Re: /proc filesystem allows bypassing directory permissions on Linux

[Tony Finch <dot@xxxxxxxx>]

Pavel Machek <pavel@xxxxxx> 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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