NAME
super - execute commands setuid root.
SYNOPSIS
To execute a command:
super [ -r reqpath] command [ args ]
super [ -r reqpath] -o path [ args ]
command [ args ]
To list available commands:
super [-H|-f] [-S]
For usage and/or version information:
super [-h] [-V]
For debugging and development:
super -b
super -c [ superfile ]
super [-d|-D|-t] [-S] [MasqOptions] [-H|-f|command...]
MasqOptions:
-F file
-T hh:mm/dayname
-G gid
-U uid
-M mach
DESCRIPTION
Super allows specified users to execute scripts (or other commands) as
if they were root; or it can set the uid, gid, and/or supplementary
groups on a per-command basis before executing the command. It is
intended to be a secure alternative to making scripts setuid root.
Super also allows ordinary users to supply commands for execution by
others; these execute with the uid, gid, and groups of the user
offering the command.
Super consults a ‘‘super.tab’’ file to see if the user is allowed to
execute the requested command. If permission is granted, super will
exec pgm [ args ], where pgm is the program that is associated with
this command. (Root is allowed execution by default, but can still be
denied if a rule excludes root. Ordinary users are disallowed
execution by default.)
The most common sort of entry in a super.tab file pairs a simple
command with a pgm path. But in fact, the command in the super.tab
file is actually treated as a pattern, and any user-entered command
that matches this pattern causes the associated pgm to be executed. If
the listed pgm contains an asterisk, then the asterisk is replaced with
the command entered by the user. One use of this is to let any program
in a certain directory be executed by a user. For example, if the
entry contains the command/pgm pairs
CommandPattern Program
scripts/* → /usr/local/super/*
* → /usr/local/somedir/*
then the translations made are
Users Command Executed Program
scripts/xyz → /usr/local/super/scripts/xyz
xyz → /usr/local/somedir/xyz
Some commands can only be run after the user enters his or her
password. These commands can then be run multiple times until some
expiration time, at which point the password needs to be re-entered.
The list of password-requiring commands and the password durations are
set in the same file that records the valid users for each command.
If command is a symbolic link (or hard link, too) to the super program,
then typing
% command args
is equivalent to typing
% super command args
(The command must not be super, or super will not recognize that it’s
being invoked via a link.)
Super without any arguments will display the list of commands that may
be executed by the user.
For security, the following precautions are taken before exec’ing:
(a) all descriptors save 0,1,2 are closed;
(b) all of the user’s environment variables are discarded, save for
TERM, LINES, and COLUMNS. If TERM contains any characters other
than {-/:+._a-zA-Z0-9}, it is discarded. If LINES or COLUMNS
contains any characters other than [0-9], it is discarded. To
these are added reasonable values for:
USER and LOGNAME: both are set to the username associated with
the real uid of the program running under super;
HOME: set to the login directory of the user running super;
ORIG_USER, ORIG_LOGNAME, ORIG_HOME: the values of USER, LOGNAME,
and HOME that refer to the user who invoked super.
(These values are computed by super, and are not the
values set by the caller, so they are a reliable
description of the caller. These are normally the same
values as USER, LOGNAME, and HOME, but they will differ
if the super command changes uid or gid before executing
the program.);
IFS: set to blank, tab, newline;
PATH: set to /bin:/usr/bin.
SUPERCMD: set to command.
additional environment variables as specified in the super.tab
file (see below).
(c) all signal handling is reset to the default.
If Super is executed without arguments, it will print the commands that
the user may execute, one command per line. Super -H prints a long-
winded description of each command that the user may execute.
The CmdPat is the command pattern, and FullPath is the full path that
will be executed. The super.tab file can specify initial arguments
that that precede any user-supplied arguments; these arguments, if any,
are printed after the FullPath column.
User-Defined Super.tab Files
Ordinary users can supply their own super files. This lets users give
well-controlled setuid/setgid access to their programs: the user who
offers the program gets the assurance of safe IFS settings, safe
environment variable settings, etc; and the user who executes the
program knows that it will execute under the uid and gid of the
offering user. If a command is entered in the form
super loginname:cmd
super looks for cmd in the file .supertab, in the home directory of
account loginname. The cmd will be executed using the uid, gid, and
supplementary groups (if any) of user loginname.
The usual super options (such as -H) can be applied to a user’s
.supertab file. For example, help information about one command can be
had by using:
super -H loginname:cmd
Likewise, help information about all of loginname’s commands can be
obtained with:
super -H loginname:
Links to per-user commands can be created and used in a manner similar
to making symlinks to super itself. If command is a symbolic link to a
user’s .supertab file, and that .supertab file is
(a) executable, and
(b) begins with
#! /path/to/super -o
then the following pair are completely equivalent:
% super loginname:command
% command
If the #!-line would be longer than the typical Unix limit of 32
characters, you can instead start the .supertab file with:
#! /bin/sh
# Keep this backslash -> \
exec /long/path/to/the/super/executable -o $0 ${1+"$@"}
(The above takes advantage of the fact that super allows comments to be
backslash-continued, but the shell doesn’t.)
