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

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Date: Sat, 24 Oct 2009 10:47:38 +1100
From: psz@...hs.usyd.edu.au
To: dan@...htwave.net.ru, pavel@....cz
Cc: bugtraq@...urityfocus.com
Subject: Re: /proc filesystem allows bypassing directory permissions on Linux

Dear Pavel,

> ... can you see a way to write to that file when /proc is unmounted?

While there is legitimate access to do so (after file is created and
before pavel issues 'chmod 700 /tmp/my_priv'), guest to use commands:

  ln /tmp/my_priv/unwritable_file /tmp/hardlink-to-object

and then use that instead of /proc/self/fd/3, should work same as
original "attack" (and am curious whether 'link counts' shown by
'ls -l /tmp/my_priv/unwritable_file' are 1 or 2 in each case).

Cheers, Paul

Paul Szabo   psz@...hs.usyd.edu.au   http://www.maths.usyd.edu.au/u/psz/
School of Mathematics and Statistics   University of Sydney    Australia