GNU bug report logs - #48872
Guix services: ‘chmod’ leaves opportunity to leak secrets

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Message #5 received at submit <at> debbugs.gnu.org (full text, mbox):

From: Xinglu Chen <public <at> yoctocell.xyz>
To: bug-guix <at> gnu.org
Subject: Guix services: ‘chmod’ leaves opportunity to
 leak secrets
Date: Sun, 06 Jun 2021 14:51:36 +0200

[Message part 1 (text/plain, inline)]

[  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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Message #8 received at 48872 <at> debbugs.gnu.org (full text, mbox):

From: Maxime Devos <maximedevos <at> telenet.be>
To: Xinglu Chen <public <at> yoctocell.xyz>, 48872 <at> debbugs.gnu.org
Subject: Re: bug#48872: Guix services: ‘chmod’
 leaves opportunity to leak secrets
Date: Tue, 08 Jun 2021 10:55:57 +0200

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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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Message #11 received at 48872 <at> debbugs.gnu.org (full text, mbox):

From: Xinglu Chen <public <at> yoctocell.xyz>
To: Maxime Devos <maximedevos <at> telenet.be>, 48872 <at> debbugs.gnu.org
Subject: Re: bug#48872: Guix services: ‘chmod’ leaves
 opportunity to leak secrets
Date: Tue, 08 Jun 2021 16:42:43 +0200

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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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