bugtraq - Re: /proc filesystem allows bypassing directory permissions on Linux

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Date: Sat, 24 Oct 2009 02:31:47 +0400
From: Dan Yefimov <dan@...htwave.net.ru>
To: Pavel Machek <pavel@....cz>
Cc: bugtraq@...urityfocus.com
Subject: Re: /proc filesystem allows bypassing directory permissions on Linux

On 24.10.2009 1:56, Pavel Machek wrote:
> Now... go back to my original email:
>
> %pavel@toy:/tmp/my_priv$ chmod 700 .
> %# relax file permissions, directory is private, so this is safe
> %# check link count on unwritable_file. We would not want someone
> %# to have a hard link to work around our permissions, would we?
> %pavel@toy:/tmp/my_priv$ chmod 666 unwritable_file
>
> Yes, you are right, open file descriptor acts as a kind of hardlink
> here. Except that
>
> a) this kind of hardlink does not exist when /proc is mounted (and on
> non-Linux)
>
> b) unlike other hardlinks, you can't see it on the link count
>
> (and c) writing to file descriptor opened read-only is bad).
>
>>> Plus, you may run traditional unix/POSIX application, expecting
>>> directory access controls to prevent the write. (Or can you see a way
>>> to write to that file when /proc is unmounted?)
>>>
>> Directory permissions control an access just to the directory
>> itself, not to the files in it, so your pretensions are in fact
>> illegitimate.
>
> Demonstrate how to get access to the file with /proc unmounted and you
> have a point. Demonstrate how to get access on anything else then
> Linux and you have a point. Otherwise there's a security hole.
>
Did you think of creating a hardlink to the file in an unrestricted location?
That is the like "security hole".
-- 

Sincerely Your, Dan.