Re: [Ksummit-discuss] [CORE TOPIC] Mainline kernel on a cellphone

[Pavel Machek <pavel@ucw.cz>]

On Fri 2015-07-31 18:52:15, Mark Brown wrote:
> On Fri, Jul 31, 2015 at 02:22:27PM +0200, Pavel Machek wrote:
> > On Wed 2015-07-29 14:32:44, Mark Brown wrote:
> > > On Thu, Jul 23, 2015 at 10:29:02PM +0200, Pavel Machek wrote:
> 
> > > ALSA is the right interface for this, this is going to be some
> > > misunderstanding on their part about how to use it or limitations in
> 
> > That's what I was arguing, but see https://lkml.org/lkml/2015/3/5/606
> 
> I can't view that link since lkml.org was down earlier today when I was
> working online and now I'm working offline.

It works for me now.

> > kai> Our take was that ALSA is not the right interface for cmt_speech. The
> > kai> cmt_speech interface in the modem is _not_ a PCM interface as modelled
> > kai> by
> > kai> ALSA. Specifically:
> > kai>
> > kai>- the interface is lossy in both directions
> 
> Yay.
> 
> > kai>- data is sent in packets, not a stream of samples (could be other
> > kai>- things
> > kai>   than PCM samples), with timing and meta-data
> 
> This is what the copy() operation in the driver is for, there are other
> DSP based devices out there as well as things like USB.
> 
> > kai>   - timing of uplink is of utmost importance
> 
> Well, ALSA doesn't impose any latency of its own so there should be no
> issue there - the overheads should be minimal.

The interface Nokia has is pretty strange. I believe you send
timestamp with buffer, and it means "I'd like this packet of data to
be sent at that time", or something like that.

Anyway, no need to argue with me, I believe ALSA should be suitable.

> > > the drivers for their particular system (and possibly some unfortunate
> > > hardware design, it sounds like they may still be bouncing the voice
> > > call through the AP which isn't the most wonderful idea ever).
> > > There
> 
> > Actually, sending data through AP is _very_ good idea from security
> > perspective. You don't want hacked baseband to eavesdrop on you, do
> > you?
> 
> Given that the baseband is a key part of the data path it's not like
> you're gaining anything there as far as I can see?   I suppose you could
> argue that the AP is actually an additional attack surface here.

Actually yes, I believe I'm gaining a lot.

If baseband is directly connected to the microphone, it can eavesdrop
on me while the phone appears to be idle.

If data is being sent through the AP, yes, it can eavesdrop on calls
(but there are probably easier ways to eavesdrop on calls), but it
won't be able to do something funny when the phone appears to be idle.

Best regards,
									Pavel
-- 
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