man2/chroot.2

   1 .\" Copyright (c) 1992 Drew Eckhardt (drew@cs.colorado.edu), March 28, 1992
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  25 .\" Modified by Michael Haardt <michael@moria.de>
  26 .\" Modified 1993-07-21 by Rik Faith <faith@cs.unc.edu>
  27 .\" Modified 1994-08-21 by Michael Chastain <mec@shell.portal.com>
  28 .\" Modified 1996-06-13 by aeb
  29 .\" Modified 1996-11-06 by Eric S. Raymond <esr@thyrsus.com>
  30 .\" Modified 1997-08-21 by Joseph S. Myers <jsm28@cam.ac.uk>
  31 .\" Modified 2004-06-23 by Michael Kerrisk <mtk.manpages@gmail.com>
  32 .\"
  33 .TH CHROOT 2 2016-07-17 "Linux" "Linux Programmer's Manual"
  34 .SH NAME
  35 chroot \- change root directory
  36 .SH SYNOPSIS
  37 .B #include <unistd.h>
  38 .sp
  39 .BI "int chroot(const char *" path );
  40 .sp
  41 .in -4n
  42 Feature Test Macro Requirements for glibc (see
  43 .BR feature_test_macros (7)):
  44 .in
  45 .sp
  46 .BR chroot ():
  47 .ad l
  48 .RS 4
  49 .PD 0
  50 .TP 4
  51 Since glibc 2.2.2:
  52 .nf
  53 _XOPEN_SOURCE && ! (_POSIX_C_SOURCE\ >=\ 200112L)
  54     || /* Since glibc 2.20: */ _DEFAULT_SOURCE
  55     || /* Glibc versions <= 2.19: */ _BSD_SOURCE
  56 .TP 4
  57 .fi
  58 Before glibc 2.2.2: none
  59 .PD
  60 .RE
  61 .ad b
  62 .SH DESCRIPTION
  63 .BR chroot ()
  64 changes the root directory of the calling process to that specified in
  65 .IR path .
  66 This directory will be used for pathnames beginning with \fI/\fP.
  67 The root directory is inherited by all children of the calling process.
  68
  69 Only a privileged process (Linux: one with the
  70 .B CAP_SYS_CHROOT
  71 capability in its user namespace) may call
  72 .BR chroot ().
  73
  74 This call changes an ingredient in the pathname resolution process
  75 and does nothing else.
  76 In particular, it is not intended to be used
  77 for any kind of security purpose, neither to fully sandbox a process nor
  78 to restrict filesystem system calls.
  79 In the past,
  80 .BR chroot ()
  81 has been used by daemons to restrict themselves prior to passing paths
  82 supplied by untrusted users to system calls such as
  83 .BR open (2).
  84 However, if a folder is moved out of the chroot directory, an attacker
  85 can exploit that to get out of the chroot directory as well.
  86 The easiest way to do that is to
  87 .BR chdir (2)
  88 to the to-be-moved directory, wait for it to be moved out, then open a
  89 path like ../../../etc/passwd.
  90
  91 .\" This is how the "slightly trickier variation" works:
  92 .\" https://github.com/QubesOS/qubes-secpack/blob/master/QSBs/qsb-014-2015.txt#L142
  93 A slightly
  94 trickier variation also works under some circumstances if
  95 .BR chdir (2)
  96 is not permitted.
  97 If a daemon allows a "chroot directory" to be specified,
  98 that usually means that if you want to prevent remote users from accessing
  99 files outside the chroot directory, you must ensure that folders are never
 100 moved out of it.
 101
 102 This call does not change the current working directory,
 103 so that after the call \(aq\fI.\fP\(aq can
 104 be outside the tree rooted at \(aq\fI/\fP\(aq.
 105 In particular, the superuser can escape from a "chroot jail"
 106 by doing:
 107 .nf
 108
 109     mkdir foo; chroot foo; cd ..
 110 .fi
 111
 112 This call does not close open file descriptors, and such file
 113 descriptors may allow access to files outside the chroot tree.
 114 .SH RETURN VALUE
 115 On success, zero is returned.
 116 On error, \-1 is returned, and
 117 .I errno
 118 is set appropriately.
 119 .SH ERRORS
 120 Depending on the filesystem, other errors can be returned.
 121 The more general errors are listed below:
 122 .TP
 123 .B EACCES
 124 Search permission is denied on a component of the path prefix.
 125 (See also
 126 .BR path_resolution (7).)
 127 .\" Also search permission is required on the final component,
 128 .\" maybe just to guarantee that it is a directory?
 129 .TP
 130 .B EFAULT
 131 .I path
 132 points outside your accessible address space.
 133 .TP
 134 .B EIO
 135 An I/O error occurred.
 136 .TP
 137 .B ELOOP
 138 Too many symbolic links were encountered in resolving
 139 .IR path .
 140 .TP
 141 .B ENAMETOOLONG
 142 .I path
 143 is too long.
 144 .TP
 145 .B ENOENT
 146 The file does not exist.
 147 .TP
 148 .B ENOMEM
 149 Insufficient kernel memory was available.
 150 .TP
 151 .B ENOTDIR
 152 A component of
 153 .I path
 154 is not a directory.
 155 .TP
 156 .B EPERM
 157 The caller has insufficient privilege.
 158 .SH CONFORMING TO
 159 SVr4, 4.4BSD, SUSv2 (marked LEGACY).
 160 This function is not part of POSIX.1-2001.
 161 .\" SVr4 documents additional EINTR, ENOLINK and EMULTIHOP error conditions.
 162 .\" X/OPEN does not document EIO, ENOMEM or EFAULT error conditions.
 163 .SH NOTES
 164 A child process created via
 165 .BR fork (2)
 166 inherits its parent's root directory.
 167 The root directory is left unchanged by
 168 .BR execve (2).
 169
 170 FreeBSD has a stronger
 171 .BR jail ()
 172 system call.
 173 .\" FIXME . eventually say something about containers,
 174 .\" virtual servers, etc.?
 175 .SH SEE ALSO
 176 .BR chdir (2),
 177 .BR pivot_root (2),
 178 .BR path_resolution (7)