Bugtraq mailing list archives
Re: /proc filesystem allows bypassing directory permissions on Linux
From: Pavel Machek <pavel () ucw cz>
Date: Tue, 27 Oct 2009 14:06:42 +0100
Hi!
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_privAttacker opens my_priv and waits.
...
pavel@toy:/tmp/my_priv$ cat unwritable_file this file should never be writableAttacker 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.
Ok, if linux honors O_SEARCH on directories, then you found problem with my example. Congratulations, you were the first one :-). I could present more complex example, pavel passing guest read-only file descriptor over unix socket. At that point, you would not be able to play with openat. OTOH, that example would be complex & ugly :-(. Pavel -- (english) http://www.livejournal.com/~pavelmachek (cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
Current thread:
- Re: /proc filesystem allows bypassing directory permissions on Linux, (continued)
- Re: /proc filesystem allows bypassing directory permissions on Linux Daryl Tester (Oct 26)
- Re: /proc filesystem allows bypassing directory permissions on Linux Pavel Kankovsky (Oct 26)
- Re: /proc filesystem allows bypassing directory permissions on Linux Pavel Kankovsky (Oct 26)
- Re: /proc filesystem allows bypassing directory permissions on Linux Tony Finch (Oct 26)
- Re: /proc filesystem allows bypassing directory permissions on Linux Matthew Dempsky (Oct 26)
- Re: /proc filesystem allows bypassing directory permissions on Linux Tony Finch (Oct 27)
- Re: /proc filesystem allows bypassing directory permissions on Linux Matthew Dempsky (Oct 28)
- Re: /proc filesystem allows bypassing directory permissions on Linux psz (Oct 28)
- Re: /proc filesystem allows bypassing directory permissions on Linux Glynn Clements (Oct 29)
- Re: /proc filesystem allows bypassing directory permissions on Linux Matthew Dempsky (Oct 26)
- Re: /proc filesystem allows bypassing directory permissions on Linux psz (Oct 27)
- Re: /proc filesystem allows bypassing directory permissions on Linux Pavel Machek (Oct 27)
- Re: /proc filesystem allows bypassing directory permissions on Linux Kinzel, David (Oct 26)
- Re: /proc filesystem allows bypassing directory permissions on Linux Dan Yefimov (Oct 26)
- Re: /proc filesystem allows bypassing directory permissions on Linux Dan Yefimov (Oct 27)