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[ISN] Linux File Permission Confusion

|  Linux Security: Tips, Tricks, and Hackery                       |
|  Published by Onsight, Inc.                                      |
|                                                                  |
|  17-April-2003                                                   |
|  http://www.hackinglinuxexposed.com/articles/20030417.html       |

This issue sponsored by Linuxfest Northwest 2003, Bellingham, WA,
April 26

LFNW is a showcase for what Northwest Linux users are doing with
Linux and open source software. It's a place for Linux enthusiasts to
get together to share their passion for what good software can do.
This year's fest will include boatloads of demonstrations, tutorials,
and lectures by gurus such as John "Mad Dog" Hall, Randal Schwartz,
and your very own Linux Security geek, Brian Hatch.

Admission is free. For more information, see www.linuxnorthwest.com.
Come join the fun!


Linux File Permission Confusion
By Brian Hatch

Summary: File permissions, the most basic form of security control
that exists on Unix-like systems, is still misunderstood by many.

Linux has many different level of security. The kernel is protected
from user processes; a user can only affect his own processes; and
user processes are protected from each other. This security model
requires that you must specifically allow users and processes to
interact, otherwise there is no avenue for interaction and thus no
avenue for attack.

The most basic security feature of any unix-like operating system is
file permissions. However, even though this is the most simple line
of defence, it is still misunderstood by many. Frequently, in an
administrator perspective, I need to re-teach users how file perms
work when they find that other users can access files they thought
were unaccessible. Worse yet is when poor file permissions are used
as part of an attack that should never have had a chance.

File permissions are those letters you see at the beginning of ls -l

 $ ls -l
 drwxrwxrwx    1 reegen   reegen      4096 Jan 22 10:05 dropbox
 drwx------    1 reegen   reegen      4096 Dec  6  8:50 private_logs
 -rw-r--r--    1 reegen   reegen      9663 Oct  8 16:03 public
 -rw-------    1 reegen   reegen      8925 Jan 12 11:17 secret

The first part of the output has 10 characters, which can be broken
up as follows (excuse the ASCII art....)

  d   rwx   rwx   rwx
  |    |     |     |
  |    |     |     \-> "Other" permissions
  |    |     |
  |    |     \-> "Group" permissions
  |    |
  |    \-> "User" permissions
  \-> Type of file

The first character says what kind of file this is. (d==directory, l=
=symlink, p==pipe, s==socket, etc). The next three groups define the
permissions of this file. An "r" means "allow read access", a "w"
means "allow write access", and "x" means "allow execute access.

On a normal file ("-" in the type-of-file field) "read" and "write"
are pretty self explanatory. "Execute" means that this file can be
executed as a program.[1]. So if the file were a shell script or
valid compiled executable (ELF format, etc) named program_name, you
could run ./program_name or, if this directory were in your path,
just program_name. Without the execute bit, you'd need to type sh
program_name for a shell script, perl program_name for a perl script,

The kernel decides if you have read/write/execute access based on
these rwx bits. Say you want to read a file -- what logic does the
kernel use to decide if you should be granted read access? Test your
intuition with the following examples:

 $ id
 uid=1000(doug) gid=1000(doug) groups=1000(doug),1001(web)

 $ ls -l foo.html
 -rw-r-----    1 www-data  web         9663 Oct  8 16:03 foo.html

Following this logic, we'd all agree that doug can read this file,
because he's part of the group web, which has read access. However
how about this situation:

 $ id
 uid=1000(doug) gid=1000(doug) groups=1000(doug),1001(web)

 $ ls -l bar.html
 -rw----r--    1 www-data  web        19838 Sep 17  4:22 bar.html

Most people's intuition would say sure, doug can read this file,
since the read bit is set for other. This is not the case!

Standard file permissions come in three flavours: user, group, and
other. Many people think that if any option is satisfied, then access
is granted. In reality, the kernel checks only the most appropriate -
it does not "check all of them, falling through when a particular
test fails"!

So, to be explicit, here is how you can think of the file permission
logic, using read access as an example:

 1. If the user is the owner of the file, see if the owner read bit
    (400) is set. If not, tough luck. Do no other tests.
 2. If the user is a member of the group that owns the file see if
    the group read bit (040) is set. If not, tough luck. Do no other
 3. If the user is not the owner of the file nor a member of the
    group that owns the file, see if the other read bit (004) is set.
    If not, tough luck. Do no other tests.

The distinction is an important one. You could create a file that you
own that is readable by group and other, but not by you[3]. Yes, this
seems somewhat counterintuitive.

We are frequently put in a position where we want to give only
certain users access to a directory or files. We put them in a
special group, and tailor the group permissions of the files/
directories to allow them this special access. Say we wanted to have
the compiler only available to developers, we may do the following:

  # ls -l /usr/bin/gcc
  -rwxr-xr-x    1 root     root        74088 Sep 23 15:13 /usr/bin/gcc

  # addgroup devel
  # chgrp devel /usr/bin/gcc
  # ls -l /usr/bin/gcc
  -rwxr-xr-x    1 root     devel       74088 Sep 23 15:13 /usr/bin/gcc

  # chmod o-rx /usr/bin/gcc
  # ls -l /usr/bin/gcc
  -rwxr-x---    1 root     devel       74088 Sep 23 15:13 /usr/bin/gcc

Now only root and the users in the devel group can run gcc. This is
the kind of group modifications we most frequently make. But what
happens if we want to have almost everyone able to use gcc except for
a few troublemakers? We can add everyone to this group, but that's a
management pain. But remember - the kernel only checks the best match
from user/group/other, not all of them. So instead, we do things the
other way around:

  # ls -l /usr/bin/gcc
  -rwxr-xr-x    1 root     root        74088 Sep 23 15:13 /usr/bin/gcc

  # addgroup jerks
  # chgrp jerks /usr/bin/gcc
  # ls -l /usr/bin/gcc
  -rwxr-xr-x    1 root     jerks       74088 Sep 23 15:13 /usr/bin/gcc

  # chmod g-rx /usr/bin/gcc
  # ls -l /usr/bin/gcc
  -rwx---r-x    1 root     jerks       74088 Sep 23 15:13 /usr/bin/gcc

What do we have now? gcc can only be run by root, and anyone not in
the group jerks. We can maintain the (hopefully short) list of jerks
much more easily than adding everyone to the devel group. The
management nightmare is over.

Next week we'll cover the finer points of directory permissions,
after which we'll get back to more interesting aspects of Linux


[1] Of course, if the file isn't an executable program, shell script,
or other recognisable form, Linux will stick it's tongue out at you.

[2] Even compiled executables can often be run without the execute
bit, such as using /lib/ld-linux.so.2 program_name. You loose the
ability to run setuid/setgid programs with their extra permissions if
you use any of these methods.

[3] Of course you could always chmod it since you own it.

Brian Hatch is Chief Hacker at Onsight, Inc and author of Hacking
Linux Exposed and Building Linux VPNs. Several years ago he ran
"chown bree /dev/heart" and has been happy ever since. Brian can be
reached at brian@xxxxxxxxxxxxxxxxxxxxxxxx

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Copyright 2003, Brian Hatch.

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