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Date:   Wed, 28 Aug 2019 17:20:41 +0200
From:   David Sterba <dsterba@suse.cz>
To:     Borislav Petkov <bp@suse.de>
Cc:     Linus Torvalds <torvalds@linux-foundation.org>,
        Pu Wen <puwen@hygon.cn>, Thomas Gleixner <tglx@linutronix.de>,
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        the arch/x86 maintainers <x86@kernel.org>
Subject: Re: [GIT pull] x86/urgent for 5.3-rc5
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On Tue, Aug 27, 2019 at 07:39:55PM +0200, Borislav Petkov wrote:
> @@ -42,5 +43,24 @@ void x86_init_rdrand(struct cpuinfo_x86 *c)
>  			return;
>  		}
>  	}
> +
> +	/*
> +	 * Stupid sanity-check whether RDRAND does *actually* generate
> +	 * some at least random-looking data.
> +	 */
> +	prev = tmp;
> +	for (i = 0; i < SANITY_CHECK_LOOPS; i++) {
> +		if (rdrand_long(&tmp)) {
> +			if (prev != tmp)
> +				changed++;

You could do some sort of weak statistical test like

		if (popcnt(prev ^ tmp) < BITS_PER_LONG / 3)
			bad++;

		if (bad > TOO_BAD)
			WARN(...);

this should catch same value, increments you mentioned and possibly
other trivial classes of not-so-random values.