CLONE_NEWUSER|CLONE_FS root exploit

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@suse.de> -----

Date: Wed, 13 Mar 2013 16:39:56 +0100
From: Sebastian Krahmer <krahmer@suse.de>
To: oss-security@lists.openwall.com
Subject: [oss-security] CLONE_NEWUSER|CLONE_FS root exploit
Envelope-To: kees@outflux.net

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@suse.de - SuSE Security Team

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

-- 
Kees Cook
Chrome OS Security

Re: CLONE_NEWUSER|CLONE_FS root exploit

[Eric W. Biederman]

Kees Cook <keescook@chromium.org> writes:

> 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

Yes.  That is a bad combination.  It let's chroot confuse privileged
processes.

Now to figure out if this is easier to squash by adding a user_namespace
to fs_struct or by just forbidding this combination.

Eric

Re: CLONE_NEWUSER|CLONE_FS root exploit

[Andy Lutomirski]

On 03/13/2013 11:35 AM, Eric W. Biederman wrote:
> Kees Cook <keescook-F7+t8E8rja9g9hUCZPvPmw@public.gmane.org> writes:
> 
>> 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
> 
> Yes.  That is a bad combination.  It let's chroot confuse privileged
> processes.
> 
> Now to figure out if this is easier to squash by adding a user_namespace
> to fs_struct or by just forbidding this combination.

It's worth making sure that setns(2) doesn't have similar issues.

Looking through other shared-but-not-a-namespace things, there are:

fs_struct: Buggy as noted.

files_struct: Probably harmless -- SCM_RIGHTS can emulate it

signal_struct: This interacts with the tty code.  Is it okay?

sighand_struct: Looks safe.  Famous last words.

FWIW, I've been alarmed in the past that struct path (e.g. the root
directory) implies an mnt_namespace (hidden in struct mount), and it's
entirely possible for the root directory's mnt_namespace not to match
nsproxy->mnt_namespace.  I'm not sure what the implications are, but
this doesn't seem healthy.

--Andy

Re: CLONE_NEWUSER|CLONE_FS root exploit

[Eric W. Biederman]

Andy Lutomirski <luto@amacapital.net> writes:

> On 03/13/2013 11:35 AM, Eric W. Biederman wrote:
>> Kees Cook <keescook-F7+t8E8rja9g9hUCZPvPmw@public.gmane.org> writes:
>> 
>>> 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
>> 
>> Yes.  That is a bad combination.  It let's chroot confuse privileged
>> processes.
>> 
>> Now to figure out if this is easier to squash by adding a user_namespace
>> to fs_struct or by just forbidding this combination.
>
> It's worth making sure that setns(2) doesn't have similar issues.

setns(2) and unshare(2) are done and merged.  See commit.

commit e66eded8309ebf679d3d3c1f5820d1f2ca332c71
Author: Eric W. Biederman <ebiederm@xmission.com>
Date:   Wed Mar 13 11:51:49 2013 -0700

    userns: Don't allow CLONE_NEWUSER | CLONE_FS


> Looking through other shared-but-not-a-namespace things, there are:
>
> fs_struct: Buggy as noted.
>
> files_struct: Probably harmless -- SCM_RIGHTS can emulate it
>
> signal_struct: This interacts with the tty code.  Is it okay?

It should be.  The tty code is heavily pid based, and CLONE_NEWPID
requires !CLONE_VM (which implies !CLONE_SIGHAND and !CLONE_VM).

> sighand_struct: Looks safe.  Famous last words.
>
> FWIW, I've been alarmed in the past that struct path (e.g. the root
> directory) implies an mnt_namespace (hidden in struct mount), and it's
> entirely possible for the root directory's mnt_namespace not to match
> nsproxy->mnt_namespace.  I'm not sure what the implications are, but
> this doesn't seem healthy.

The calls to check_mnt prevent abuse of the files found with fs_struct
not matching the current mount namespace.

static inline int check_mnt(struct mount *mnt)
{
	return mnt->mnt_ns == current->nsproxy->mnt_ns;
}

Thanks for looking I know I did the same double take and wondered if I
had missed anything else by accident when this bug showed up.

So far even just looking it all over again I can't see anything.  But I
have clearly been blind before.

Eric

Re: CLONE_NEWUSER|CLONE_FS root exploit

[Andy Lutomirski]

On Thu, Mar 14, 2013 at 1:29 PM, Eric W. Biederman
<ebiederm@xmission.com> wrote:
> Andy Lutomirski <luto@amacapital.net> writes:
>
>> On 03/13/2013 11:35 AM, Eric W. Biederman wrote:
>>> Kees Cook <keescook-F7+t8E8rja9g9hUCZPvPmw@public.gmane.org> writes:
>>>
>>>> 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
>>>
>>> Yes.  That is a bad combination.  It let's chroot confuse privileged
>>> processes.
>>>
>>> Now to figure out if this is easier to squash by adding a user_namespace
>>> to fs_struct or by just forbidding this combination.
>>
>> It's worth making sure that setns(2) doesn't have similar issues.
>
> setns(2) and unshare(2) are done and merged.  See commit.
>
> commit e66eded8309ebf679d3d3c1f5820d1f2ca332c71
> Author: Eric W. Biederman <ebiederm@xmission.com>
> Date:   Wed Mar 13 11:51:49 2013 -0700
>
>     userns: Don't allow CLONE_NEWUSER | CLONE_FS
>
>
>> Looking through other shared-but-not-a-namespace things, there are:
>>
>> fs_struct: Buggy as noted.
>>
>> files_struct: Probably harmless -- SCM_RIGHTS can emulate it
>>
>> signal_struct: This interacts with the tty code.  Is it okay?
>
> It should be.  The tty code is heavily pid based, and CLONE_NEWPID
> requires !CLONE_VM (which implies !CLONE_SIGHAND and !CLONE_VM).
>
>> sighand_struct: Looks safe.  Famous last words.
>>
>> FWIW, I've been alarmed in the past that struct path (e.g. the root
>> directory) implies an mnt_namespace (hidden in struct mount), and it's
>> entirely possible for the root directory's mnt_namespace not to match
>> nsproxy->mnt_namespace.  I'm not sure what the implications are, but
>> this doesn't seem healthy.
>
> The calls to check_mnt prevent abuse of the files found with fs_struct
> not matching the current mount namespace.
>
> static inline int check_mnt(struct mount *mnt)
> {
>         return mnt->mnt_ns == current->nsproxy->mnt_ns;
> }
>
> Thanks for looking I know I did the same double take and wondered if I
> had missed anything else by accident when this bug showed up.
>
> So far even just looking it all over again I can't see anything.  But I
> have clearly been blind before.

This is way too fun.  Got another one :/

I'll follow up in a sec off-list.

--Andy