~azzar1/unity/add-show-desktop-key

# IVLE

# Copyright (C) 2007-2008 The University of Melbourne

#

# This program is free software; you can redistribute it and/or modify

# it under the terms of the GNU General Public License as published by

# the Free Software Foundation; either version 2 of the License, or

# (at your option) any later version.

#

# 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, write to the Free Software

# Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA  02110-1301  USA

# Module: Interpret

# Author: Matt Giuca

# Date: 18/1/2008

# Runs a student script in a safe execution environment.

#

# NOTE: This script currently disables cookies. This means students will be

# unable to write session-based or stateful web applications. This is done for

# security reasons (we do not want the students to see the IVLE cookie of

# whoever is visiting their pages).

# This can be resolved but needs careful sanitisation. See fixup_environ.

from common import studpath

from common import db

import conf

import functools

import os

import pwd

import subprocess

import cgi

# TODO: Make progressive output work

# Question: Will having a large buffer size stop progressive output from

# working on smaller output

CGI_BLOCK_SIZE = 65535

uids = {}

def get_uid(login):

    """Get the unix uid corresponding to the given login name.

       If it is not in the dictionary of uids, then consult the

       database and retrieve an update of the user table."""

    global uids

    if login in uids:

        return uids[login]

    conn = db.DB()

    res = conn.get_all('login', ['login', 'unixid'])

    def repack(flds):

        return (flds['login'], flds['unixid'])

    uids = dict(map(repack,res))

    return uids[login]

def interpret_file(req, owner, jail_dir, filename, interpreter):

    """Serves a file by interpreting it using one of IVLE's builtin

    interpreters. All interpreters are intended to run in the user's jail. The

    jail location is provided as an argument to the interpreter but it is up

    to the individual interpreters to create the jail.

    req: An IVLE request object.

    owner: Username of the user who owns the file being served.

    jail_dir: Absolute path to the user's jail.

    filename: Absolute filename within the user's jail.

    interpreter: A function object to call.

    """

    # We can't test here whether or not the target file actually exists,

    # because the apache user may not have permission. Instead we have to

    # rely on the interpreter generating an error.

    if filename.startswith(os.sep):

        filename_abs = filename

        filename_rel = filename[1:]

    else:

        filename_abs = os.path.join(os.sep, filename)

        filename_rel = filename

    # Get the UID of the owner of the file

    # (Note: files are executed by their owners, not the logged in user.

    # This ensures users are responsible for their own programs and also

    # allows them to be executed by the public).

    uid = get_uid(owner)

    # Split up req.path again, this time with respect to the jail

    (working_dir, _) = os.path.split(filename_abs)

    # jail_dir is the absolute jail directory.

    # path is the filename relative to the user's jail.

    # working_dir is the directory containing the file relative to the user's

    # jail.

    # (Note that paths "relative" to the jail actually begin with a '/' as

    # they are absolute in the jailspace)

    return interpreter(uid, jail_dir, working_dir, filename_abs, req)

class CGIFlags:

    """Stores flags regarding the state of reading CGI output."""

    def __init__(self):

        self.started_cgi_body = False

        self.got_cgi_headers = False

        self.wrote_html_warning = False

        self.linebuf = ""

        self.headers = {}       # Header names : values

def execute_cgi(interpreter, trampoline, uid, jail_dir, working_dir,

                script_path, req):

    """

    trampoline: Full path on the local system to the CGI wrapper program

        being executed.

    uid: User ID of the owner of the file.

    jail_dir: Absolute path of owner's jail directory.

    working_dir: Directory containing the script file relative to owner's

        jail.

    script_path: CGI script relative to the owner's jail.

    req: IVLE request object.

    The called CGI wrapper application shall be called using popen and receive

    the HTTP body on stdin. It shall receive the CGI environment variables to

    its environment.

    """

    # Get the student program's directory and execute it from that context.

    (tramp_dir, _) = os.path.split(trampoline)

    # TODO: Don't create a file if the body length is known to be 0

    # Write the HTTP body to a temporary file so it can be passed as a *real*

    # file to popen.

    f = os.tmpfile()

    body = req.read()

    if body is not None:

        f.write(body)

        f.flush()

        f.seek(0)       # Rewind, for reading

    # Set up the environment

    # This automatically asks mod_python to load up the CGI variables into the

    # environment (which is a good first approximation)

    old_env = os.environ.copy()

    for k in os.environ.keys():

        del os.environ[k]

    for (k,v) in req.get_cgi_environ().items():

        os.environ[k] = v

    fixup_environ(req)

