linux-kernel - Re: [lxc-devel] [RFC PATCH 00/11] Add support for devtmpfs in user namespaces

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Message-ID: <87ioow9ls9.fsf@x220.int.ebiederm.org>
Date:	Fri, 23 May 2014 15:23:50 -0700
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Serge Hallyn <serge.hallyn@...ntu.com>
Cc:	Seth Forshee <seth.forshee@...onical.com>,
	Jens Axboe <axboe@...nel.dk>,
	Serge Hallyn <serge.hallyn@...onical.com>,
	Arnd Bergmann <arnd@...db.de>, linux-kernel@...r.kernel.org,
	LXC development mailing-list 
	<lxc-devel@...ts.linuxcontainers.org>,
	James Bottomley <James.Bottomley@...senPartnership.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: Re: [lxc-devel] [RFC PATCH 00/11] Add support for devtmpfs in user namespaces

Serge Hallyn <serge.hallyn@...ntu.com> writes:

> Quoting Eric W. Biederman (ebiederm@...ssion.com):
>> 
>> 
>> >> Ultimately the technical challenge is how do we create a block device
>> >> that is safe for a user who does not have any capabilities to use, and
>> >> what can we do with that block device to make it useful.
>> >
>> > Yes, and I'd like to get started solving those challenges. But I also
>> > don't think we can address these two points (support partition blkdevs,
>> > help prevent more priveleged users from using a namespace's loop
>> > devices) sufficiently while having an implementation completely
>> > contained within the loop driver as Greg is requesting.
>> 
>> My key take away from the conversation is that we should reduce the
>> scope of what is being done to something that makes sense and the
>> propblems are immediately visible.
>> 
>> Part of me would like to suggest that fuse and it's ability to imitate
>> device nodes might be a more appropriate solution, to something that
>
> Do you have a link to more info on this?  Some googling got me to an
> interesting but old thread on CUSE, but nothing specifically about fuse
> doing this.

CUSE is probably what I was thinking of.  It is all part of the fuse
code base in the kernel.  And now that I am reminded it is called CUSE
I go Duh that is a character device...

Fuse and everything it can do is definitely the filesystem I would like
to see most have the audits to be enabled in user namespace.  Fuse
was built to be sufficiently paranoid to allow this and so it should not
take a lot to take fuse the rest of the way.

>> just needs block device access and nothing else.
>> 
>> For purposes of discussion let's call it unprivloopfs.  That can reuse
>> code from the loop device or not as appropriate.  Not supporting
>> paritioning I think is a very reasonable first step until it is shown
>> that we can make good use of partitioning support, and there are not
>> better ways of solving the problem.
>> 
>> I expect the most productive thing to talk about is what is your
>> immediate goal?  Mounting a filesystem?  Building an iso?
>
> For me it would be taking an iso and making some changes to it to
> localize it (i.e. take an install iso and add preseed file).
>
> Now of course in the end there is no reason why we can't do all of
> this with a new suite of libraries which simply uses read/write with
> knowledge of the fs layouts to parse and modify the backing files.
> My concern there is that duplicating all of the fs code seems unlikely
> to improve the soundness of either implementation.  Perhaps we can
> autogenerate this from the kernel source?  Does fuse already do
> something like that?

I am not aware of that.  But I have not worked extensively with fuse.

I do agree that finding a way to perform a read-only mount of an ISO by
an unprivielged user is a very interesting use case.  Given it's
interchange medium nature isofs should be as hardened as human possible,
and that is likely easier with a read-only filesystem.  And at less than
4000 lines of code isofs is auditable.

So as a target for unprivileged mounts of a block device isofs looks
like a good place to start.

Eric
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