linux-kernel - Re: [PATCH V2 0/1] ubd: add io_uring based userspace block driver

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Date:   Thu, 19 May 2022 10:46:41 +0100
From:   Stefan Hajnoczi <stefanha@...hat.com>
To:     Ming Lei <ming.lei@...hat.com>
Cc:     Jens Axboe <axboe@...nel.dk>, linux-block@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Harris James R <james.r.harris@...el.com>,
        io-uring@...r.kernel.org,
        Gabriel Krisman Bertazi <krisman@...labora.com>,
        ZiyangZhang <ZiyangZhang@...ux.alibaba.com>,
        Xiaoguang Wang <xiaoguang.wang@...ux.alibaba.com>
Subject: Re: [PATCH V2 0/1] ubd: add io_uring based userspace block driver

On Thu, May 19, 2022 at 10:42:22AM +0800, Ming Lei wrote:
> On Wed, May 18, 2022 at 04:49:03PM +0100, Stefan Hajnoczi wrote:
> > On Wed, May 18, 2022 at 08:53:54PM +0800, Ming Lei wrote:
> > > On Wed, May 18, 2022 at 11:45:32AM +0100, Stefan Hajnoczi wrote:
> > > > On Wed, May 18, 2022 at 03:09:46PM +0800, Ming Lei wrote:
> > > > > On Tue, May 17, 2022 at 03:06:34PM +0100, Stefan Hajnoczi wrote:
> > > > > > Here are some more thoughts on the ubd-control device:
> > > > > > 
> > > > > > The current patch provides a ubd-control device for processes with
> > > > > > suitable permissions (i.e. root) to create, start, stop, and fetch
> > > > > > information about devices.
> > > > > > 
> > > > > > There is no isolation between devices created by one process and those
> > > > > 
> > > > > I understand linux hasn't device namespace yet, so can you share the
> > > > > rational behind the idea of device isolation, is it because ubd device
> > > > > is served by ubd daemon which belongs to one pid NS? Or the user creating
> > > > > /dev/ubdbN belongs to one user NS?
> > > > 
> > > > With the current model a process with access to ubd-control has control
> > > > over all ubd devices. This is not desirable for most container use cases
> > > > because ubd-control usage within a container means that container could
> > > > stop any ubd device on the system.
> > > > 
> > > > Even for non-container use cases it's problematic that two applications
> > > > that use ubd can interfere with each other. If an application passes the
> > > > wrong device ID they can stop the other application's device, for
> > > > example.
> > > > 
> > > > I think it's worth supporting a model where there are multiple ubd
> > > > daemons that are not cooperating/aware of each other. They should be
> > > > isolated from each other.
> > > 
> > > Maybe I didn't mention it clearly, I meant the following model in last email:
> > > 
> > > 1) every user can send UBD_CMD_ADD_DEV to /dev/ubd-control
> > > 
> > > 2) the created /dev/ubdcN & /dev/udcbN are owned by the user who creates
> > > it
> > 
> > How does this work? Does userspace (udev) somehow get the uid/gid from
> > the uevent so it can set the device node permissions?
> 
> We can let 'ubd list' export the owner info, then udev may override the default
> owner with exported info.
> 
> Or it can be done inside devtmpfs_create_node() by passing ubd's uid/gid
> at default.
> 
> For /dev/ubdcN, I think it is safe, since the driver is only
> communicating with the userspace daemon, and both belong to same owner.
> Also ubd driver is simple enough to get full audited.
> 
> For /dev/ubdbN, even though FS isn't allowed to mount, there is still
> lots of kernel code path involved, and some code path may not be run
> with unprivileged user before, that needs careful audit.
> 
> So the biggest problem is if it is safe to export block disk to unprivileged
> user, and that is the one which can't be bypassed for any approach.

Okay.

Stefan

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