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

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Message-ID: <CAHmME9oJvbZgT4yT9Vydc2ZQVSo3Ea65G5aVK7gFxphkV00BnQ@mail.gmail.com>
Date: Wed, 31 Jan 2024 14:14:14 +0100
From: "Jason A. Donenfeld" <Jason@...c4.com>
To: "Reshetova, Elena" <elena.reshetova@...el.com>
Cc: "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>, 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" <x86@...nel.org>, "Theodore Ts'o" <tytso@....edu>, 
	Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@...ux.intel.com>, 
	"Nakajima, Jun" <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-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, Jan 31, 2024 at 8:56 AM Reshetova, Elena
<elena.reshetova@...el.com> wrote:
> So given this, I would personally consider the virtual guest TSC value
> observable by host/VMM.
> [2] TDX module source code:
> https://www.intel.com/content/www/us/en/download/738875/782152/intel-trust-domain-extension-intel-tdx-module.html

Thanks for the explanation and link. Indeed if this is all mediated by
the host, we're in bad shape.

> For the high resolution timers, host controls guest apic timers and interrupts fully.
> So, it has the power to see and even affect when a certain interrupt happens
> or doesnt happen in the guest. It can delay guest timers at its will on pretty
> extensive time periods. This seems powerful enough for me.
> Things like HPET are also fully under host control.

And I suppose RDPMC is similar?

And it's not like the guest can just take an excessive amount of TSC
samples and randomly select which ones it uses because chickens and
eggs...

The situation you paint is that all of our entropy inputs -- timers,
rdrand, etc -- are either host controllable, host observable, or host
(and guest sibling) DoS'able, so if you don't trust the host, there
are no good inputs. That's not a great position to be in, and I wonder
if something can be done on the hardware side to remedy it, as this
seems like a major shortcoming in TDX. So far, all of the proposed
mitigations introduce some other DoS.

> Yes, in practice there will be physical hw underneath, but the problem imo is
> that the host is in between and still very powerful when it comes to interrupts and
> timers at the moment.

Sure sounds like it.

Jason