linux-kernel - RE: [PATCH v2 2/2] x86/coco: Require seeding RNG with RDRAND on CoCo systems

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Date: Fri, 16 Feb 2024 07:57:19 +0000
From: "Reshetova, Elena" <elena.reshetova@...el.com>
To: "Jason A. Donenfeld" <Jason@...c4.com>
CC: "x86@...nel.org" <x86@...nel.org>, "linux-coco@...ts.linux.dev"
	<linux-coco@...ts.linux.dev>, "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>, Borislav Petkov <bp@...en8.de>,
	Daniel P . Berrangé <berrange@...hat.com>, Dave Hansen
	<dave.hansen@...ux.intel.com>, "H . Peter Anvin" <hpa@...or.com>, Ingo Molnar
	<mingo@...hat.com>, "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
	Theodore Ts'o <tytso@....edu>, Thomas Gleixner <tglx@...utronix.de>
Subject: RE: [PATCH v2 2/2] x86/coco: Require seeding RNG with RDRAND on CoCo
 systems

> Hi Elena,
> 
> On Thu, Feb 15, 2024 at 07:30:32AM +0000, Reshetova, Elena wrote:
> > Should we just go back to the approach to add one more check in
> random_init_early()
> > to panic in the CoCo case if both rdseed and rdrand fails to give us anything?
> 
> Yea, no, definitely not. That is, in my opinion, completely backwards
> and leads to impossible maintainability. CoCo is not some special
> snowflake that gets to sprinkle random conditionals in generic code.
> 
> First, consider the motivation for doing this:
> - This is to abort on a physical defective CPU bug -- presumably a
>   highly implausible thing to ever happen.
> - This is for a threat model that few people are really compelled by
>   anyway, e.g. it's whack-a-mole season with host->guest vectors.
> - This is for a single somewhat obscure configuration of a single
>   architecture with a feature only available on certain chipsets.
> - This is not an "intrinsic" problem that necessitates plumbing complex
>   policy logic all over the place, but for a very special
>   driver-specific case.
> 
> Rather, what this patch does is...
> 
> > Now with this patch, the logic becomes
> 
> Your description actually wasn't quite accurate so I'll write it out
> (and I'll clarify in the commit message/comments for v3 - my fault for
> being vague):
> 
> 1. At early boot, x86/CoCo is initialized. As part of that
>    initialization, it makes sure it can get 256 bits of RDRAND output
>    and adds it to the pool, in exactly the same way that the SD card
>    driver adds inserted memory card serial numbers to the pool. 

Yes, my mental picture that random_init_early() is called before 
setup_arch() was obviously wrong, I should have checked it explicitly.
So, yes, coco_random_init() happens first, which actually now has a nice
side-effect that on coco platforms we drop HW CPU output even earlier
in the entropy pool (Yay!).
Which at this point would be almost perfect, *if* we could also
count this entropy drop and allow ChaCha seeding to benefit straight from
this early drop of entropy. 

How about changing this to use add_hwgenerator_randomness()? 
And adjust cc_random_init() to try rdseed first and only fallback to rdrand
if it fails? 
I envision that a counter argument to this would be "we only count
entropy from HW CPU RNG, if we trust CPU RNG", but in CoCo case
we *do trust CPU* and this is the output of true HW RNG that we are
mixing in the pool per definition. 

Best Regards,
Elena.