linux-kernel - Re: [PATCH 1/2] x86/random: Retry on RDSEED failure

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Date: Wed, 14 Feb 2024 15:11:03 -0500
From: "Theodore Ts'o" <tytso@....edu>
To: "Jason A. Donenfeld" <Jason@...c4.com>
Cc: Tom Lendacky <thomas.lendacky@....com>,
        "Reshetova, Elena" <elena.reshetova@...el.com>,
        Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>, "x86@...nel.org" <x86@...nel.org>,
        Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@...ux.intel.com>,
        "Nakajima, Jun" <jun.nakajima@...el.com>,
        "Kalra, Ashish" <ashish.kalra@....com>,
        Sean Christopherson <seanjc@...gle.com>,
        "linux-coco@...ts.linux.dev" <linux-coco@...ts.linux.dev>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/2] x86/random: Retry on RDSEED failure

On Wed, Feb 14, 2024 at 09:04:34PM +0100, Jason A. Donenfeld wrote:
> AMD people, Intel people: what are the fullest statements we can rely
> on here? Do the following two statements work?
> 
> 1) On newer chips, RDRAND never fails.
> 2) On older chips, RDRAND never fails if you try 10 times in a loop,
> unless you consider host->guest attacks, which we're not, because CoCo
> is only a thing on the newer chips.
> 
> If those hold true, then the course of action would be to just add a
> WARN_ON(!ok) but keep the loop as-is.

I think we may only want to do the WARN_ON in early boot.  Otherwise,
on older chips, if a userspace process executes RDRAND is a tight
loop, it might cause the WARN_ON to trigger, which is considered
undesirable (and is certainly going to be something that could result
in a syzbot complaint).

					- Ted