sup

sup

a "small is beautiful" tool for UNIX privilege escalation

sup

a "small is beautiful" tool for unix privilege escalation

sup is a very small and secure c application. it is designed to run as root (with suid bit on) to facilitate the privilege escalation of users to execute certain programs as superuser.

all settings in sup are hard-coded at compile time. sup is very portable and self-contained, designed for production use as a static binary. sup is a sort of hard-coded sudo: it is an ideal companion for artisans building small containers and embedded systems.

sup's new homepage is sup.dyne.org

sup code can be read and reviewed on-line

stable releases can be downloaded from files.dyne.org/sup

releases are signed with the maintainer's keys

the develpment version of sup can be found at git.devuan.org/jaromil/sup

of course there is also a github mirror

software by Dyne.org

why are you whispering?

as you may have realised already, sup is so minimal that all its documentation is written lowercase. it was originally written in 2009 by pancake of nopcode.org and maintained until 2011 as part of the suckless tools. in 2016 sup is being adopted by jaromil of dyne.org, extending its features to support static build with musl-libc and hardcoded sha256 hashing of binary files.

configure

sup's configuration resides in config.h and should be set before building. here below an intuitive example:

// sup's configuration file
// need sup to be re-compiled for any change to be effective

/// un/comment flags below to remove functionalities
#define HASH 1
#define DAEMON 1
// #define DEBUG 1

#ifndef FLAGSONLY

#define USER 1000
#define GROUP -1

#define SETUID 0
#define SETGID 0

#define CHROOT ""
#define CHRDIR ""

static struct rule_t rules[] = {
    // allow user to run these programs when found at a specific path location
    { USER, GROUP, "whoami",   "/usr/bin/whoami", "" },
    { USER, GROUP, "ifconfig", "/sbin/ifconfig",  "" },
    { USER, GROUP, "ls",       "/bin/ls",         "" },
    { USER, GROUP, "wifi",     "/root/wifi.sh",   "" },
    // allow to run id when found with matching hash anywhere in PATH
    { USER, GROUP, "id",       "*", "db533b77fc9e262209a46e0f.." },
     // allow to run any program found in PATH
    { USER, GROUP, "*",        "*"},
    { 0 }, // end of configuration
};
#endif

fields are organized as follows:

| USER | GROUP | binary name | binary path | hash (optional) |

  • USER is the numeric id (UID) of the user authorized to execute the binary as superuser
  • GROUP is the numeric group (GID) of the user authorized to execute the binary as superuser (use -1 for none)
  • binary name is the command to run with setuid/setgid, e.g., ls
  • binary path is the path to the program, e.g., /bin/ls
  • hash can be computed before build using sha256sum (from GNU coreutils)

binary name and binary path accept the wildcard character * to mean any executable, and anywhere in PATH, respectively.

running sup -l will display the compiled-in configuration.

build

sup requires a c compiler and the gnu make tool to be built.

a simple make command will build a sup binary good enough for evaluation purposes, with dynamic links to the libm and libc libraries installed system-wide.

for production use, sup should be built as a static binary: this is easily done by first installing musl-libc in its default location (/usr/local/musl) and then using the make musl command inside sup.

technical details

sup consists of 3 files:

  • sup.c is the main source
  • config.h is the configuration, hardcoded at compile time
  • sha256.c is optional and provides the hashing functionality if # define HASH 1 is set.

sup is written in ansi c with posix1.b compliance for gnu/linux and bsd systems. it uses setuid/gid for privilege escalation and execv() to launch processes as superuser. daemon mode uses fork() to send processes in the background with NOTTY and stdin/out/err file descriptors set to /dev/null.

frequently asked questions

is there a package for sup?

it does not make sense to have a sup package. sup is configured at build-time and its built binaries are specific to the task they have been built and set suid for. sup is a tool for people distributing containers, online services, embedded devices, and such.

why not use doas (openbsd)?

sup is different from doas, because doas is configured at runtime.

why not use sudo?

same reason as above, sudo is configured at runtime.

why not use su?

sup is made so that people (or scripts) don't have to type passwords every time they need to execute something they are entitled to execute as superusers. with su one has to type the root password every time. also scripts won't work without interaction.

is sup still a suckless tool?

this new code hasn't been grinded by the merry folks at suckless yet, but pancake has acknowledged this development and, having left maintainance, is happy to hand it over to jaromil.

licensing

sup is copyleft software licensed as gnu lesser public license (lgplv3). when compiled with hashing capability, its license turns into gnu gplv2+ because of the sha256 component.

sup is copyleft (c) 2009-2011 by pancake of nopcode.org
.               (c) 2016      by jaromil of dyne.org
the fips-180-2 sha-256 implementation optionally included in sup is
copyleft (c) 2001-2003 by christophe cevine