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Date:	Mon, 16 May 2016 09:12:57 -0500
From:	Seth Forshee <>
To:	"Eric W. Biederman" <>
Cc:	James Bottomley <>,
	Djalal Harouni <>,
	Alexander Viro <>,
	Chris Mason <>,,
	Serge Hallyn <>,
	Josh Triplett <>,
	Andy Lutomirski <>,,,,
	Dongsu Park <>,
	David Herrmann <>,
	Miklos Szeredi <>,
	Alban Crequy <>,
	Dave Chinner <>
Subject: Re: [RFC v2 PATCH 0/8] VFS:userns: support portable root filesystems

On Sat, May 14, 2016 at 09:21:55PM -0500, Eric W. Biederman wrote:
>   I have slowly been working with Seth Forshee on these issues as
>   the last thing I want is to introduce more security bugs right now.
>   Seth being a braver man than I am has already merged his changes into
>   the Ubuntu kernel.

Maybe not quite so brave as you think. I also threw on a patch to
disable the feature unless explicitly enabled by a sys admin.

> James I think you are missing the fact that all filesystems already have
> the make_kuid and make_kgid calls right where the data comes off disk,
> and the from_kuid and from_kgid calls right where the on-disk data is
> being created just before it goes on disk.  Which means that the actual
> impact on filesystems of the translation is trivial.

It is fairly simple but a there's bit more that just id conversions to
change. With ext4 I found that there were mount options which needed to
be restricted, some capability checks to update, and access to external
journal devices must be checked. In all it wasn't a whole lot of changes
to the filesystem though. Fuse was a bit more involved, but the
complexities there won't apply to other filesystems.

> Djalal if you could work with Seth I think that would be very useful.  I
> know I am dragging my heels there but I really hope I can dig in and get
> everything reviewed and merged soonish.

That would make me very happy :-)

I'm happy to look with Djalal for commonalities. I did skim his patches
before, and based on that all I really expect to find are things related
to permission checks when ids don't map. The rest seems fundamentally


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