bug#48872: Guix services: ‘chmod’ leaves opportunity to leak secrets

[Xinglu Chen]

[  This was reported on the Nixpkgs bug tracker a few weeks ago
   <https://github.com/NixOS/nixpkgs/issues/121293>  ]

When doing something like

  (call-with-output-file FILE
    (lambda (port)
      (display SECRET port)))
  (chmod FILE #o400)

an unpriviliged user could open FILE before FILE had been chmod’ed, and
then read the contents of FILE.

One solution to this problem would be to use

  (mkdir (dirname FILE) #o400)

before writing SECRET to FILE.

I have identified at least two services which are vulnerable to this:

* ‘wireguard-service-type’ in (gnu services vpn)
* ‘patchwork-service-type’ in (gnu servicse web)

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