linux-kernel - Re: [PATCH v1 1/2] binderfs: implement "max" mount option

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Date:   Mon, 24 Dec 2018 12:48:24 +0100
From:   Christian Brauner <christian@...uner.io>
To:     Greg KH <gregkh@...uxfoundation.org>
Cc:     tkjos@...roid.com, devel@...verdev.osuosl.org,
        linux-kernel@...r.kernel.org, arve@...roid.com, maco@...roid.com,
        joel@...lfernandes.org, Todd Kjos <tkjos@...gle.com>
Subject: Re: [PATCH v1 1/2] binderfs: implement "max" mount option

On Mon, Dec 24, 2018 at 12:45:59PM +0100, Greg KH wrote:
> On Mon, Dec 24, 2018 at 12:09:37PM +0100, Christian Brauner wrote:
> > On Sun, Dec 23, 2018 at 03:35:49PM +0100, Christian Brauner wrote:
> > > Since binderfs can be mounted by userns root in non-initial user namespaces
> > > some precautions are in order. First, a way to set a maximum on the number
> > > of binder devices that can be allocated per binderfs instance and second, a
> > > way to reserve a reasonable chunk of binderfs devices for the initial ipc
> > > namespace.
> > > A first approach as seen in [1] used sysctls similiar to devpts but was
> > > shown to be flawed (cf. [2] and [3]) since some aspects were unneeded. This
> > > is an alternative approach which avoids sysctls completely and instead
> > > switches to a single mount option.
> > > 
> > > Starting with this commit binderfs instances can be mounted with a limit on
> > > the number of binder devices that can be allocated. The max=<count> mount
> > > option serves as a per-instance limit. If max=<count> is set then only
> > > <count> number of binder devices can be allocated in this binderfs
> > > instance.
> > > 
> > > This allows to safely bind-mount binderfs instances into unprivileged user
> > > namespaces since userns root in a non-initial user namespace cannot change
> > > the mount option as long as it does not own the mount namespace the
> > > binderfs mount was created in and hence cannot drain the host of minor
> > > device numbers
> > > 
> > > [1]: https://lore.kernel.org/lkml/20181221133909.18794-1-christian@brauner.io/
> > > [2]; https://lore.kernel.org/lkml/20181221163316.GA8517@kroah.com/
> > > [3]: https://lore.kernel.org/lkml/CAHRSSEx+gDVW4fKKK8oZNAir9G5icJLyodO8hykv3O0O1jt2FQ@mail.gmail.com/
> > > [4]: https://lore.kernel.org/lkml/20181221192044.5yvfnuri7gdop4rs@brauner.io/
> > > 
> > > Cc: Todd Kjos <tkjos@...gle.com>
> > > Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
> > > Signed-off-by: Christian Brauner <christian.brauner@...ntu.com>
> > 
> > Right, I forgot to ask. Do we still have time to land this alongside the
> > other patches in 4.21? :)
> 
> It's too late for 4.21-rc1, but let's see what happens after that :)

Sweet! Much appreciated. :)

Christian