CLONE_NEWUSER|CLONE_FS root exploit

[Kees Cook]

Hi,

It seem like we should block (at least) this combination. On 3.9, this
exploit works once uidmapping is added.

http://www.openwall.com/lists/oss-security/2013/03/13/10

-Kees

----- Forwarded message from Sebastian Krahmer <krahmer@xxxxxxx> -----

Date: Wed, 13 Mar 2013 16:39:56 +0100
From: Sebastian Krahmer <krahmer@xxxxxxx>
To: oss-security@xxxxxxxxxxxxxxxxxx
Subject: [oss-security] CLONE_NEWUSER|CLONE_FS root exploit
Envelope-To: kees@xxxxxxxxxxx

Hi,

Seems like CLONE_NEWUSER|CLONE_FS might be a forbidden
combination.
During evaluating the new user namespace thingie, it turned out
that its trivially exploitable to get a (real) uid 0,
as demonstrated here:

http://stealth.openwall.net/xSports/clown-newuser.c

The trick is to setup a chroot in your CLONE_NEWUSER,
but also affecting the parent, which is running
in the init_user_ns, but with the chroot shared.
Then its trivial to get a rootshell from that.

Tested on a openSUSE12.1 with a custom build 3.8.2 (x86_64).

I hope I didnt make anything wrong, mixing up the UIDs,
or disabled important checks during kernel build on my test
system. ;)

regards,
Sebastian

-- 

~ perl self.pl
~ $_='print"\$_=\47$_\47;eval"';eval
~ krahmer@xxxxxxx - SuSE Security Team

----- End forwarded message -----

-- 
Kees Cook
Chrome OS Security
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