Re: /proc filesystem allows bypassing directory permissions on Linux

From: Dan Yefimov <>
To: Pavel Machek <>
Subject: Re: /proc filesystem allows bypassing directory permissions on Linux

On 24.10.2009 2:05, Pavel Machek wrote:
> On Sat 2009-10-24 01:12:51, Dan Yefimov wrote:
>> On 24.10.2009 0:35, Matthew Bergin wrote:
>>> doesnt look like the original owner is trying to write to it. Shows it
>>> cant, it had guest write to it via the proc folders bad permissions.
>>> Looks legitimate
>> Please tell me, who issued 'chmod 0666 unwritable_file'? Was that an
>> attacker? No, that was the owner of 'unwritable_file', nobody else.
>> What the 0666 file mode means? It means, that everybody can write to
>> the file, can't he? So why do you believe that pretension
>> legitimate?
> Original owner did chmod 666... after making sure traditional unix
> permissions protect the file. Please look at original mail; it was
> subtle but I believe I got it right, and file would not be writable
> with /proc unmounted.
I remember the original mail content. You're right, you can't reach the file if 
the procfs is not mounted, but you forget about the race, allowing the guest to 
create a hardlink to the file in an unrestricted location before the directory 
access becomes restricted. Again, procfs is just another, specific kind of 

Sincerely Your, Dan.

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