Per-user .supertab linking works as follows: if /path/to/xyz is a
symlink to some user’s .supertab file, and the .supertab file begins
with #! /path/to/super -o, then the shell will invoke super with
arguments something like
super -o /path/to/xyz [args]
Super checks that /path/to/xyz is a link to a real .supertab file, and
then always turns the last part of the path (here xyz) into the command
to execute.
** Security Warning **
Note that if you use symlinks to a per-user .supertab file, then you
must trust that the .supertab file will actually execute a super
command, instead of doing something nasty. That is because super
itself isn’t invoked until the shell has opened the .supertab file and
done whatever the .supertab file tells it to do. By contrast, the
direct command super loginname:cmd doesn’t involve any shell processing
of the .supertab file.
REGULAR OPTIONS
-V Print the super version number.
-S When super prompts for a password, this forces it to prompt on
stdin, even if the default (/dev/tty) is readable and writable.
Note: This only applies to password-type authentication — that
is, the older type of authentication wherein super itself
prompts for the password; PAM authentication is handled by your
system’s PAM modules.
-f This requests a list of available commands in a terse format
useful for processing by scripts. (-f stands for facts, as in
‘‘just the facts, m’am’’).
-rreqpath
Tells super to generate an error if the program associated with
this command is not reqpath. This helps you write scripts that
ensure that super only executes what they expect it to execute.
See step 4 of the section, ‘‘Creating Super Scripts’’, for an
example of its use.
-H Causes super to print a verbose listing of the commands
available to the user. It prints both the command and its
translation to a program pgm. If the displayed pgm contains an
asterisk, then the actual program executed is formed by
replacing the asterisk with the command entered by the user.
The following examples show the kinds of lines that may be
displayed with the -H option:
Example 1.
super skill → /usr/local/bin/skill
Typing super skill will execute /usr/local/bin/skill.
Example 2.
super {lp*} → /usr/bin/*
This example contains asterisks on both the left and right
sides. The left side shows the valid pattern you must match to
execute the command shown on the right-hand side. Usually, the
right-hand side has no asterisk, just a full path to a command
to execute. If there is an asterisk present, it is replaced by
the command you entered, thereby forming the actual executed
command. Thus, if you type super lpxxx (where xxx is any
string), super will execute /usr/bin/lpxxx.
Example 3.
super {co*} → /usr/bin/compress
The asterisk on the left-hand side means you can enter
super coxxx (where xxx is any string), but since the right-hand
side doesn’t contain an asterisk, coxxx will always execute
/usr/bin/compress.
-t This enables ‘‘test’’ mode. It does all normal checks except
for those requiring user input (passwords and variables that the
user must enter), but doesn’t execute any command. Instead, it
exits with status code 0 if the command is ok to execute, else
1. All normal error message output is generated in the usual
way, but no special debug messages are generated. Thus, it is a
useful means for a script to check if a command is likely to
work, and hence reasonable to exec super. Let’s say that a
script /usr/local/bin/foo wants to invoke itself using super foo
(See the section ‘‘Creating Super Scripts’’ for how to avoid
infinite loops when doing this!) the script can use the -r
option to ensure that super foo refers to the correct file, and
it can use test mode to ensure that super foo is a valid
command:
prog=`basename $0`
/usr/local/bin/super -t -r $0 $prog
case $? in
0 ) exec /usr/local/bin/super -t -r $0 $prog ;;
* ) echo "Super $prog doesn’t work!"
... So take appropriate action ...
;;
esac
DEBUG AND DEVELOPMENT OPTIONS
These options are useful when creating and debugging super.tab files.
They have little or no value to the everyday user. With the exception
of the -b option, they can be combined with the regular options, above.
-b Print the names and values of built-in variables, then exit.
Useful for administrators to learn the values against which
builtin variables can be tested.
-c[superfile]
Tells super to check the syntax of the entries in the superfile,
but not to execute any command. If no superfile is given, the
regular super.tab is checked. The exit code is 0 if the file’s
syntax is ok; otherwise the exit code is 1 (and an error message
is printed). After modifying a super file, you should use this
option to check its integrity.
Note that super -c isn’t a complete check that you’ve correctly
set up an entry, because you can create syntactically valid
entries that don’t do exactly what you want. Therefore, you
should also use super -d cmd to make sure that the command
you’ve entered will be executed with the correct arguments, uid,
gid, umask, and so on.
