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

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Date: Thu, 29 Oct 2009 20:20:51 +0100
From: Pavel Machek <pavel@....cz>
To: Dan Yefimov <dan@...htwave.net.ru>
Cc: bugtraq@...urityfocus.com
Subject: Re: /proc filesystem allows bypassing directory permissions on
	Linux

On Thu 2009-10-29 18:24:01, Dan Yefimov wrote:
> On 29.10.2009 0:27, Pavel Machek wrote:
>>>> That race is easily fixed.
>>>
>>> No, you're not right.
>>>
>>>> After chmodding the directory to 0700, *first*
>>>> check the link count, *then* chmod the file to 0666:
>>>>
>>>>      User1 creates file with permissions 0644
>>>>                      User2 opens file for read access on file descriptor 4
>>>>      User1 chmod's directory to 0700
>>>>      User1 verifies no hard links to file
>>>
>>> Here's a window, during which User2 is able to create a hardlink and
>>> that will remain unnoticed by User1. There's no way to perform link
>>> check and conditionally do chmod in an atomic manner.
>>
>> 0700 on directory prevents hardlink creation, see?
>>
> Do you still remember about openat()? If the directory was created with 

No, you said you can do hardlink, and you can't. Try
it. openat(O_SEARCH) does not seem to exist.  

> 0700 mode from the origin, you would be right, and procfs wouldn't
>allow 
> opening files in that directory too, but if you let others to traverse 
> that directory and open your believed to be secure files from the origin, 
> it's your fault.

I can do the example with fd passing and 700 directory, but it would
be lot of C code. Feel free to play, my example was not nearly the
only way to demonstrate it, and no, it was not racy.
									Pavel
-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html