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/* IVLE - Informatics Virtual Learning Environment
* 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
*
* Program: Trampoline
* Author: Tom Conway, Matt Giuca
* Date: 20/12/2007
*
* This program runs a given program in a given working dir.
* First, it chroots to a jail path and setuids to a given user ID.
* This is intented to provide a safe execution environment for arbitrary
* programs and scripts.
*
* Scripts (such as Python programs) should be executed by supplying
* "/usr/bin/python" as the program, and the script as the first argument.
*
* Usage: trampoline uid jail-path working-path program [args...]
* Must run as root. Will safely setuid to the supplied uid, checking that it
* is not root. Recommended that the file is set up as follows:
* sudo chown root:root trampoline; sudo chroot +s trampoline
*/
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/stat.h>
/* conf.h is admin-configured by the setup process.
* It defines jail_base.
*/
#include "conf.h"
/* Returns TRUE if the given uid is allowed to execute trampoline.
* Only root or the web server should be allowed to execute.
* This is determined by the whitelist allowed_uids in conf.h.
*/
int uid_allowed(int uid)
{
int i;
/* root is always allowed to execute trampoline */
if (uid == 0)
return 1;
/* loop over all allowed_uids */
for (i=(sizeof(allowed_uids)/sizeof(*allowed_uids))-1; i>=0; i--)
{
if (allowed_uids[i] == uid)
return 1;
}
/* default to disallowing */
return 0;
}
/* Turn the process into a daemon using the standard
* 2-fork technique.
*/
void daemonize(void)
{
pid_t pid, sid;
/* already a daemon */
if ( getppid() == 1 ) return;
/* Fork off the parent process */
pid = fork();
if (pid < 0) {
exit(1);
}
/* If we got a good PID, then we can exit the parent process. */
if (pid > 0) {
exit(0);
}
/* At this point we are executing as the child process */
/* Change the file mode mask */
umask(0);
/* Create a new SID for the child process */
sid = setsid();
if (sid < 0) {
exit(1);
}
/* Change the current working directory. This prevents the current
directory from being locked; hence not being able to remove it. */
if ((chdir("/")) < 0) {
exit(1);
}
/* Redirect standard files to /dev/null */
freopen( "/dev/null", "r", stdin);
freopen( "/dev/null", "w", stdout);
freopen( "/dev/null", "w", stderr);
}
static void usage(const char* nm)
{
fprintf(stderr,
"usage: %s [-d] <uid> <jail> <cwd> <program> [args...]\n", nm);
exit(1);
}
int main(int argc, char* const argv[])
{
char* jailpath;
char* work_dir;
char* prog;
char* const * args;
int uid;
int arg_num = 1;
int daemon_mode = 0;
/* Disallow execution from all users but the whitelisted ones, and root */
if (!uid_allowed(getuid()))
{
fprintf(stderr, "only the web server may execute trampoline\n");
exit(1);
}
/* Args check and usage */
if (argc < 5)
{
usage(argv[0]);
}
if (strcmp(argv[arg_num], "-d") == 0)
{
if (argc < 6)
{
usage(argv[0]);
}
daemon_mode = 1;
arg_num++;
}
uid = atoi(argv[arg_num++]);
jailpath = argv[arg_num++];
work_dir = argv[arg_num++];
prog = argv[arg_num];
args = argv + arg_num;
/* Disallow suiding to the root user */
if (uid == 0)
{
fprintf(stderr, "cannot set up a jail as root\n");
exit(1);
}
/* Jail path must:
* Be non-empty
* Start with a '/'
* Not contain "/.."
* Begin with jail_base
*/
if (strlen(jailpath) < 1 || jailpath[0] != '/'
|| strstr(jailpath, "/..")
|| strncmp(jailpath, jail_base, strlen(jail_base)))
{
fprintf(stderr, "bad jail path: %s\n", jailpath);
exit(1);
}
/* chroot into the jail.
* Henceforth this process, and its children, cannot see anything above
* jailpath. */
if (chroot(jailpath))
{
perror("could not chroot");
exit(1);
}
/* chdir into the specified working directory */
if (chdir(work_dir))
{
perror("could not chdir");
exit(1);
}
/* setuid to the given user ID.
* Henceforth we will be running as this user instead of root.
*/
if (setuid(uid))
{
perror("could not setuid");
exit(1);
}
if (daemon_mode)
{
daemonize();
}
/* exec (replace this process with the a new instance of the target
* program). Pass along all the arguments.
* Note that for script execution, the "program" will be the interpreter,
* and the first argument will be the script. */
execv(prog, args);
/* XXX if (daemon_mode) use syslog? */
/* nb exec won't return unless there was an error */
perror("could not exec");
return 1;
}
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