    # usage: tramp uid jail_dir working_dir script_path

    pid = subprocess.Popen(

        [trampoline, str(uid), jail_dir, working_dir, interpreter,

        script_path],

        stdin=f, stdout=subprocess.PIPE, stderr=subprocess.STDOUT,

        cwd=tramp_dir)

    # Restore the environment

    for k in os.environ.keys():

        del os.environ[k]

    for (k,v) in old_env.items():

        os.environ[k] = v

    # process_cgi_line: Reads a single line of CGI output and processes it.

    # Prints to req, and also does fancy HTML warnings if Content-Type

    # omitted.

    cgiflags = CGIFlags()

    # Read from the process's stdout into req

    data = pid.stdout.read(CGI_BLOCK_SIZE)

    while len(data) > 0:

        process_cgi_output(req, data, cgiflags)

        data = pid.stdout.read(CGI_BLOCK_SIZE)

    # If we haven't processed headers yet, now is a good time

    if not cgiflags.started_cgi_body:

        process_cgi_output(req, '\n', cgiflags)

    # If we wrote an HTML warning header, write the footer

    if cgiflags.wrote_html_warning:

        req.write("""</pre>

  </div>

</body>

</html>""")

def process_cgi_output(req, data, cgiflags):

    """Processes a chunk of CGI output. data is a string of arbitrary length;

    some arbitrary chunk of output written by the CGI script."""

    if cgiflags.started_cgi_body:

        if cgiflags.wrote_html_warning:

            # HTML escape text if wrote_html_warning

            req.write(cgi.escape(data))

        else:

            req.write(data)

    else:

        # Break data into lines of CGI header data. 

        linebuf = cgiflags.linebuf + data

        # First see if we can split all header data

        # We need to get the double CRLF- or LF-terminated headers, whichever

        # is smaller, as either sequence may appear somewhere in the body.

        usplit = linebuf.split('\n\n', 1)

        wsplit = linebuf.split('\r\n\r\n', 1)

        split = len(usplit[0]) > len(wsplit[0]) and wsplit or usplit

        if len(split) == 1:

            # Haven't seen all headers yet. Buffer and come back later.

            cgiflags.linebuf = linebuf

            return

        headers = split[0]

        data = split[1]

        cgiflags.linebuf = ""

        cgiflags.started_cgi_body = True

        # Process all the header lines

        split = headers.split('\r\n', 1)

        if len(split) == 1:

            split = headers.split('\n', 1)

        while True:

            process_cgi_header_line(req, split[0], cgiflags)

            if len(split) == 1: break

            headers = split[1]

            if cgiflags.wrote_html_warning:

                # We're done with headers. Treat the rest as data.

                data = headers + '\n' + data

                break

            split = headers.split('\r\n', 1)

            if len(split) == 1:

                split = headers.split('\n', 1)

        # Check to make sure the required headers were written

        if cgiflags.wrote_html_warning:

            # We already reported an error, that's enough

            pass

        elif "Content-Type" in cgiflags.headers:

            pass

        elif "Location" in cgiflags.headers:

            if ("Status" in cgiflags.headers and req.status >= 300

                and req.status < 400):

                pass

            else:

                message = """You did not write a valid status code for

the given location. To make a redirect, you may wish to try:</p>

<pre style="margin-left: 1em">Status: 302 Found

Location: &lt;redirect address&gt;</pre>"""

                write_html_warning(req, message)

                cgiflags.wrote_html_warning = True

        else:

            message = """You did not print a Content-Type header.

CGI requires that you print a "Content-Type". You may wish to try:</p>

<pre style="margin-left: 1em">Content-Type: text/html</pre>"""

            write_html_warning(req, message)

            cgiflags.wrote_html_warning = True

        # Call myself to flush out the extra bit of data we read

        process_cgi_output(req, data, cgiflags)

def process_cgi_header_line(req, line, cgiflags):

    """Process a line of CGI header data. line is a string representing a

    complete line of text, stripped and without the newline.

    """

    try:

        name, value = line.split(':', 1)

    except ValueError:

        # No colon. The user did not write valid headers.

        if len(cgiflags.headers) == 0:

            # First line was not a header line. We can assume this is not

            # a CGI app.

            message = """You did not print a CGI header.

