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/*
* This program creates a new filesystem namespace, bind mounts /bin/bash
* over /bin/sh and runs its arguments.
* It needs to be installed setuid root and will drop the extra privileges
* before running the argument.
*
* gcc -O2 -s switchsh.c -o switchsh
* chown root:root switchsh
* chmod u+s switchsh
* ./switchsh /bin/sh --version
*
* Written by Marco d'Itri <md@linux.it>, released to the public domain.
*/
/* Needed for CLONE_NEWNS */
#define _GNU_SOURCE
#include <stdio.h>
#include <stdlib.h>
#include <stdarg.h>
#include <unistd.h>
#include <string.h>
#include <errno.h>
#include <pwd.h>
#include <grp.h>
#include <sys/syscall.h> /* syscall */
#include <signal.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <sys/wait.h> /* waitpid */
#include <sys/mount.h> /* mount */
# include <sched.h> /* unshare */
#ifndef USE_CLONE
/* old kernel headers may lack the system call number */
# if defined(__NR_unshare)
# elif defined(__i386__)
# define __NR_unshare 310
# elif defined(__x86_64__)
# define __NR_unshare 272
# elif defined(__powerpc__) || defined(__powerpc64__)
# define __NR_unshare 282
# else
/* update the kernel headers or add the number for the architecture */
# error "unshare unsupported on this architecture!"
# endif
# ifndef SYS_unshare
# define SYS_unshare __NR_unshare
# endif
# if !(__GLIBC__ >= 2 && __GLIBC_MINOR__ >= 4)
static inline int unshare(int flags)
{
return syscall(SYS_unshare, flags);
}
# endif
#endif
/* Prototypes */
void err_quit(const char *, ...)
__attribute__ ((noreturn, format(printf, 1, 2)));
void err_sys(const char *, ...)
__attribute__ ((noreturn, format(printf, 1, 2)));
int main(int argc, char *argv[])
{
int pid;
if (!*++argv)
err_quit("Usage: switchsh PROGRAM [ARG]...");
#ifdef USE_CLONE
/* think about this as fork(2)... */
pid = syscall(SYS_clone, CLONE_NEWNS | SIGCHLD, 0);
#else
pid = fork();
#endif
if (pid < 0) { /* error */
if (errno == EPERM)
err_quit("This program must be setuid root!");
err_sys("fork");
}
if (pid > 0) {
/* parent */
int status;
/* wait for the child */
waitpid(pid, &status, 0);
exit(WIFEXITED(status) ? WEXITSTATUS(status) : WTERMSIG(status) + 128);
}
/* child */
#ifndef USE_CLONE
if (unshare(CLONE_NEWNS) < 0) {
if (errno == ENOSYS)
err_quit("unshare requires a kernel >= 2.6.16");
else
err_sys("unshare");
}
#endif
if ((mount("", "/", NULL, MS_SLAVE|MS_REC, NULL) < 0) ||
(mount("/bin/bash", "/bin/sh", NULL, MS_BIND, NULL) < 0)) {
if (errno == EPERM)
err_quit("This program must be setuid root!");
err_sys("mount");
}
/* permanently drop privileges */
if (setgid(getgid()) < 0)
err_sys("cannot drop GID 0");
if (setuid(getuid()) < 0)
err_sys("cannot drop UID 0");
execvp(*argv, argv);
err_sys("Can't exec %s", argv[0]);
}
/* Error routines */
void err_sys(const char *fmt, ...)
{
va_list ap;
va_start(ap, fmt);
vfprintf(stderr, fmt, ap);
fprintf(stderr, ": %s\n", strerror(errno));
va_end(ap);
exit(1);
}
void err_quit(const char *fmt, ...)
{
va_list ap;
va_start(ap, fmt);
vfprintf(stderr, fmt, ap);
fputs("\n", stderr);
va_end(ap);
exit(1);
}
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