Re: /proc filesystem allows bypassing directory permissions on Linux

[Tony Finch <dot () dotat at>]

Pavel Machek <pavel () ucw cz> wrote:
> 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

Attacker opens my_priv and waits.

> 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

Attacker uses openat() to open and modify the "private" file.

> pavel@toy:/tmp/my_priv$ cat unwritable_file 
> got you
> # Security problem here
>
> Unexpected? Well, yes, to me anyway. Linux specific? Yes, I think so.

Not quite, as described above: there's a permissions race which
allowed the attacker to open the my_priv directory. Once you
have an fd on a directory it's possible to open any file inside
without a full-path permissions check. If you created the directory
using `mkdir -m 0700` (eliminating the race) then you should be safe.

Tony.
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