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Date: Wed, 21 Feb 2024 14:47:18 -0800
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Subject: Re: [PATCH v3] x86/coco: Require seeding RNG with RDRAND on CoCo
 systems
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To: "Jason A. Donenfeld" <Jason@zx2c4.com>
Cc: x86@kernel.org, linux-coco@lists.linux.dev, linux-kernel@vger.kernel.org,
 Borislav Petkov <bp@alien8.de>, =?UTF-8?Q?Daniel_P_=2E_Berrang=C3=A9?=
 <berrange@redhat.com>, Dave Hansen <dave.hansen@linux.intel.com>,
 Elena Reshetova <elena.reshetova@intel.com>, "H . Peter Anvin"
 <hpa@zytor.com>, Ingo Molnar <mingo@redhat.com>,
 "Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>,
 Theodore Ts'o <tytso@mit.edu>, Thomas Gleixner <tglx@linutronix.de>
References: <20240221123248.25570-1-Jason@zx2c4.com>
 <eada8acd-9341-401b-b6c9-a05848eb91d6@intel.com>
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From: Dave Hansen <dave.hansen@intel.com>
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On 2/21/24 09:19, Jason A. Donenfeld wrote:
> On Wed, Feb 21, 2024 at 5:55 PM Dave Hansen <dave.hansen@intel.com> wrote:
>> On 2/21/24 04:32, Jason A. Donenfeld wrote:
>>> +__init void cc_random_init(void)
>>> +{
>>> +     unsigned long rng_seed[32 / sizeof(long)];
>> My only nit with this is the magic "32".
>>
>> Why not 16?  Or 64?
> 32 bytes = 256-bits = what we're targeting. Very normal thing to see
> places in the RNG, used all over random.c and lots of platform
> drivers. Pretty obvious and straightforward to anyone familiar with
> this kind of code. Not the kind of thing you'd want to replace with
> some abstracted constant that makes you search.

OK, so we're trying to get 256 bits of seed data from RDRAND?

There's an entire section of the Intel whitepaper[1]: "Generating Seeds
from RDRAND".  It describes one "method of turning 512 128-bit samples
from the DRNG into a 128-bit seed value".  I was naively thinking that
if the kernel wants 256 bits of seed data from RDRAND, it might take
2*(512 128-bit samples).

I'm not suggesting that we use the exact construction from that
whitepaper, but I'm reasonably sure I could actually explain to someone
where a magic 1024 came from.

I also went through a smattering of add_device_randomness() users.  I
didn't see much of a pattern there that seemed to line up with a
256-bits convention.  If anything they seemed to just use what they had
laying around.  I saw byte counts of 16, 21, 12, 8, 1, strlen(), 56.
But no pattern I could discern.  Did you mean something different by
"platform drivers"?

If we're going to have arch/x86-specific crud, it would be great to make
it obvious and straightforward to those of us simple folk that are
familiar with arch/x86 code.

1.
https://www.intel.com/content/www/us/en/developer/articles/guide/intel-digital-random-number-generator-drng-software-implementation-guide.html