In modern phones, doesn't the main chip control the baseband chip rather than the other way? What is the threat model?

Obviously since phone calls and text messages are unencrypted the baseband chip could send them elsewhere. But your internet communication should already be encrypted before it hits the baseband chip. So is the threat model Motorola has intentionally introduced a backdoor to leak data from the main chip to the baseband chip?

"control" is mostly a gentlemans agreement. It sends commands. The OS running in the baseband can decide how to handle the commands. Assuming a malicious vendor, motivated by its host nation, that would be where they could place the most effective backdoors.

Vendors with nefarious intentions or operating on directives from a state agency are a real and valid concern, but the handset under consideration makes use of a Qualcomm baseband chipset, so the OS it runs on is supplied by Qualcomm, not the manufacturer.

If I understand it correctly (but please do correct me if I am wrong), Qualcomm supplies the baseband OS as a signed binary blob.

My knowledge is third-hand as I haven't worked at Motorola for over 20 years (and back then I worked on "server" side, not device side), but I think you are correct. What I'm unclear on is how it differs from the chips (and associated blobs) sold for use in China, where the legal requirements differ, and how reliable the "signing" really is. I do know that some groups get special permission from Qualcomm to load other firmware and expose certain features to the user-land OS. My threat analysis is mostly theoretical and I can't really say that Qualcomms control is lacking or co-opted.

Graphene doesn't expect to have control of the RF. It makes all the RF stuff happen on the other side of a firewall.

The userspace OS needs to be where control of any radio (bluetooth, wi-fi, cellular, nfc, uwb etc.) components happens, otherwise it would be difficult to be compliant in terms of regulatory requirements like a working Flight Mode.

Baseband firmware/processors are still making strides in terms of defending against attacks so I don't think we are at the point where backdoors are needed. More likely that undocumented AT commands, plausibly deniable bugdoors and existing memory corruption vulnerabilities are already convenient reality. However, part of that reality is also that once the cellular modem is exploited OTA it is still a massive task to traverse IOMMU restrictions and get device owner data/secrets from the userspace OS. It doesn't even grant visibility into application traffic which has been aggressively TLS encrypted since the mid 2010s.

> The userspace OS needs to be where control of any radio (bluetooth, wi-fi, cellular, nfc, uwb etc.) components happens, otherwise it would be difficult to be compliant in terms of regulatory requirements like a working Flight Mode.

The entities that are putting in backdoors don't respect the law, and obviously will do what they can to obfuscate the backdoor enough to pass regulatory reviews.

My threat model is precisely entities that don't respect law or are unethical. So, the question then boils down to whether current regulatory review processes can catch such backdoors and the people running these processes cannot be bribed or otherwise forced to skip proper review?

It's my understanding that you cannot really defend against backdoors. You can somewhat mitigate their impact through good security/privacy design. Your energies may be better invested in a position that assumes any and all of the radios have been compromised or exploited and work from there (this is what mainstream flagship smartphone SoC/OS designers already do).

The OS absolutely controls functionality of the baseband. But it doesn't actually matter that much at all. Even if the baseband were fully open any serious, truly secure device is still going to use IOMMUs, etc to segregate the systems from each other and compartmentalize them. This is already true today, you can find tons of vendors who own all the IP in multiple subsystems on a bus, and they will still insert IOMMUs inbetween, etc.

Any practical and secure design for a cellular phone has to assume the network is totally compromised and hostile. From this perspective, adding the radio chip and its software to that category is not actually a huge leap; it is the bare minimum in terms of secure hardware design. GrapheneOS is where the trust is placed, not the baseband, so its source code is more or less not relevant.