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

[Xinglu Chen]

On Tue, Jun 08 2021, Maxime Devos wrote:

> 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.

Indeed, this sounds like a better approach!

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