Comment Re:Figures (Score 5, Informative) 453
In a phone, the GSM modem has its own CPU (and its own memory).
Most phones are based on SoCs (Systems on a Chip); everything's interconnected on the same silicon. Usually the GSM modem processor has access to the memory and I/O busses of the main processor (but not the other way around), can reset the main processor, and often boots before the main processor and must explicitly turn on the main processor before it runs. I believe that in some designs the modem processor actually sets up the boot loader for the main processor as well. The modem processor can definitely rewrite the flash where the main processor's operating system is stored.
The result of this is that the modem has total control of the phone. It can do anything it wants to any data on the phone, including the internals of the main OS, and there's basically nothing the main processor can do about it other than maybe be too obscure and complicated to manipulate easily.
The firmware in the modem is invariably closed source and secret. The modem will only boot firmware that's crypto-signed by the manufacturer, and anyway the hardware is totally undocumented.
The modems have "over the air" command sets that let the carrier manipulate the phone remotely without going through the main OS. Those command sets can be very rich... and can include the ability to reflash the main OS, or even to peek and poke its memory while it's running.
So on most (all?) phones, it basically doesn't matter what your OS is. The carrier (possibly together with the SoC manufacturer) can do whatever it wants if it's willing to figure out the complexity of doing so. And of course governments lean on carriers and SoC manufacturers to get access to that capability, and commercial "partners" also have influence.