On SoCs with an on-chip modem, the modem can write to any part of host memory it likes. No IOMMU.
On the (very) few remaining phones that use two separate chips, what do you think the odds are that the host OS is hardened against attacks originating from its own modem? That is a hideously complicated protocol spoken between the modem and the host processor. Plenty of validation+overflow footguns. Finding exploits here isn't going to get security researchers promoted, if they can find them at all. "Exploit is available only to modem manufacturer" does not engender a high CVE score.
Oh, and, just to top it off, that interface between the modem and the host processor when they aren't on the same chip is... drum roll... USB. As in, BadUSB. As in, Mr. Phone says: "wow, somebody plugged in a USB keyboard! And a USB mouse!".
But hey, the modem doesn't even need to own the CPU. It can record and exfiltrate your GPS location quite happily via LTE, all on its own. And buffer up a nearly unlimited amount of location data on that private NAND flash it has in case you're out of cell range. All while pinky-swearing that location services are definitely absolutely turned off, promise. I don't know why people believe that location-tracking is ever turned off on a phone; it's just absurd to think that.
On the (very) few remaining phones that use two separate chips, what do you think the odds are that the host OS is hardened against attacks originating from its own modem? That is a hideously complicated protocol spoken between the modem and the host processor. Plenty of validation+overflow footguns. Finding exploits here isn't going to get security researchers promoted, if they can find them at all. "Exploit is available only to modem manufacturer" does not engender a high CVE score.
Oh, and, just to top it off, that interface between the modem and the host processor when they aren't on the same chip is... drum roll... USB. As in, BadUSB. As in, Mr. Phone says: "wow, somebody plugged in a USB keyboard! And a USB mouse!".
But hey, the modem doesn't even need to own the CPU. It can record and exfiltrate your GPS location quite happily via LTE, all on its own. And buffer up a nearly unlimited amount of location data on that private NAND flash it has in case you're out of cell range. All while pinky-swearing that location services are definitely absolutely turned off, promise. I don't know why people believe that location-tracking is ever turned off on a phone; it's just absurd to think that.