NAME
super - execute commands setuid root.SYNOPSIS
To execute a command:-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- (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.
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
(a) executable,
and
(b) begins
with
#! /path/to/super -o
% 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 ${1+"$@"}
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.
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.
- /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_userN.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 argsYou 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 DeichNOTES
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.)| local |