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Filename : ssh_util.py
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# Copyright (C) 2012 Canonical Ltd.
# Copyright (C) 2012 Hewlett-Packard Development Company, L.P.
#
# Author: Scott Moser <scott.moser@canonical.com>
# Author: Juerg Hafliger <juerg.haefliger@hp.com>
#
# This file is part of cloud-init. See LICENSE file for license information.

import os
import pwd

from cloudinit import log as logging
from cloudinit import util

LOG = logging.getLogger(__name__)

# See: man sshd_config
DEF_SSHD_CFG = "/etc/ssh/sshd_config"

# this list has been filtered out from keytypes of OpenSSH source
# openssh-8.3p1/sshkey.c:
# static const struct keytype keytypes[] = {
# filter out the keytypes with the sigonly flag, eg:
# { "rsa-sha2-256", "RSA", NULL, KEY_RSA, 0, 0, 1 },
# refer to the keytype struct of OpenSSH in the same file, to see
# if the position of the sigonly flag has been moved.
#
# dsa, rsa, ecdsa and ed25519 are added for legacy, as they are valid
# public keys in some old distros. They can possibly be removed
# in the future when support for the older distros is dropped
#
# When updating the list, also update the _is_printable_key list in
# cloudinit/config/cc_ssh_authkey_fingerprints.py
VALID_KEY_TYPES = (
    "dsa",
    "rsa",
    "ecdsa",
    "ed25519",
    "ecdsa-sha2-nistp256-cert-v01@openssh.com",
    "ecdsa-sha2-nistp256",
    "ecdsa-sha2-nistp384-cert-v01@openssh.com",
    "ecdsa-sha2-nistp384",
    "ecdsa-sha2-nistp521-cert-v01@openssh.com",
    "ecdsa-sha2-nistp521",
    "sk-ecdsa-sha2-nistp256-cert-v01@openssh.com",
    "sk-ecdsa-sha2-nistp256@openssh.com",
    "sk-ssh-ed25519-cert-v01@openssh.com",
    "sk-ssh-ed25519@openssh.com",
    "ssh-dss-cert-v01@openssh.com",
    "ssh-dss",
    "ssh-ed25519-cert-v01@openssh.com",
    "ssh-ed25519",
    "ssh-rsa-cert-v01@openssh.com",
    "ssh-rsa",
    "ssh-xmss-cert-v01@openssh.com",
    "ssh-xmss@openssh.com",
)

_DISABLE_USER_SSH_EXIT = 142

DISABLE_USER_OPTS = (
    "no-port-forwarding,no-agent-forwarding,"
    "no-X11-forwarding,command=\"echo \'Please login as the user \\\"$USER\\\""
    " rather than the user \\\"$DISABLE_USER\\\".\';echo;sleep 10;"
    "exit " + str(_DISABLE_USER_SSH_EXIT) + "\"")


class AuthKeyLine(object):
    def __init__(self, source, keytype=None, base64=None,
                 comment=None, options=None):
        self.base64 = base64
        self.comment = comment
        self.options = options
        self.keytype = keytype
        self.source = source

    def valid(self):
        return (self.base64 and self.keytype)

    def __str__(self):
        toks = []
        if self.options:
            toks.append(self.options)
        if self.keytype:
            toks.append(self.keytype)
        if self.base64:
            toks.append(self.base64)
        if self.comment:
            toks.append(self.comment)
        if not toks:
            return self.source
        else:
            return ' '.join(toks)


class AuthKeyLineParser(object):
    """
    AUTHORIZED_KEYS FILE FORMAT
     AuthorizedKeysFile specifies the file containing public keys for public
     key authentication; if none is specified, the default is
     ~/.ssh/authorized_keys.  Each line of the file contains one key (empty
     (because of the size of the public key encoding) up to a limit of 8 kilo-
     bytes, which permits DSA keys up to 8 kilobits and RSA keys up to 16
     kilobits.  You don't want to type them in; instead, copy the
     identity.pub, id_dsa.pub, or the id_rsa.pub file and edit it.

     sshd enforces a minimum RSA key modulus size for protocol 1 and protocol
     2 keys of 768 bits.

