linux-kernel - Re: Revisit AF_BUS: is it a better way to implement KDBUS?

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Message-ID: <CALCETrXfHg3SdRrfsUpRQp9MQzptJsfKJ2Wqn38_95DWrqXc7g@mail.gmail.com>
Date:	Fri, 31 Jul 2015 14:15:46 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	cee1 <fykcee1@...il.com>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Greg KH <gregkh@...uxfoundation.org>,
	Lennart Poettering <lennart@...ttering.net>,
	David Herrmann <dh.herrmann@...il.com>,
	One Thousand Gnomes <gnomes@...rguk.ukuu.org.uk>
Subject: Re: Revisit AF_BUS: is it a better way to implement KDBUS?

On Fri, Jul 31, 2015 at 9:25 AM, cee1 <fykcee1@...il.com> wrote:
> 2015-07-31 2:12 GMT+08:00 Andy Lutomirski <luto@...capital.net>:
>>
>> I find myself wondering whether an in-kernel *bus* is a good idea at
>> all.  Creating a bus that unprivileged programs are allowed to
>> broadcast on (which is kind of the point) opens up big cans of worms.
>
> This can be solved in this AF_BUS like this:
> * Becoming a bus master needs a proper CAP.
> * Impose a bus endpoint to join multicast address "maddr1" first, if
> it wants to send to multicast address "maddr2".
>
> The bus endpoint sends the request of joining maddr1, and the bus
> master grants it with replying a cmsg(control message) and setting up
> a proper eBPF.
>
> Next time, the bus endpoint sends to maddr2, the kernel will allow this if:
> 1) maddr1 & maddr2 == maddr1
> And 2) the eBPF allows it.
>  (i.e. the same multicast match logic in this AF_BUS)
>

I don't understand.

If the endpoint is unprivileged (i.e. random untrusted things can send
multicast), then you have the scaling problem.  If the endpoint is
privileged, then it's much less clear to me that this thing is useful.

--Andy
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