How much protection does an add-on GSM modem give me vs. built into phone ?

Peter Zotov whitequark at whitequark.org
Thu Oct 4 11:15:09 UTC 2012


Denis 'GNUtoo' Carikli писал 04.10.2012 15:07:
> On Thu, 04 Oct 2012 14:53:08 +0400
> Peter Zotov <whitequark at whitequark.org> wrote:
>
>> Denis 'GNUtoo' Carikli писал 04.10.2012 14:26:
>> > On Thu, 4 Oct 2012 00:32:48 +0200
>> > Peter Stuge <peter at stuge.se> wrote:
>> >
>> >> John Case wrote:
>> >> > the real trick I am interested is isolating (or at least
>> >> > controlling) the interaction between the baseband processor and
>> >> the
>> >> > application processor.  Using a computer with a USB dongle 
>> gives
>> >> me
>> >> > that control ... would I have that same level of control if we
>> >> > had free software running on the baseband processor, or is 
>> there
>> >> > still additional bleeding possible simpy by virtue of being
>> >> > built into the computer ?
>> >>
>> >> In a smartphone it's almost not possible to distinguish the
>> >> "computer" from the "GSM modem" anymore, because of how the
>> >> hardware is constructed, so yes.
>> > In some yes, in some no... it depend on how the smartphone was
>> > designed:
>> >
>> > On one end some smartphones (openmoko GTA02,golden delicious
>> > GTA04), the
>> > baseband is isolated(tough on GTA04 it has access to a GPS with no
>> > antenna(so it can't work)) . And on the other end there are
>> > smartphones
>> > with qualcomm System on a chip...where the modem and the CPU are 
>> in
>> > a single chip:
>> > The modem part has the audio DSP connected to it, the GPS.
>> > And the baseband uses shared RAM memory and shared NAND(if I
>> > remember well)...
>> > And I'm not sure but maybe the baseband is even needed for booting
>> > the
>> > main CPU...
>> >
>> > There are also systems in between like the galaxy S/Neuxs S that
>> > uses shared memory but do not have other problems...
>>
>> In addition to the above, there are some phones where baseband is
>> completely
>> submissive to the AP, namely Galaxy SII. Basically it's exactly the
>> same
>> as the USB dongle situation, but the dongle is integrated on the
>> phone's
>> PCB.
> Did you check what the modem transport was(shared memory, high speed
> serial etc...)?
>
> Denis.

HSIC. It's basically USB but with a slightly altered physical layer to
acommodate the unusual topology.
http://www.synopsys.com/dw/dwtb/hsic_usb2_device/hsic_usb2_device.html

There is no shared memory or, in fact, any other connections between BP
and interfaces of the phone. Audio is transferred via the same USB, for
example.

GPS technically has some relation with the BP, I'm not absolutely sure
which precisely, but you can a) upload reference SiRF firmware to the 
GPS,
thus rendering any changes Samsung put to the latter void and b) AP
controls !RESET pins of both GPS and BP. It's trivial to not allow both 
to
run simultaneously.

-- 
   WBR, Peter Zotov.




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