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Re: /proc filesystem allows bypassing directory permissions on Linux
From: Tony Finch <dot () dotat at>
Date: Mon, 26 Oct 2009 16:01:32 +0000

Pavel Machek <pavel () ucw cz> wrote:

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$ echo this file should never be writable > unwritable_file
# lock down directory
pavel () toy:/tmp/my_priv$ chmod 700 .
# relax file permissions, directory is private, so this is safe
# check link count on unwritable_file. We would not want someone 
# to have a hard link to work around our permissions, would we?
pavel () toy:/tmp/my_priv$ chmod 666 unwritable_file 
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.

Tony.
-- 
f.anthony.n.finch  <dot () dotat at>  http://dotat.at/
DOVER WIGHT: WEST OR NORTHWEST 5 TO 7 DECREASING 4, THEN BACKING SOUTHEAST
LATER. MODERATE OR ROUGH. DRIZZLE LATER. GOOD, BECOMING MODERATE LATER.


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