     The options (if present) consist of comma-separated option specifica-
     tions.  No spaces are permitted, except within double quotes.  The fol-
     lowing option specifications are supported (note that option keywords are
     case-insensitive):
    """

    def _extract_options(self, ent):
        """
        The options (if present) consist of comma-separated option specifica-
         tions.  No spaces are permitted, except within double quotes.
         Note that option keywords are case-insensitive.
        """
        quoted = False
        i = 0
        while (i < len(ent) and
               ((quoted) or (ent[i] not in (" ", "\t")))):
            curc = ent[i]
            if i + 1 >= len(ent):
                i = i + 1
                break
            nextc = ent[i + 1]
            if curc == "\\" and nextc == '"':
                i = i + 1
            elif curc == '"':
                quoted = not quoted
            i = i + 1

        options = ent[0:i]

        # Return the rest of the string in 'remain'
        remain = ent[i:].lstrip()
        return (options, remain)

    def parse(self, src_line, options=None):
        # modeled after opensshes auth2-pubkey.c:user_key_allowed2
        line = src_line.rstrip("\r\n")
        if line.startswith("#") or line.strip() == '':
            return AuthKeyLine(src_line)

        def parse_ssh_key(ent):
            # return ketype, key, [comment]
            toks = ent.split(None, 2)
            if len(toks) < 2:
                raise TypeError("To few fields: %s" % len(toks))
            if toks[0] not in VALID_KEY_TYPES:
                raise TypeError("Invalid keytype %s" % toks[0])

            # valid key type and 2 or 3 fields:
            if len(toks) == 2:
                # no comment in line
                toks.append("")

            return toks

        ent = line.strip()
        try:
            (keytype, base64, comment) = parse_ssh_key(ent)
        except TypeError:
            (keyopts, remain) = self._extract_options(ent)
            if options is None:
                options = keyopts

            try:
                (keytype, base64, comment) = parse_ssh_key(remain)
            except TypeError:
                return AuthKeyLine(src_line)

        return AuthKeyLine(src_line, keytype=keytype, base64=base64,
                           comment=comment, options=options)


def parse_authorized_keys(fnames):
    lines = []
    parser = AuthKeyLineParser()
    contents = []
    for fname in fnames:
        try:
            if os.path.isfile(fname):
                lines = util.load_file(fname).splitlines()
                for line in lines:
                    contents.append(parser.parse(line))
        except (IOError, OSError):
            util.logexc(LOG, "Error reading lines from %s", fname)

    return contents


def update_authorized_keys(old_entries, keys):
    to_add = list([k for k in keys if k.valid()])
    for i in range(0, len(old_entries)):
        ent = old_entries[i]
        if not ent.valid():
            continue
        # Replace those with the same base64
        for k in keys:
            if k.base64 == ent.base64:
                # Replace it with our better one
                ent = k
                # Don't add it later
                if k in to_add:
                    to_add.remove(k)
        old_entries[i] = ent

    # Now append any entries we did not match above
    for key in to_add:
        old_entries.append(key)

    # Now format them back to strings...
    lines = [str(b) for b in old_entries]

    # Ensure it ends with a newline
    lines.append('')
    return '\n'.join(lines)


def users_ssh_info(username):
    pw_ent = pwd.getpwnam(username)
    if not pw_ent or not pw_ent.pw_dir:
        raise RuntimeError("Unable to get SSH info for user %r" % (username))
    return (os.path.join(pw_ent.pw_dir, '.ssh'), pw_ent)


def render_authorizedkeysfile_paths(value, homedir, username):
    # The 'AuthorizedKeysFile' may contain tokens
    # of the form %T which are substituted during connection set-up.
    # The following tokens are defined: %% is replaced by a literal
    # '%', %h is replaced by the home directory of the user being
    # authenticated and %u is replaced by the username of that user.
    macros = (("%h", homedir), ("%u", username), ("%%", "%"))
    if not value:
        value = "%h/.ssh/authorized_keys"
    paths = value.split()
    rendered = []
    for path in paths:
        for macro, field in macros:
            path = path.replace(macro, field)
        if not path.startswith("/"):
            path = os.path.join(homedir, path)
        rendered.append(path)
    return rendered


def extract_authorized_keys(username, sshd_cfg_file=DEF_SSHD_CFG):
    (ssh_dir, pw_ent) = users_ssh_info(username)
    default_authorizedkeys_file = os.path.join(ssh_dir, 'authorized_keys')
    auth_key_fns = []
    with util.SeLinuxGuard(ssh_dir, recursive=True):
        try:
            ssh_cfg = parse_ssh_config_map(sshd_cfg_file)
            auth_key_fns = render_authorizedkeysfile_paths(
                ssh_cfg.get("authorizedkeysfile", "%h/.ssh/authorized_keys"),
                pw_ent.pw_dir, username)

        except (IOError, OSError):
            # Give up and use a default key filename
            auth_key_fns[0] = default_authorizedkeys_file
            util.logexc(LOG, "Failed extracting 'AuthorizedKeysFile' in SSH "
                        "config from %r, using 'AuthorizedKeysFile' file "
                        "%r instead", DEF_SSHD_CFG, auth_key_fns[0])

