linux-kernel - Re: [PATCH v2 00/28] user_namespace: introduce fsid mappings

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Date:   Mon, 17 Feb 2020 14:35:38 -0800
From:   James Bottomley <James.Bottomley@...senPartnership.com>
To:     Christian Brauner <christian.brauner@...ntu.com>
Cc:     linux-security-module@...r.kernel.org,
        Kees Cook <keescook@...omium.org>,
        Jonathan Corbet <corbet@....net>,
        Alexey Dobriyan <adobriyan@...il.com>,
        linux-api@...r.kernel.org, containers@...ts.linux-foundation.org,
        Jann Horn <jannh@...gle.com>, linux-kernel@...r.kernel.org,
        smbarber@...omium.org, Seth Forshee <seth.forshee@...onical.com>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        linux-fsdevel@...r.kernel.org,
        Alexander Viro <viro@...iv.linux.org.uk>
Subject: Re: [PATCH v2 00/28] user_namespace: introduce fsid mappings

On Mon, 2020-02-17 at 22:20 +0100, Christian Brauner wrote:
> On Mon, Feb 17, 2020 at 01:06:08PM -0800, James Bottomley wrote:
> > On Fri, 2020-02-14 at 19:35 +0100, Christian Brauner wrote:
> > [...]
> > > People not as familiar with user namespaces might not be aware
> > > that fsid mappings already exist. Right now, fsid mappings are
> > > always identical to id mappings. Specifically, the kernel will
> > > lookup fsuids in the uid mappings and fsgids in the gid mappings
> > > of the relevant user namespace.
> > 
> > This isn't actually entirely true: today we have the superblock
> > user namespace, which can be used for fsid remapping on filesystems
> > that support it (currently f2fs and fuse).  Since this is a single
> > shift,
> 
> Note that this states "the relevant" user namespace not the caller's
> user namespace. And the point is true even for such filesystems. fuse
> does call make_kuid(fc->user_ns, attr->uid) and hence looks up the
> mapping in the id mappings.. This would be replaced by make_kfsuid().
> 
> > how is it going to play with s_user_ns?  Do you have to understand
> > the superblock mapping to use this shift, or are we simply using
> > this to replace s_user_ns?
> 
> I'm not sure what you mean by understand the superblock mapping. The
> case is not different from the devpts patch in this series.

So since devpts wasn't originally a s_user_ns consumer, I assume you're
thinking that this patch series just replaces the whole of s_user_ns
for fuse and f2fs and we can remove it?

> Fuse needs to be changed to call make_kfsuid() since it is mountable
> inside user namespaces at which point everthing just works.

The fuse case is slightly more complicated because there are sound
reasons to run the daemon in a separate user namespace regardless of
where the end fuse mount is.

James