GNU bug report logs - #50960
[PATCH 00/10] Add 'guix shell' to subsume 'guix environment'

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From: Maxime Devos <maximedevos <at> telenet.be>
To: Ludovic Courtès <ludo <at> gnu.org>, 50960 <at> debbugs.gnu.org
Subject: [bug#50960] [PATCH 04/10] DRAFT shell: By default load the local 'guix.scm' or 'manifest.scm' file.
Date: Sat, 02 Oct 2021 16:15:21 +0200

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Ludovic Courtès schreef op za 02-10-2021 om 12:22 [+0200]:
> +(define (find-file-in-parent-directories candidates)
> +  "Find one of CANDIDATES in the current directory or one of its ancestors."
> +  (let loop ((directory (getcwd)))
> +    (and (= (stat:uid (stat directory)) (getuid))
> +         (or (any (lambda (candidate)
> +                    (let ((candidate (string-append directory "/" candidate)))
> +                      (and (file-exists? candidate) candidate)))
> +                  candidates)
> +             (loop (string-append directory "/..")))))) ;Unix ".." resolution

I do not recommend this.  What would happen if someone creates a temporary directory
"/tmp/stuff" do things in to throw away later (setting permissions appropriately),
tries to create a guix.scm in that directory but misspells it as, say, guix.sm, and runs
"guix shell" from within /tmp/stuff?  Then find-file-in-parent-directories would
load /tmp/guix.scm (possibly created by a local attacker, assuming a multi-user system),
-- if it weren't for the (= (stat:uid (stat directory)) (getuid)).

Because of the (= (stat:uid ...) (getuid)), this attack method is not possible.
However, it causes other issues.  Now it isn't possible for two users (that trust
each other), to set up a directory writable by both (e.g. with ACLs, or by making
the directory group-writable and placing the two users in the same group), for
working together, with a guix.scm usable by both.

These can be two users on the same machine, or remotely via something like NFS,
or a single person having multiple user accounts used for different purposes.

(I once created multiple user accounts on Debian: one regular purpose, one for reading
and games, and one for school, and made the ‘for-reading’ and ‘school’ home directory
readable by the ‘regular-purpose’ account.  It was occasionally useful.)

Greetings,
Maxime.

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