bug#28751: GuixSD setuid-programs handling creates setuid binaries in the store

[ludo@gnu.org (Ludovic Courtès)]

On GuixSD, ‘activate-setuid-programs’ in (gnu build activation) would
create setuid-root binaries under /gnu/store for all the programs listed
under ‘setuid-programs’ in the ‘operating-system’ declaration.

‘activate-setuid-programs’ in (gnu build activation) does this:

  (define (make-setuid-program prog)
    (let ((target (string-append %setuid-directory
                                 "/" (basename prog))))
      (link-or-copy prog target)
      (chown target 0 0)
      (chmod target #o6555)))

which amounts to:

  1. ln /gnu/store/…/bin/su /run/setuid-programs/su
  2. chmod +s /run/setuid-programs/su

meaning that *both* ‘su’ files become setuid root.

This leads to setuid-root files in the store, which is a violation of a
fundamental assumption that setuid files cannot exist in the store.

Detailed announcement and fix coming.

Ludo’.