESS;

}

#if DROPBEAR_FUZZ

int fuzz_checkpubkey_line(buffer* line, int line_num, char* filename,

    const char* algo, unsigned int algolen,

    const unsigned char* keyblob, unsigned int keybloblen) {

  return checkpubkey_line(line, line_num, filename, algo, algolen, keyblob, keybloblen, NULL);

}

#endif

#endif