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Re: /proc filesystem allows bypassing directory permissions on Linux
From: Anton Ivanov <anton.ivanov () kot-begemot co uk>
Date: Sat, 24 Oct 2009 19:05:35 +0100

On Sat, 2009-10-24 at 21:39 +0400, Dan Yefimov wrote:
On 24.10.2009 20:59, Anton Ivanov wrote:
Not to tell about
that /proc/<PID>/fd/ contains only symbolic links, not files, so I can't
understand, how the original reporter managed to gain access to the file in the
restricted directory using that symlink.

The perms are definitely broken and without a code audit on procfs I
would not bet that this is limited just to this rather obscure test

To be honest, I hope that it is limited to this rather obscure test
case. If it is not there may be entertaining ramifications.

Given my citation above (I personally use Linux), that obscure test case looks 
doubtful. If the original reporter uses some patched kernel, that doesn't matter 

It works on Debian 2.6.26 out of the box. It is not an obscure patched
kernel case I am afraid. 

If you redir an FD to a file using thus redir-ed FD in /proc allows you
to bypass directory permissions for where the file is located.
Thankfully, file permissions still apply so you need an app which has
silly file perms in a bolted down directory for this.

Symlinking the same file to a link on a normal ext3 or nfs filesystem as
a sanity check shows correct permission behaviour. If you try to write
to that symlink you get permission denied so the permissions on the fs
actually work.

No need to be root, nothing. It is not a case of "forget to drop EID or
something else like that either". It looks like what it says on the tin
- permission bypass. 

Not that I would have expected anything different considering who posted
it in the first place.

   Understanding is a three-edged sword:
            your side, their side, and the truth. --Kosh Naranek

A. R. Ivanov
E-mail:  anton.ivanov () kot-begemot co uk
WWW:     http://www.kot-begemot.co.uk/

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