linux-kernel - Re: ioctl CAP_LINUX_IMMUTABLE is checked in the wrong namespace

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Message-ID: <CALCETrVh0hCtiK=9fmEYfUWuz4C1dPq22kvyd=OWFMH5pXGaZw@mail.gmail.com>
Date:	Tue, 29 Apr 2014 17:23:57 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Serge Hallyn <serge.hallyn@...ntu.com>
Cc:	Stéphane Graber <stgraber@...ntu.com>,
	"Ted Ts'o" <tytso@....edu>,
	Linux Containers <containers@...ts.linux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	lxc-devel <lxc-devel@...ts.sourceforge.net>,
	"Eric W. Biederman" <ebiederm@...ssion.com>
Subject: Re: ioctl CAP_LINUX_IMMUTABLE is checked in the wrong namespace

On Tue, Apr 29, 2014 at 5:21 PM, Serge Hallyn <serge.hallyn@...ntu.com> wrote:
> Quoting Andy Lutomirski (luto@...capital.net):
>> > It should be a nonissue so long as we make sure that a file owned by a
>> > uid outside the scope of the container may not be changed even though
>> > fs_owner_uid is set. Otherwise, it's just a matter of chmod +S on say
>> > a shell and anyone who can see the fs from the host will be getting a
>> > root shell (assuming said file is owned by the host's uid 0).
>>
>> I feel like that's too fragile.  I'd rather add a rule that one of
>
> yeah I don't wnat to rush something like that.  I'd rather stash
> the userns of the task which did the mounting and check against
> that.  Note that would make it worthless unless and until we allowed
> mounting from non-init userns, but then we can only claim "our fs
> superblock readers suck and therefore containers can't mount an fs"
> so long before we start to feel some shame and audit them...
>
>> these filesystems always acts like it's nosuid unless you're inside a
>> user namespace that matches fs_owner_uid.
>>
>> Maybe even that is too weird.  How about setuid, setgid, and fcaps
>> only work on mounts that are in mount namespaces that are owned by the
>> current user namespace or one of its parents?  IOW, a struct mount is
>> only trusted if mnt->mnt_ns->user_ns == current user ns or one of its
>> parents?
>>
>> Untrusted mounts would act like they are nosuid,nodev.  Someone can
>> try to figure out a safe way to relax nodev at some point.

Do you like this variant?  We could add a way for global root to mount
an fs on behalf of a userns.  I'd rather this be more explicit than
just mounting it in a mount ns owned by the user namespace, though.

--Andy
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