linux-kernel - Re: [PATCH v2 0/3] fuse: Add support for mounts from pid/user namespaces

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Date:	Tue, 23 Sep 2014 19:03:47 +0200
From:	Miklos Szeredi <miklos@...redi.hu>
To:	Miklos Szeredi <miklos@...redi.hu>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	Serge Hallyn <serge.hallyn@...ntu.com>,
	fuse-devel <fuse-devel@...ts.sourceforge.net>,
	Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Linux-Fsdevel <linux-fsdevel@...r.kernel.org>
Subject: Re: [PATCH v2 0/3] fuse: Add support for mounts from pid/user namespaces

On Tue, Sep 23, 2014 at 6:26 PM, Seth Forshee
<seth.forshee@...onical.com> wrote:
> On Tue, Sep 23, 2014 at 06:07:35PM +0200, Miklos Szeredi wrote:
>> On Tue, Sep 2, 2014 at 5:44 PM, Seth Forshee <seth.forshee@...onical.com> wrote:
>> > Here's an updated set of patches for allowing fuse mounts from pid and
>> > user namespaces. I discussed some of the issues we debated with the last
>> > patch set (and a few others) with Eric at LinuxCon, and the updates here
>> > mainly reflect the outcome of those discussions.
>> >
>> > The stickiest issue in the v1 patches was the question of where to get
>> > the user and pid namespaces from that are used for translating ids for
>> > communication with userspace. Eric told me that for user namespaces at
>> > least we need to grab a namespace at open or mount time and use only
>> > that namespace to prevent certain types of attacks.
>>
>> I'm not convinced.  Let us have the gory details, please.
>
> I'll do my best, and hopefully Eric will fill in any details I miss.
>
> I think there may have been more than one possible scenario that Eric
> was describing to me, but this is the one I remember. A user could
> create a namespace and mount a fuse filesystem without nosuid. It could
> then pass the /dev/fuse fd to a process in init_user_ns, which could
> expose a suid file owned by root (or any other user) and use it to gain
> elevated privileges.
>
> On the other hand, if file ownership is always interpreted in the
> context of the namespace from which the filesystem is mounted then suid
> can only be used to become another uid already under that user's
> control.

Much simpler solution: don't allow SUID in unprivileged namespaces.
SUID is a really ugly hack with many problems, just get rid of it.

Thanks,
Miklos
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