10.9 Setuid Programs

Some programs need to run with “root” privileges, even when they are launched by unprivileged users. A notorious example is the passwd program, which users can run to change their password, and which needs to access the /etc/passwd and /etc/shadow files—something normally restricted to root, for obvious security reasons. To address that, these executables are setuid-root, meaning that they always run with root privileges (see How Change Persona in The GNU C Library Reference Manual, for more info about the setuid mechanism).

The store itself cannot contain setuid programs: that would be a security issue since any user on the system can write derivations that populate the store (see The Store). Thus, a different mechanism is used: instead of changing the setuid bit directly on files that are in the store, we let the system administrator declare which programs should be setuid root.

The setuid-programs field of an operating-system declaration contains a list of <setuid-program> denoting the names of programs to have a setuid or setgid bit set (see Using the Configuration System). For instance, the passwd program, which is part of the Shadow package, with a setuid root can be designated like this:

(setuid-program
  (program (file-append shadow "/bin/passwd")))

Data Type: setuid-program

This data type represents a program with a setuid or setgid bit set.

program

A file-like object having its setuid and/or setgid bit set.

setuid? (default: #t)

Whether to set user setuid bit.

setgid? (default: #f)

Whether to set group setgid bit.

user (default: 0)

UID (integer) or user name (string) for the user owner of the program, defaults to root.

group (default: 0)

GID (integer) goup name (string) for the group owner of the program, defaults to root.

A default set of setuid programs is defined by the %setuid-programs variable of the (gnu system) module.

Scheme Variable: %setuid-programs

A list of <setuid-program> denoting common programs that are setuid-root.

The list includes commands such as passwd, ping, su, and sudo.

Under the hood, the actual setuid programs are created in the /run/setuid-programs directory at system activation time. The files in this directory refer to the “real” binaries, which are in the store.