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

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Date: Mon, 29 Jan 2024 14:18:50 -0800
From: Dave Hansen <dave.hansen@...el.com>
To: "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.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 1/29/24 13:33, Kirill A. Shutemov wrote:
>> Let's assume buggy userspace exists.  Is that userspace *uniquely*
>> exposed to a naughty VMM or is that VMM just added to the list of things
>> that can attack buggy userspace?
> This is good question.
> 
> VMM has control over when a VCPU gets scheduled and on what CPU which
> gives it tighter control over the target workload. It can make a
> difference if there's small window for an attack before RDRAND is
> functional again.

This is all a bit too theoretical for my taste.  I'm fine with doing
some generic mitigation (WARN_ON_ONCE(hardware_is_exhausted)), but we're
talking about a theoretical attack with theoretical buggy software when
in a theoretically unreachable hardware state.

Until it's clearly much more practical, we have much bigger problems to
worry about.