Subject: Re: [bug#67175] [PATCH 4/9] least-authority: Add support for changing UIDs/GIDs before exec.

From: Maxim Cournoyer <[email-masked]>
Date: Sun, 03 Dec 2023 21:13:54 -0500

Ludovic Courtès <[email-masked]> writes:

> * guix/least-authority.scm (least-authority-wrapper): Add #:user
> and #:group.
> [code]: Add calls to ‘setgid’ and ‘setuid’ when appropriate.
>
> Change-Id: I2aad8e5686b42b5c92fc306b114c5c60cb8bc551

This should mention it fixes bug #67175 :-).

> ---
>  guix/least-authority.scm | 25 +++++++++++++++++++++++--
>  1 file changed, 23 insertions(+), 2 deletions(-)
>
> diff --git a/guix/least-authority.scm b/guix/least-authority.scm
> index bfd7275e7c..3465fe9a48 100644
> --- a/guix/least-authority.scm
> +++ b/guix/least-authority.scm
> @@ -1,5 +1,5 @@
>  ;;; GNU Guix --- Functional package management for GNU
> -;;; Copyright © 2022 Ludovic Courtès <[email-masked]>
> +;;; Copyright © 2022-2023 Ludovic Courtès <[email-masked]>
>  ;;;
>  ;;; This file is part of GNU Guix.
>  ;;;
> @@ -41,6 +41,8 @@ (define %precious-variables
>  
>  (define* (least-authority-wrapper program
>                                    #:key (name "pola-wrapper")
> +                                  (user #f)
> +                                  (group #f)
>                                    (guest-uid 1000)
>                                    (guest-gid 1000)
>                                    (mappings '())
> @@ -55,7 +57,11 @@ (define* (least-authority-wrapper program
>  <file-system-mapping> records indicating directories mirrored inside the
>  execution environment of PROGRAM.  DIRECTORY is the working directory of the
>  wrapped process.  Each environment listed in PRESERVED-ENVIRONMENT-VARIABLES
> -is preserved; other environment variables are erased."
> +is preserved; other environment variables are erased.
> +
> +When USER and GROUP are set and NAMESPACES does not include 'user, change UIDs
> +and GIDs to these prior to executing PROGRAM.  This usually requires that the
> +resulting wrapper be executed as root so it can call setgid(2) and
>  setuid(2)."

About "usually"; in which case could a programm call to setgid and
setuid without being root?

>    (define code
>      (with-imported-modules (source-module-closure
>                              '((gnu system file-systems)
> @@ -113,6 +119,10 @@ (define* (least-authority-wrapper program
>                                  #$program signal)
>                          (exit (+ 128 signal))))))
>  
> +          (define namespaces '#$namespaces)
> +          (define host-group '#$group)
> +          (define host-user '#$user)
> +
>            ;; Note: 'call-with-container' creates a sub-process that this one
>            ;; waits for.  This might seem suboptimal but unshare(2) isn't
>            ;; really applicable: the process would still run in the same PID
> @@ -123,6 +133,17 @@ (define* (least-authority-wrapper program
>               (lambda ()
>                 (chdir #$directory)
>                 (environ variables)
> +
> +               (unless (memq 'user namespaces)
> +                 ;; This process lives in its parent user namespace,
> +                 ;; presumably as root; now is the time to setgid/setuid if
> +                 ;; asked for it (the 'clone' call would fail with EPERM if we
> +                 ;; changed UIDs/GIDs beforehand).

Related to my previous interrogation, should we check if the current
user id is 0 (root), and fail otherwise with an informative message?

-- 
Thanks,
Maxim