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

From: Maxime Devos <[email-masked]>
Date: Tue, 08 Jun 2021 10:55:57 +0200

Xinglu Chen schreef op zo 06-06-2021 om 14:51 [+0200]:
> [  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.

Alternatively, a variant of call-with-output-file
could be defined that has a #:perms argument.

This new procedure, let's call it call-with-output-file*,
could create a file with the right permissions with
(open "/etc/...-secret" (bitwise-ior O_WRONLY O_CREAT) #o400)
or something like that.

Then the vulnerable code above would become ...

  (call-with-output-file* FILE
    (lambda (port)
      (display SECRET port))
    #:perms #o400)

This seems a bit easier in usage to me!
No need to worry if changing the permissions of the parent
directory would break anything this way.

Greetings,
Maxime.