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* bug#48872: Guix services: ‘chmod’ leaves opportunity to leak secrets
@ 2021-06-06 12:51 Xinglu Chen
  2021-06-08  8:55 ` Maxime Devos
  0 siblings, 1 reply; 3+ messages in thread
From: Xinglu Chen @ 2021-06-06 12:51 UTC (permalink / raw)
  To: 48872

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[  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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^ permalink raw reply	[flat|nested] 3+ messages in thread

* bug#48872: Guix services: ‘chmod’ leaves opportunity to leak secrets
  2021-06-06 12:51 bug#48872: Guix services: ‘chmod’ leaves opportunity to leak secrets Xinglu Chen
@ 2021-06-08  8:55 ` Maxime Devos
  2021-06-08 14:42   ` Xinglu Chen
  0 siblings, 1 reply; 3+ messages in thread
From: Maxime Devos @ 2021-06-08  8:55 UTC (permalink / raw)
  To: Xinglu Chen, 48872

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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.

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^ permalink raw reply	[flat|nested] 3+ messages in thread

* bug#48872: Guix services: ‘chmod’ leaves opportunity to leak secrets
  2021-06-08  8:55 ` Maxime Devos
@ 2021-06-08 14:42   ` Xinglu Chen
  0 siblings, 0 replies; 3+ messages in thread
From: Xinglu Chen @ 2021-06-08 14:42 UTC (permalink / raw)
  To: Maxime Devos, 48872

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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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^ permalink raw reply	[flat|nested] 3+ messages in thread

end of thread, other threads:[~2021-06-08 14:44 UTC | newest]

Thread overview: 3+ messages (download: mbox.gz / follow: Atom feed)
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2021-06-06 12:51 bug#48872: Guix services: ‘chmod’ leaves opportunity to leak secrets Xinglu Chen
2021-06-08  8:55 ` Maxime Devos
2021-06-08 14:42   ` Xinglu Chen

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