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Date:   Tue, 05 Jul 2022 14:50:34 -0700
From:   "H. Peter Anvin" <hpa@zytor.com>
To:     Borislav Petkov <bp@alien8.de>,
        "Jason A. Donenfeld" <Jason@zx2c4.com>
CC:     linux-arm-kernel@lists.infradead.org,
        linuxppc-dev@lists.ozlabs.org, linux-s390@vger.kernel.org,
        linux-kernel@vger.kernel.org, x86@kernel.org,
        Catalin Marinas <catalin.marinas@arm.com>,
        Will Deacon <will@kernel.org>,
        Michael Ellerman <mpe@ellerman.id.au>,
        Heiko Carstens <hca@linux.ibm.com>,
        Alexander Gordeev <agordeev@linux.ibm.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Arnd Bergmann <arnd@arndb.de>
Subject: Re: [PATCH] random: remove CONFIG_ARCH_RANDOM and "nordrand"
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On July 5, 2022 12:57:04 PM PDT, Borislav Petkov <bp@alien8=2Ede> wrote:
>On Tue, Jul 05, 2022 at 09:44:17PM +0200, Jason A=2E Donenfeld wrote:
>> Oh, huh=2E Maybe in that case I should adjust the message to say "consi=
der
>> using `random=2Etrust_cpu=3D0`," which is the thing that would actually=
 make
>> a security difference=2E
>
>Why isn't that option documented in
>Documentation/admin-guide/kernel-parameters=2Etxt?
>
>> But actually, one thing that wasn't clear to me was: does `nordrand`
>> affect what userspace sees? While random=2Ec is okay in lots of
>> circumstances, I could imagine `nordrand` playing a role in preventing
>> userspace from using it, which might be desirable=2E Is this the case? =
If
>> so, I can remove the nordrand chunk from this patch for v2=2E If not, I=
'll
>> adjust the text to mention `random=2Etrust_cpu=3D0`=2E
>
>Unfortunately, it doesn't disable the instruction=2E It would be lovely i=
f
>we had a switch like that=2E=2E=2E
>
>That's why this message is supposed to be noisy so that people can pay
>attention at least=2E
>
>> In the sense that random=2Ec can handle mostly any input without making
>> the quality worse=2E So, you can't accidentally taint it=2E The only ri=
sk is
>> if it thinks RDRAND is good and trustable when it isn't, but that's wha=
t
>> `random=2Etrust_cpu=3D0` is for=2E
>
>And that's why I'm saying that if you detect RDRAND returning the
>same thing over and over again, you should simply stop using it=2E
>Automatically=2E Not rely on the user to do anything=2E
>

It's just math=2E The only variable is your confidence level, i=2Ee=2E at =
what level do you decide that the likelihood of pure chance is way smaller =
than the likelihood of hardware failure=2E For example, the likelihood of m=
 n-bit samples in a row being identical is 2^-(n*(m-3/2)), and the likeliho=
od of the CPU being destroyed by a meterorite in the same microsecond is ab=
out 2^-100=2E