    # always store all the keys in the user's private file
    return (default_authorizedkeys_file, parse_authorized_keys(auth_key_fns))


def setup_user_keys(keys, username, options=None):
    # Make sure the users .ssh dir is setup accordingly
    (ssh_dir, pwent) = users_ssh_info(username)
    if not os.path.isdir(ssh_dir):
        util.ensure_dir(ssh_dir, mode=0o700)
        util.chownbyid(ssh_dir, pwent.pw_uid, pwent.pw_gid)

    # Turn the 'update' keys given into actual entries
    parser = AuthKeyLineParser()
    key_entries = []
    for k in keys:
        key_entries.append(parser.parse(str(k), options=options))

    # Extract the old and make the new
    (auth_key_fn, auth_key_entries) = extract_authorized_keys(username)
    with util.SeLinuxGuard(ssh_dir, recursive=True):
        content = update_authorized_keys(auth_key_entries, key_entries)
        util.ensure_dir(os.path.dirname(auth_key_fn), mode=0o700)
        util.write_file(auth_key_fn, content, mode=0o600)
        util.chownbyid(auth_key_fn, pwent.pw_uid, pwent.pw_gid)


class SshdConfigLine(object):
    def __init__(self, line, k=None, v=None):
        self.line = line
        self._key = k
        self.value = v

    @property
    def key(self):
        if self._key is None:
            return None
        # Keywords are case-insensitive
        return self._key.lower()

    def __str__(self):
        if self._key is None:
            return str(self.line)
        else:
            v = str(self._key)
            if self.value:
                v += " " + str(self.value)
            return v


def parse_ssh_config(fname):
    if not os.path.isfile(fname):
        return []
    return parse_ssh_config_lines(util.load_file(fname).splitlines())


def parse_ssh_config_lines(lines):
    # See: man sshd_config
    # The file contains keyword-argument pairs, one per line.
    # Lines starting with '#' and empty lines are interpreted as comments.
    # Note: key-words are case-insensitive and arguments are case-sensitive
    ret = []
    for line in lines:
        line = line.strip()
        if not line or line.startswith("#"):
            ret.append(SshdConfigLine(line))
            continue
        try:
            key, val = line.split(None, 1)
        except ValueError:
            key, val = line.split('=', 1)
        ret.append(SshdConfigLine(line, key, val))
    return ret


def parse_ssh_config_map(fname):
    lines = parse_ssh_config(fname)
    if not lines:
        return {}
    ret = {}
    for line in lines:
        if not line.key:
            continue
        ret[line.key] = line.value
    return ret


def update_ssh_config(updates, fname=DEF_SSHD_CFG):
    """Read fname, and update if changes are necessary.

    @param updates: dictionary of desired values {Option: value}
    @return: boolean indicating if an update was done."""
    lines = parse_ssh_config(fname)
    changed = update_ssh_config_lines(lines=lines, updates=updates)
    if changed:
        util.write_file(
            fname, "\n".join(
                [str(line) for line in lines]
            ) + "\n", preserve_mode=True)
    return len(changed) != 0


def update_ssh_config_lines(lines, updates):
    """Update the SSH config lines per updates.

    @param lines: array of SshdConfigLine.  This array is updated in place.
    @param updates: dictionary of desired values {Option: value}
    @return: A list of keys in updates that were changed."""
    found = set()
    changed = []

    # Keywords are case-insensitive and arguments are case-sensitive
    casemap = dict([(k.lower(), k) for k in updates.keys()])

    for (i, line) in enumerate(lines, start=1):
        if not line.key:
            continue
        if line.key in casemap:
            key = casemap[line.key]
            value = updates[key]
            found.add(key)
            if line.value == value:
                LOG.debug("line %d: option %s already set to %s",
                          i, key, value)
            else:
                changed.append(key)
                LOG.debug("line %d: option %s updated %s -> %s", i,
                          key, line.value, value)
                line.value = value

    if len(found) != len(updates):
        for key, value in updates.items():
            if key in found:
                continue
            changed.append(key)
            lines.append(SshdConfigLine('', key, value))
            LOG.debug("line %d: option %s added with %s",
                      len(lines), key, value)
    return changed

# vi: ts=4 expandtab