linux-kernel - Re: [PATCH 1/2] x86/random: Retry on RDSEED failure

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Date: Thu, 1 Feb 2024 10:46:06 -0800
From: Dave Hansen <dave.hansen@...el.com>
To: "Jason A. Donenfeld" <Jason@...c4.com>, Theodore Ts'o <tytso@....edu>,
 "Reshetova, Elena" <elena.reshetova@...el.com>,
 Dave Hansen <dave.hansen@...ux.intel.com>
Cc: "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
 Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>,
 Borislav Petkov <bp@...en8.de>, "H. Peter Anvin" <hpa@...or.com>,
 "x86@...nel.org" <x86@...nel.org>,
 Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@...ux.intel.com>,
 "Nakajima, Jun" <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-coco@...ts.linux.dev>,
 "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/2] x86/random: Retry on RDSEED failure

On 2/1/24 10:09, Jason A. Donenfeld wrote:
> Question ii) Just how DoS-able is RDRAND? From host to guest, where
> the host controls scheduling, that seems easier, but how much so, and
> what's the granularity of these operations, and could retries still
> help, or not at all? What about from guest to guest, where the
> scheduling is out of control; in that case is there a value of N for
> which N retries makes it actually impossible to DoS? What about from
> userspace to kernelspace; good value of N?

So far, in practice, I haven't seen a single failure of RDRAND.  It's
been limited to RDSEED.  In a perfect world, I'd change the architecture
docs to say, "RDRAND only fails when the hardware breaks" and leave
RDSEED defined to be the one that fails easily.

Dealing with a fragile RDSEED seems like a much easier problem than
dealing with a fragile RDRAND since RDSEED is used _much_ more sparingly
in the kernel today.

But I'm not sure if the hardware implementations fit into this perfect
world I've conjured up.  We're going to wrangle up the folks at Intel
who can hopefully tell me if I'm totally deluded.

Has anyone seen RDRAND failures in practice?  Or just RDSEED?

> Question iii) How likely is Intel to actually fix this in a
> satisfactory way (see "specifying this is an interesting question" in
> [1])? And if they would, what would the timeline even be?

If the fix is pure documentation, it's on the order of months.  I'm
holding out hope that some kind of anti-DoS claims like you mentioned:

> Specifying this is an interesting question. What exactly might our
> requirements be for a "non-broken" RDRAND? It seems like we have two
> basic ones:
> 
> - One VMX (or host) context can't DoS another one.
> - Ring 3 can't DoS ring 0.

are still possible on existing hardware, at least for RDRAND.