I can't speak for Google devices but per Apple's iOS security whitepaper (page 41) [1]:
"To protect the device from vulnerabilities in network processor firmware, network interfaces including Wi-Fi and baseband have limited access to application processor memory. When USB or SDIO is used to interface with the network processor, the network processor can’t initiate Direct Memory Access (DMA) transactions to the application processor. When PCIe is used, each network processor is on its own isolated PCIe bus. An IOMMU on each PCIe bus limits the network processor’s DMA access to pages of memory containing its network packets or control structures."
Oh, totally. It's an important part of modern defense in depth. 'Just because they said they turned on the IOMMU doesn't make it bulletproof' is all I'm saying.
You'd be surprised how unsafe those are too, even though they aren't RDMA protocols. USB and SDIO stacks aren't really designed with malicious input in mind; it's like the 90s all over again if you think of those as the network protocols they are.
That's right, any kind of communication opens up some form of attack surface. Eliminating DMA is still worthwhile by reducing its total size, and shifting more of the implementation to the softare stack gives the device owner (hopefully the user) more control over the attack surface.
"To protect the device from vulnerabilities in network processor firmware, network interfaces including Wi-Fi and baseband have limited access to application processor memory. When USB or SDIO is used to interface with the network processor, the network processor can’t initiate Direct Memory Access (DMA) transactions to the application processor. When PCIe is used, each network processor is on its own isolated PCIe bus. An IOMMU on each PCIe bus limits the network processor’s DMA access to pages of memory containing its network packets or control structures."
[1]: https://www.apple.com/business/site/docs/iOS_Security_Guide....