linux-kernel - Re: [PATCH random v2] x86/rdrand: Remove "nordrand" flag in favor of "random.trust

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Date:   Sat, 9 Jul 2022 13:30:52 +0200
From:   Borislav Petkov <bp@...e.de>
To:     "Jason A. Donenfeld" <Jason@...c4.com>
Cc:     linux-kernel@...r.kernel.org, x86@...nel.org,
        Theodore Ts'o <tytso@....edu>,
        "H . Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH random v2] x86/rdrand: Remove "nordrand" flag in favor of
 "random.trust_cpu"

On Sat, Jul 09, 2022 at 12:43:06PM +0200, Jason A. Donenfeld wrote:
> The decision of whether or not to trust RDRAND is controlled by the
> "random.trust_cpu" boot time parameter or the CONFIG_RANDOM_TRUST_CPU
> compile time default. The "nordrand" flag was added during the early
> days of RDRAND, when there were worries that merely using its values
> could compromise the RNG. However, these days, RDRAND values are not
> used directly but always go through the RNG's hash function, making
> "nordrand" no longer useful.
> 
> Rather, the correct switch is "random.trust_cpu", which not only handles
> the relevant trust issue directly, but also is general to multiple CPU
> types, not just x86.
> 
> However, x86 RDRAND does have a history of being occasionally
> problematic. Prior, when the kernel would notice something strange, it'd
> warn in dmesg and suggest enabling "nordrand". We can improve on that by
> making the test a little bit better and then taking the step of
> automatically disabling RDRAND if we detect it's problematic.
> 
> Also extend the basic sanity test to RDSEED in addition to RDRAND, and
> disable both if either one fails.
> 
> Cc: x86@...nel.org
> Cc: Theodore Ts'o <tytso@....edu>
> Suggested-by: H. Peter Anvin <hpa@...or.com>
> Suggested-by: Borislav Petkov <bp@...e.de>
> Signed-off-by: Jason A. Donenfeld <Jason@...c4.com>
> ---
> This is a tip-ish commit, but it relies on the CONFIG_ARCH_RANDOM commit
> in the random tree, so I'll take this through the random tree to avoid
> conflicts.

Acked-by: Borislav Petkov <bp@...e.de>

-- 
Regards/Gruss,
    Boris.

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