linux-kernel - Re: [RFC] Randomness on confidential computing platforms

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Message-ID: <dqiaimv3qqh77cfm2huzja4vsho3jls7vjmnwgda7enw633ke2@qiqrdnno75a7>
Date: Mon, 29 Jan 2024 18:41:49 +0200
From: "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
To: Dave Hansen <dave.hansen@...el.com>
Cc: Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>, 
	Borislav Petkov <bp@...en8.de>, Dave Hansen <dave.hansen@...ux.intel.com>, 
	"H. Peter Anvin" <hpa@...or.com>, x86@...nel.org, Theodore Ts'o <tytso@....edu>, 
	"Jason A. Donenfeld" <Jason@...c4.com>, 
	Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@...ux.intel.com>, Elena Reshetova <elena.reshetova@...el.com>, 
	Jun Nakajima <jun.nakajima@...el.com>, Tom Lendacky <thomas.lendacky@....com>, 
	"Kalra, Ashish" <ashish.kalra@....com>, Sean Christopherson <seanjc@...gle.com>, 
	linux-coco@...ts.linux.dev, linux-kernel@...r.kernel.org
Subject: Re: [RFC] Randomness on confidential computing platforms

On Mon, Jan 29, 2024 at 08:30:11AM -0800, Dave Hansen wrote:
> On 1/26/24 05:42, Kirill A. Shutemov wrote:
> > 3. Panic after enough re-tries of RDRAND/RDSEED instructions fail.
> >    Another DoS variant against the Guest.
> 
> I think Sean was going down the same path, but I really dislike the idea
> of having TDX-specific (or CoCo-specific) policy here.
> 
> How about we WARN_ON() RDRAND/RDSEED going bonkers?  The paranoid folks
> can turn on panic_on_warn, if they haven't already.

Sure, we can do it for kernel, but we have no control on what userspace
does.

Sensible userspace on RDRAND/RDSEED failure should fallback to kernel
asking for random bytes, but who knows if it happens in practice
everywhere.

Do we care?

-- 
  Kiryl Shutsemau / Kirill A. Shutemov