CGI requires that you print a "Content-Type". You may wish to try:</p>

<pre style="margin-left: 1em">Content-Type: text/html</pre>"""

        else:

            # They printed some header at least, but there was an invalid

            # header.

            message = """You printed an invalid CGI header. You need to leave

a blank line after the headers, before writing the page contents."""

        write_html_warning(req, message)

        cgiflags.wrote_html_warning = True

        # Handle the rest of this line as normal data

        process_cgi_output(req, line + '\n', cgiflags)

        return

    # Read CGI headers

    value = value.strip()

    if name == "Content-Type":

        req.content_type = value

    elif name == "Location":

        req.location = value

    elif name == "Status":

        # Must be an integer, followed by a space, and then the status line

        # which we ignore (seems like Apache has no way to send a custom

        # status line).

        try:

            req.status = int(value.split(' ', 1)[0])

        except ValueError:

            message = """The "Status" CGI header was invalid. You need to

print a number followed by a message, such as "302 Found"."""

            write_html_warning(req, message)

            cgiflags.wrote_html_warning = True

            # Handle the rest of this line as normal data

            process_cgi_output(req, line + '\n', cgiflags)

    else:

        # Generic HTTP header

        # FIXME: Security risk letting users write arbitrary headers?

        req.headers_out[name] = value

    cgiflags.headers[name] = value

def write_html_warning(req, text):

    """Prints an HTML warning about invalid CGI interaction on the part of the

    user. text may contain HTML markup."""

    req.content_type = "text/html"

    req.write("""<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN"

"http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">

<html xmlns="http://www.w3.org/1999/xhtml">

<head>

  <meta http-equiv="Content-Type"

    content="text/html; charset=utf-8" />

</head>

<body style="margin: 0; padding: 0; font-family: sans-serif;">

  <div style="background-color: #faa; border-bottom: 1px solid black;

    padding: 8px;">

    <p><strong>Warning</strong>: %s

  </div>

  <div style="margin: 8px;">

    <pre>

""" % text)

location_cgi_python = os.path.join(conf.ivle_install_dir,

    "bin/trampoline")

# Mapping of interpreter names (as given in conf/app/server.py) to

# interpreter functions.

interpreter_objects = {

    'cgi-python'

        : functools.partial(execute_cgi, "/usr/bin/python",

            location_cgi_python),

    # Should also have:

    # cgi-generic

    # python-server-page

}

def fixup_environ(req):

    """Assuming os.environ has been written with the CGI variables from

    apache, make a few changes for security and correctness.

    Does not modify req, only reads it.

    """

    env = os.environ

    # Comments here are on the heavy side, explained carefully for security

    # reasons. Please read carefully before making changes.

    # Remove HTTP_COOKIE. It is a security risk to have students see the IVLE

    # cookie of their visitors.

    try:

        del env['HTTP_COOKIE']

    except: pass

    # Remove DOCUMENT_ROOT and SCRIPT_FILENAME. Not part of CGI spec and

    # exposes unnecessary details about server.

    try:

        del env['DOCUMENT_ROOT']

    except: pass

    try:

        del env['SCRIPT_FILENAME']

    except: pass

    # Remove PATH. The PATH here is the path on the server machine; not useful

    # inside the jail. It may be a good idea to add another path, reflecting

    # the inside of the jail, but not done at this stage.

    try:

        del env['PATH']

    except: pass

    # Remove SCRIPT_FILENAME. Not part of CGI spec (see SCRIPT_NAME).

    # PATH_INFO is wrong because the script doesn't physically exist.

    # Apache makes it relative to the "serve" app. It should actually be made

    # relative to the student's script.

    # TODO: At this stage, it is not possible to add a path after the script,

    # so PATH_INFO is always "".

    path_info = ""

    env['PATH_INFO'] = path_info

    # PATH_TRANSLATED currently points to a non-existant location within the

    # local web server directory. Instead make it represent a path within the

    # student jail.

    (username, _, path_translated) = studpath.url_to_jailpaths(req.path)

    if len(path_translated) == 0 or path_translated[0] != os.sep:

        path_translated = os.sep + path_translated

    env['PATH_TRANSLATED'] = path_translated

    # CGI specifies that REMOTE_HOST SHOULD be set, and MAY just be set to

    # REMOTE_ADDR. Since Apache does not appear to set this, set it to

    # REMOTE_ADDR.

    if 'REMOTE_HOST' not in env and 'REMOTE_ADDR' in env:

        env['REMOTE_HOST'] = env['REMOTE_ADDR']

    # SCRIPT_NAME is the path to the script WITHOUT PATH_INFO.

    script_name = req.uri

    if len(path_info) > 0:

        script_name = script_name[:-len(path_info)]

    env['SCRIPT_NAME'] = script_name

    # SERVER_SOFTWARE is actually not Apache but IVLE, since we are

    # custom-making the CGI request.

    env['SERVER_SOFTWARE'] = "IVLE/" + str(conf.ivle_version)

    # Additional environment variables

    env['HOME'] = os.path.join('/home', username)