You can use an IOMMU to isolate DMA to a separate security domain. This is how Qubes OS works.
No iommu on the imx8mq.
That's unfortunate, since an IOMMU is an excellent solution to this kind of problem, especially when one considers all the potential Linux USB stack vulnerabilities that can be exploited if you assume the modem is untrustworthy.
Sadly, you're wrong, and the phone modem is on the USB bus precisely for that reason.
Sadly I am happy to be corrected! :D
Yea, if feels like if you wanted that kind of isolation, you'd actually need a second SoC on there with its own memory and some kind of communication back to the main processor.