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

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Date:   Mon, 17 Feb 2020 22:20:22 +0100
From:   Christian Brauner <christian.brauner@...ntu.com>
To:     James Bottomley <James.Bottomley@...senPartnership.com>
Cc:     Stéphane Graber <stgraber@...ntu.com>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Aleksa Sarai <cyphar@...har.com>, Jann Horn <jannh@...gle.com>,
        Kees Cook <keescook@...omium.org>,
        Jonathan Corbet <corbet@....net>, linux-kernel@...r.kernel.org,
        containers@...ts.linux-foundation.org, smbarber@...omium.org,
        Seth Forshee <seth.forshee@...onical.com>,
        linux-security-module@...r.kernel.org,
        Alexander Viro <viro@...iv.linux.org.uk>,
        linux-api@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        Alexey Dobriyan <adobriyan@...il.com>
Subject: Re: [PATCH v2 00/28] user_namespace: introduce fsid mappings

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