-d This enables debug mode, in which case (a) debugging information
is printed while checking a user for validity, and (b) the
command isn’t actually executed. Useful to check if a new entry
in the super.tab file (see below) has been handled properly.
-D Same as -d, plus prints more information about variables defined
in the super.tab file.
-Fsuperfile
This option is only used for debugging, and lets you test a
superfile before installing it. No command will actually be
executed. It also turns on a non-verbose debugging, showing the
matched command names and reasons for accepting or rejecting the
command.
-Ggid This option is also used for debugging, and tells super to act
as if the caller’s groupid or groupname was gid. It carries the
same restrictions and debug info as the -F option.
-Uuid This option is also used for debugging, and tells super to act
as if the caller’s uid or username was uid. It carries the same
restrictions and debug info as the -F option.
-Mmach This option is also used for debugging, and tells super to act
as if the caller’s host (machine) was mach. It carries the same
restrictions and debug info as the -F option.
-Thh:mm/dayname
This option is also used for debugging, and tells super to act
as if the execution time is hh:mm/dayname. This lets you check
if a time specification in the super.tab file is properly
restricting execution. It carries the same restrictions and
debug info as the -F option.
FILES
/etc/super.tab
contains the list of commands that super may execute, along with
the names of the user/group combinations who may execute each
command. The valid-user line can restrict use to particular
users or groups on different hosts, so a single super.tab file
can be used across a network.
/var/run/superstamps/username
is used as a timestamp for the last time that the user entered
his or her password.
CREATING SUPER SCRIPTS
You must be exceedingly careful when writing scripts for super. A
surprising variety of ordinary commands can, when run setuid-root, be
exploited for nasty purposes. Always make your scripts do as little as
possible, and give the user as few options as possible.
Think twice about side-effects and alternative uses of these scripts.
For instance, make sure your script doesn’t quietly invoke the user’s
.cshrc or similar file. Or, you might write a script to allow users to
mount cd-rom’s by executing mount(8). But if you don’t write it
carefully, a user could mount a floppy disk containing, say, a setuid-
root shell.
Security issues aside, here are some hints on creating super scripts:
1. Scripts must begin with #! interpreter-path.
2. Some variants of csh will not run setuid scripts unless the -b
flag (force a "break" from option processing) is set:
#!/bin/csh -fb
Similarly, if your super.tab file starts a shell such as csh or
tcsh, you may want to include the -b option in the super.tab
file, so that you don’t have to remember to type it on the
command line every time; use a line like the following in the
super.tab file:
SHELL "/usr/bin/csh -fb" some_priv_user
N.B. This is by way of example only; it’s not a very good idea
to really let somebody become root without any password check.
3. Better still, avoid csh scripts entirely -- they are harder to
write safely than Bourne-shell scripts.
4. It’s nice to make the super call transparent to users, so that
they can type
% cdmount args
instead of
% super cdmount args
You can make a script super itself by beginning the script in
the following way:
#!/bin/sh
prog=`basename $0`
test "X$SUPERCMD" = "X$prog" ||
exec /usr/local/bin/super -r $0 $prog ${1+"$@"}
Here, the path that is exec’d should be replaced with the path
at your site that leads to super. The option -r$0 is a sanity-
check option: it tells super that it’s an error if ‘‘super
$prog’’ doesn’t execute ‘‘$0’’, ie this self-same program.
(Also, see the -t option for how a script can check that super
$prog will work before doing an exec super.)
5. Some programs need certain directories in the path. Your super
scripts may have to add directories like /etc or /usr/etc to
make commands work. For instance, SunOS 4.1 needs /usr/etc in
the path before it can mount filesystems of type ‘‘hsfs’’.
6. By default, super only changes the effective uid. Some programs
(e.g. exportfs under SunOS 4.1.x) require the real uid to be
root. In that case, you should put an option like ‘‘uid=root’’
or ‘‘u+g=root’’ into the super.tab file.
SEE ALSO
super.tab(5).
AUTHOR
Will Deich
will@ucolick.org
NOTES
If the super.tab file isn’t owned by root, or if it is group- or world-
writable, super won’t run setuid-root. (If the user’s real uid is
root, super won’t run at all; otherwise, the effective uid reverts to
real uid.)
There is a race condition when using password-requiring commands, but
it doesn’t affect security: if a user is running two copies of super
simultaneously, and both processes try to update the user’s password
timestamp file at the same time, then it is possible for one of the
super commands to fail. Workaround: a single user shouldn’t execute
two password-requiring super programs simultaneously.
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