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Date: Mon, 29 Jan 2024 22:26:06 +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 10:55:38AM -0800, Dave Hansen wrote:
> On 1/29/24 08:41, Kirill A. Shutemov wrote:
> > On Mon, Jan 29, 2024 at 08:30:11AM -0800, Dave Hansen wrote:
> >> On 1/26/24 05:42, Kirill A. Shutemov wrote:
> >>> 3. Panic after enough re-tries of RDRAND/RDSEED instructions fail.
> >>>    Another DoS variant against the Guest.
> >>
> >> I think Sean was going down the same path, but I really dislike the idea
> >> of having TDX-specific (or CoCo-specific) policy here.
> >>
> >> How about we WARN_ON() RDRAND/RDSEED going bonkers?  The paranoid folks
> >> can turn on panic_on_warn, if they haven't already.
> > 
> > Sure, we can do it for kernel, but we have no control on what userspace
> > does.
> > 
> > Sensible userspace on RDRAND/RDSEED failure should fallback to kernel
> > asking for random bytes, but who knows if it happens in practice
> > everywhere.
> > 
> > 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.

-- 
  Kiryl Shutsemau / Kirill A. Shutemov