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

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Date: Mon, 29 Jan 2024 15:32:52 -0800
From: "H. Peter Anvin" <hpa@...or.com>
To: Dave Hansen <dave.hansen@...el.com>,
        "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>, 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 January 29, 2024 2:18:50 PM PST, Dave Hansen <dave.hansen@...el.com> wrote:
>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.

Again, do we even have a problem with the "hold the boot until we have entropy"option?