linux-kernel - Re: [PATCH v3] x86/coco: Require seeding RNG with RDRAND on CoCo systems

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Date: Thu, 22 Feb 2024 00:09:32 +0100
From: "Jason A. Donenfeld" <Jason@...c4.com>
To: Dave Hansen <dave.hansen@...el.com>
Cc: x86@...nel.org, linux-coco@...ts.linux.dev, linux-kernel@...r.kernel.org, 
	Borislav Petkov <bp@...en8.de>, Daniel P . Berrangé <berrange@...hat.com>, 
	Dave Hansen <dave.hansen@...ux.intel.com>, Elena Reshetova <elena.reshetova@...el.com>, 
	"H . Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...hat.com>, 
	"Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>, "Theodore Ts'o" <tytso@....edu>, 
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH v3] x86/coco: Require seeding RNG with RDRAND on CoCo systems

On Wed, Feb 21, 2024 at 11:47 PM Dave Hansen <dave.hansen@...el.com> wrote:
> OK, so we're trying to get 256 bits of seed data from RDRAND?

Yes. This fills a 32-byte buffer of longs up with output from
arch_get_random_longs().

> But no pattern I could discern.  Did you mean something different by
> "platform drivers"?

https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/arch/um/kernel/um_arch.c#n413
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/drivers/char/random.c#n655
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/scripts/gcc-plugins/latent_entropy_plugin.c#n90


> 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.

If you insist, I'll stick a local `enum { RANDOM_BYTES_COUNT = 32 }`
in that function or something. Seems unnecessary to me but if that's
what you need no problem. Would that suffice? Or a different variable
name? Or a comment on "what is 32 about?"

Jason