Re: For review: user_namespace(7) man page

[Andy Lutomirski]

On Tue, Sep 9, 2014 at 9:05 AM, Eric W. Biederman <ebiederm@xxxxxxxxxxxx> wrote:
>
> We may also want to discuss the specific restrictions on chroot.
>
> The text about chroot at least gives people a strong hint that the
> chroot rules are affected by user namespaces.
>
> The restrictions that we have settled on to avoid chroot being a problem
> are the creator of a user namespace must not be chrooted in their
> current mount namespace, and the creator of the user namespace must not
> be threaded.
>
> Andy can you check me on this it looks like unshare is currently buggy
> in that it will allow a threaded application to create a user namespace.

I think it's this code in unshare:

    /*
     * If unsharing a user namespace must also unshare the thread.
     */
    if (unshare_flags & CLONE_NEWUSER)
        unshare_flags |= CLONE_THREAD | CLONE_FS;

I suppose that this should be documented.

CLONE_FS prevents the chroot from leaking out of the namespace.  (But
see the other thread that I'm about to start...)

--Andy
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