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Re: /proc filesystem allows bypassing directory permissions on Linux
From: Pavel Machek <pavel () ucw cz>
Date: Sat, 24 Oct 2009 21:02:50 +0200

On Sat 2009-10-24 10:47:38, psz () maths usyd edu au wrote:
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).

You are welcome to try it. link count is 2 for "ln" case and 1 for
"/proc" case -- and that's exactly the problem. "pavel" has no chance
to know that backdoor exists.
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html

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