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

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Date: Fri, 26 Jan 2024 16:46:50 +0200
From: Nikolay Borisov <nik.borisov@...e.com>
To: "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,
 Theodore Ts'o <tytso@....edu>, "Jason A. Donenfeld" <Jason@...c4.com>
Cc: 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 26.01.24 г. 15:42 ч., Kirill A. Shutemov wrote:
> 4. Exit to the host/VMM with an error indication after a Confidential
>     Computing Guest failed to obtain random input from RDRAND/RDSEED
>     instructions after reasonable number of retries. This option allows
>     host/VMM to take some correction action for cases when the load on
>     RDRAND/RDSEED instructions has been put by another actor, i.e. the
>     other guest VM. The exit to host/VMM in such cases can be made
>     transparent for the Confidential Computing Guest in the TDX case with
>     the assistance of the TDX module component.

But is this really a viable solution in the face of malicious VMM? It 
assumes that if the VMM is signaled that randomness has been exhausted 
it will try to rectify it, what if such a signal can instead be 
repurposed for malicious purposes? Could it perhaps be used as some sort 
of a side channel attack ?