zlacker

>>fghtr+(OP)
This blogpost propagates what I'd like to describe as an urban legend about baseband processors and main memory. The story originates from old times where even fancy phones allowed the baseband to write everywhere in main memory. The myth then becomes that you need the baseband physically separated from your main application processor.

But the world's moved on since those reports were made. It's FUD: https://www.reddit.com/r/CopperheadOS/comments/6wtul0/on_sen...

>>DCKing+ua
I know very little about the topic so bearing that in mind:

We're already in a world were we can't quite trust our CPUs, so why trusting baseband chips?

If it does make the design more complicated, it may also reduce the potential attack surface.

>>alex_d+8b
> If it does make the design more complicated, it may also reduce the potential attack surface.

an increase in complexity would rule out reduction of attack surface. in fact attack surface would be guaranteed to increase

>>Dyslex+wb
Well, that isn't generally true if the complexity is actually a security boundary. After all, all security designs are based on layers -- it's hard to add a layer of security without adding complexity.

As a counter-example -- removing all of Linux's privilege checking would make the code a lot less complicated, but the attack surface would increase a million-fold. In this case, the Librem 5's separation of the baseband such that communication is done over USB (a protocol which doesn't have DMA) is a security improvement over giving the baseband DMA access.

>>cyphar+Ed
USB protocols are often times handled in SW, some in Linux kernel, some in userspace. So if someone discovers RCE over USB in Linux USB stack, modem will have direct memory access, or even RCE on the main CPU with kernel privileges.

I have no experience with PCIe so maybe it's harder with USB to abuse the host system, than with PCIe these days.

You can think of USB as being similar to using a TCP/IP protocol between multiple machines capable of executing code, and having to execute code to handle higher level protocols, like HTTP or whatnot. If there's a code execution bug anywhere, the USB capable device will be able to exploit it.

And by default, there's a code-execution bug on all normally configured Linux machines. If you'll not create a USB "firewall", modem can just create a virtual keyboard and kernel will happily accept all input from it, for example. So modem can just type whatever it wants to your shell. It will be obvious, but, it's still device->host RCE.

>>megous+Cp
We're not in disagreement (I never claimed or even implied that USB is bug-free) -- but in order to get an RCE or DMA-like access you first need to exploit the USB stack. PCIe gives you that kind of access for free by design (almost -- there is IOMMU these days but there is little evidence that it is nearly as secure as hardware vendors claim, and you'd need to have phone hardware which supports it).