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

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Message-ID: <CAGRGNgV2EALagFHKhBwDaySEFMufeK=7shF-yTRtJkFuF+W1Gg@mail.gmail.com>
Date:   Tue, 14 Feb 2023 20:11:17 +1100
From:   Julian Calaby <julian.calaby@...il.com>
To:     Hector Martin <marcan@...can.st>
Cc:     Arend van Spriel <aspriel@...il.com>,
        Franky Lin <franky.lin@...adcom.com>,
        Hante Meuleman <hante.meuleman@...adcom.com>,
        Kalle Valo <kvalo@...nel.org>,
        "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        Sven Peter <sven@...npeter.dev>,
        Alyssa Rosenzweig <alyssa@...enzweig.io>,
        Linus Walleij <linus.walleij@...aro.org>,
        asahi@...ts.linux.dev, linux-wireless@...r.kernel.org,
        brcm80211-dev-list.pdl@...adcom.com,
        SHA-cyfmac-dev-list@...ineon.com, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] brcmfmac: pcie: Provide a buffer of random bytes to
 the device

Hi Hector,

On Tue, Feb 14, 2023 at 8:08 PM Hector Martin <marcan@...can.st> wrote:
>
> On 14/02/2023 18.00, Julian Calaby wrote:
> > Hi Arend,
> >
> > On Tue, Feb 14, 2023 at 7:04 PM Hector Martin <marcan@...can.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.

Sorry, I should have been more clear, I wasn't expecting you to know,
I was asking Arend if he knew.

Thanks,

-- 
Julian Calaby

Email: julian.calaby@...il.com
Profile: http://www.google.com/profiles/julian.calaby/