linux-kernel - Re: [PATCH v4 18/21] fuse: Add support for pid namespaces

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Date:	Thu, 21 Jul 2016 09:25:17 +0200
From:	Miklos Szeredi <mszeredi@...hat.com>
To:	Seth Forshee <seth.forshee@...onical.com>
Cc:	Sheng Yang <sheng@...ker.org>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Miklos Szeredi <miklos@...redi.hu>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	Serge Hallyn <serge.hallyn@...onical.com>,
	Richard Weinberger <richard.weinberger@...il.com>,
	Austin S Hemmelgarn <ahferroin7@...il.com>,
	Pavel Tikhomirov <ptikhomirov@...tuozzo.com>,
	kernel list <linux-kernel@...r.kernel.org>,
	linux-bcache@...r.kernel.org, dm-devel@...hat.com,
	linux-raid@...r.kernel.org, linux-mtd@...ts.infradead.org,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	fuse-devel@...ts.sourceforge.net,
	linux-security-module@...r.kernel.org, selinux@...ho.nsa.gov,
	cgroups@...r.kernel.org
Subject: Re: [PATCH v4 18/21] fuse: Add support for pid namespaces

On Wed, Jul 20, 2016 at 2:52 PM, Seth Forshee
<seth.forshee@...onical.com> wrote:

> It sounds like we need to remove the restriction on accessing the
> filesystem from a different pid namespace. I don't think this poses a
> security problem. However there's no pid mapping that is usable by the
> userspace fuse process, so what do we put in the fuse request? Probably
> the only candidates are 0 and 0xffffffff.
>
> So a question for the fuse developers - is one value or the other
> preferrable for fuse_in_header.pid when the pid cannot be mapped, and is
> this going to cause problems for any fuse filesystems? I suspect that
> few filesystems actually look at the pid anyway, and already for a
> filesystem mounted in a pid namespace the values being given to
> userspace won't be correct for the namespace of the fuse process.

pid = 0 sounds good.

The pid from the request is used for example to get the auxiliary
group list by libfuse (fuse_req_getgroups()).   That's not used by all
filesystems and it will return an error in case it can't find the proc
entry (which it won't for pid == 0).

It would be nice if we could transfer the group list through the
userspace/kernel protocol, since then it wouldn't depend on proc and
on being in the same pid namespace.  But that's another story.

Thanks,
Miklos