Re: [PATCH 2/2] brcmfmac: pcie: Provide a buffer of random bytes to the device

[Hector Martin <marcan@marcan.st>]

On 14/02/2023 18.00, Julian Calaby wrote:
> Hi Arend,
> 
> On Tue, Feb 14, 2023 at 7:04 PM Hector Martin <marcan@marcan.st> wrote:
>>
>> Newer Apple firmwares on chipsets without a hardware RNG require the
>> host to provide a buffer of 256 random bytes to the device on
>> initialization. This buffer is present immediately before NVRAM,
>> suffixed by a footer containing a magic number and the buffer length.
>>
>> This won't affect chips/firmwares that do not use this feature, so do it
>> unconditionally for all Apple platforms (those with an Apple OTP).
> 
> Following on from the conversation a year ago, is there a way to
> detect chipsets that need these random bytes? While I'm sure Apple is
> doing their own special thing for special Apple reasons, it seems
> relatively sensible to omit a RNG on lower-cost chipsets, so would
> other chipsets need it?

I think we could include a list of chips known not to have the RNG (I
think it's only the ones shipped on T2 machines). The main issue is I
don't have access to those machines so it's hard for me to test exactly
which ones need it. IIRC Apple's driver unconditionally provides the
randomness. I could at least test the newer chips on AS platforms and
figure out if they need it to exclude them... but then again, all I can
do is test whether they work without the blob, but they might still want
it (and simply become less secure without it).

So I guess the answer is "maybe, I don't know, and it's kind of hard to
know for sure"... the joys of reverse engineering hardware without
vendor documentation.

If you mean whether other chips with non-apple firmware can use this, I
have no idea. That's probably something for Arend to answer. My gut
feeling is Apple added this as part of a hardening mechanism and
non-Apple firmware does not use it (and Broadcom then probably started
shipping chips with a hardware RNG and firmware that uses it directly
across all vendors), in which case the answer is no.

- Hector