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Re: /proc filesystem allows bypassing directory permissions on Linux
From: Pavel Machek <pavel () ucw cz>
Date: Tue, 27 Oct 2009 14:06:42 +0100


pavel () toy:/tmp$ uname -a
Linux toy.ucw.cz 2.6.32-rc3 #21 Mon Oct 19 07:32:02 CEST 2009 armv5tel GNU/Linux
pavel () toy:/tmp mkdir my_priv; cd my_priv

Attacker opens my_priv and waits.
pavel () toy:/tmp/my_priv$ cat unwritable_file 
this file should never be writable

Attacker uses openat() to open and modify the "private" file.

pavel () toy:/tmp/my_priv$ cat unwritable_file 
got you
# Security problem here

Unexpected? Well, yes, to me anyway. Linux specific? Yes, I think so.

Not quite, as described above: there's a permissions race which
allowed the attacker to open the my_priv directory. Once you
have an fd on a directory it's possible to open any file inside
without a full-path permissions check. If you created the directory
using `mkdir -m 0700` (eliminating the race) then you should be safe.

Ok, if linux honors O_SEARCH on directories, then you found problem
with  my example. Congratulations, you were the first one :-).

I could present more complex example, pavel passing guest read-only
file descriptor over unix socket. At that point, you would not be able
to play with openat. OTOH, that example would be complex & ugly :-(.
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html

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