Re: [Qemu-devel] [PATCH 0/4] net-bridge: rootless bridge support for qemu

[Avi Kivity]

On 11/04/2009 02:28 AM, Anthony Liguori wrote:
> This series solves a problem that I've been struggling with for a few years now.
> One of the best things about qemu is that it's possible to run guests as an
> unprivileged user to improve security.  However, if you want to have your guests
> communicate with the outside world, you're pretty much forced to run qemu as
> root.
>    

Only is you run an unmanaged system.

> At least with KVM support, this is probably the most common use case which means
> that most of our users are running qemu as root.  That's terrible.
>    

Most of our users run managed systems.

> We address this problem by introducing a new network backend: -net bridge.  This
> backend is less flexible than -net tap because it relies on a helper with
> elevated privileges to do the heavy lifting of allocating and attaching a tap
> device to a bridge.  We use a special purpose helper because we don't want
> to elevate the privileges of more generic tools like brctl.
>    

But you're essentially allowing any user to send packets through the 
physical interface.  Including, say, nobody (who's just broken in 
through an unrelated service).

>  From a user perspective, to use bridged networking with a guest, you simply 
> use:
>
>    qemu -hda linux.img -net bridge -net nic
>
> And assuming a bridge is defined named qemubr0 and the administrator has setup
> permissions accordingly, it will Just Work.  My hope is that distributions will
> do this work as part of the qemu packaging process such that for most users,
> the out-of-the-box experience will also Just Work.
>    

For most distributions, the ootb experience is already much better than 
running qemu on the command line.  I'm not sure who we're optimizing for 
here, and concerned that we're loosening security for qemu non-users.

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