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

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Date: Mon, 29 Jan 2024 13:04:23 -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 12:26, Kirill A. Shutemov wrote:
>>> Do we care?
>> I want to make sure I understand the scenario:
>>
>>  1. We're running in a guest under TDX (or SEV-SNP)
>>  2. The VMM (or somebody) is attacking the guest by eating all the
>>     hardware entropy and RDRAND is effectively busted
>>  3. Assuming kernel-based panic_on_warn and WARN_ON() rdrand_long()
>>     failure, that rdrand_long() never gets called.
> Never gets called during attack. It can be used before and after.
> 
>>  4. Userspace is using RDRAND output in some critical place like key
>>     generation and is not checking it for failure, nor mixing it with
>>     entropy from any other source
>>  5. Userspace uses the failed RDRAND output to generate a key
>>  6. Someone exploits the horrible key
>>
>> Is that it?
> Yes.

Is there something that fundamentally makes this a VMM vs. TDX guest
problem?  If a malicious VMM can exhaust RDRAND, why can't malicious
userspace do the same?

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?