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Date: Mon, 29 Jan 2024 23:33:00 +0200
From: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
To: Dave Hansen <dave.hansen@intel.com>
Cc: Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@redhat.com>, 
	Borislav Petkov <bp@alien8.de>, Dave Hansen <dave.hansen@linux.intel.com>, 
	"H. Peter Anvin" <hpa@zytor.com>, x86@kernel.org, Theodore Ts'o <tytso@mit.edu>, 
	"Jason A. Donenfeld" <Jason@zx2c4.com>, 
	Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@linux.intel.com>, Elena Reshetova <elena.reshetova@intel.com>, 
	Jun Nakajima <jun.nakajima@intel.com>, Tom Lendacky <thomas.lendacky@amd.com>, 
	"Kalra, Ashish" <ashish.kalra@amd.com>, Sean Christopherson <seanjc@google.com>, 
	linux-coco@lists.linux.dev, linux-kernel@vger.kernel.org
Subject: Re: [RFC] Randomness on confidential computing platforms
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On Mon, Jan 29, 2024 at 01:04:23PM -0800, Dave Hansen wrote:
> 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?

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.

Admittedly, I don't find my own argument very convincing :)

-- 
  Kiryl Shutsemau / Kirill A. Shutemov