man2/chroot.2

.\" Copyright (c) 1992 Drew Eckhardt (drew@cs.colorado.edu), March 28, 1992
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.\" Modified by Michael Haardt <michael@moria.de>
.\" Modified 1993-07-21 by Rik Faith <faith@cs.unc.edu>
.\" Modified 1994-08-21 by Michael Chastain <mec@shell.portal.com>
.\" Modified 1996-06-13 by aeb
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.\" Modified 2004-06-23 by Michael Kerrisk <mtk.manpages@gmail.com>
.\"
.TH CHROOT 2 2021-03-22 "Linux" "Linux Programmer's Manual"
.SH NAME
chroot \- change root directory
.SH SYNOPSIS
.nf
.B #include <unistd.h>
.PP
.BI "int chroot(const char *" path );
.fi
.PP
.RS -4
Feature Test Macro Requirements for glibc (see
.BR feature_test_macros (7)):
.RE
.PP
.BR chroot ():
.nf
    Since glibc 2.2.2:
        _XOPEN_SOURCE && ! (_POSIX_C_SOURCE >= 200112L)
            || /* Since glibc 2.20: */ _DEFAULT_SOURCE
            || /* Glibc <= 2.19: */ _BSD_SOURCE
    Before glibc 2.2.2:
        none
.fi
.SH DESCRIPTION
.BR chroot ()
changes the root directory of the calling process to that specified in
.IR path .
This directory will be used for pathnames beginning with \fI/\fP.
The root directory is inherited by all children of the calling process.
.PP
Only a privileged process (Linux: one with the
.B CAP_SYS_CHROOT
capability in its user namespace) may call
.BR chroot ().
.PP
This call changes an ingredient in the pathname resolution process
and does nothing else.
In particular, it is not intended to be used
for any kind of security purpose, neither to fully sandbox a process nor
to restrict filesystem system calls.
In the past,
.BR chroot ()
has been used by daemons to restrict themselves prior to passing paths
supplied by untrusted users to system calls such as
.BR open (2).
However, if a folder is moved out of the chroot directory, an attacker
can exploit that to get out of the chroot directory as well.
The easiest way to do that is to
.BR chdir (2)
to the to-be-moved directory, wait for it to be moved out, then open a
path like ../../../etc/passwd.
.PP
.\" This is how the "slightly trickier variation" works:
.\" https://github.com/QubesOS/qubes-secpack/blob/master/QSBs/qsb-014-2015.txt#L142
A slightly
trickier variation also works under some circumstances if
.BR chdir (2)
is not permitted.
If a daemon allows a "chroot directory" to be specified,
that usually means that if you want to prevent remote users from accessing
files outside the chroot directory, you must ensure that folders are never
moved out of it.
.PP
This call does not change the current working directory,
so that after the call \(aq\fI.\fP\(aq can
be outside the tree rooted at \(aq\fI/\fP\(aq.
In particular, the superuser can escape from a "chroot jail"
by doing:
.PP
.in +4n
.EX
mkdir foo; chroot foo; cd ..
.EE
.in
.PP
This call does not close open file descriptors, and such file
descriptors may allow access to files outside the chroot tree.
.SH RETURN VALUE
On success, zero is returned.
On error, \-1 is returned, and
.I errno
is set to indicate the error.
.SH ERRORS
Depending on the filesystem, other errors can be returned.
The more general errors are listed below:
.TP
.B EACCES
Search permission is denied on a component of the path prefix.
(See also
.BR path_resolution (7).)
.\" Also search permission is required on the final component,
.\" maybe just to guarantee that it is a directory?
.TP
.B EFAULT
.I path
points outside your accessible address space.
.TP
.B EIO
An I/O error occurred.
.TP
.B ELOOP
Too many symbolic links were encountered in resolving
.IR path .
.TP
.B ENAMETOOLONG
.I path
is too long.
.TP
.B ENOENT
The file does not exist.
.TP
.B ENOMEM
Insufficient kernel memory was available.
.TP
.B ENOTDIR
A component of
.I path
is not a directory.
.TP
.B EPERM
The caller has insufficient privilege.
.SH CONFORMING TO
SVr4, 4.4BSD, SUSv2 (marked LEGACY).
This function is not part of POSIX.1-2001.
.\" SVr4 documents additional EINTR, ENOLINK and EMULTIHOP error conditions.
.\" X/OPEN does not document EIO, ENOMEM or EFAULT error conditions.
.SH NOTES
A child process created via
.BR fork (2)
inherits its parent's root directory.
The root directory is left unchanged by
.BR execve (2).
.PP
The magic symbolic link,
.IR /proc/[pid]/root ,
can be used to discover a process's root directory; see
.BR proc (5)
for details.
.PP
FreeBSD has a stronger
.BR jail ()
system call.
.SH SEE ALSO
.BR chroot (1),
.BR chdir (2),
.BR pivot_root (2),
.BR path_resolution (7),
.BR switch_root (8)