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 1:08, Pavel Machek wrote:
>> That can hardly be called a real security hole, since the behaviour
>> described above is expected, and is as it was conceived by design.
>> If the file owner in fact allows writing to it, why should Linux
>> prevent that from happening?
> No, I do not think this is expected. You could not write to that file
> under traditional unix, and you can not write into that file when
> /proc is unmounted.
> I do not think mounting /proc should change access control semantics.
It didn't in fact change anything. If the guest created hardlink to that file in 
a unrestricted location, what would you say? Procfs is in that respect just 
another sort of hardlinks, whether you like that or not. If you didn't in fact 
restrict an access to the file, you're on your own.

> 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. Anyway, you're free 
to consider that a security hole, but remember, that nobody is obliged to agree 
with you in that or help you solving problems invented by yourself.

Sincerely Your, Dan.

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