Re: [PATCH 1/2] x86/rdrand: implement sanity check for RDSEED

[Borislav Petkov]

On Wed, Mar 12, 2025 at 03:31:29PM +0300, Mikhail Paulyshka wrote:
> +/*
> + * RDSEED has Built-In-Self-Test (BIST) that runs on every invocation.
> + * Run the instruction a few times as a sanity check. Also make sure
> + * it's not outputting the same value over and over, which has happened
> + * as a result of past CPU bugs.
> + *
> + * If it fails, it is simple to disable RDSEED here.
> + */
> +
> +void x86_init_rdseed(struct cpuinfo_x86 *c)
> +{
> +	unsigned long sample, prev;
> +	bool failure = false;
> +	size_t i, changed;
> +
> +	if (!cpu_has(c, X86_FEATURE_RDSEED))
> +		return;
> +
> +	for (changed = 0, i = 0; i < SAMPLES; ++i) {
> +		if (!rdseed_long(&sample)) {
> +			failure = true;
> +			break;
> +		}
> +		changed += i && sample != prev;
> +		prev = sample;
> +	}
> +	if (changed < MIN_CHANGE)
> +		failure = true;
> +
> +	if (failure) {
> +		clear_cpu_cap(c, X86_FEATURE_RDSEED);
> +		pr_emerg("RDSEED is not reliable on this platform; disabling.\n");
> +	}
> +}

This one basically duplicates x86_init_rdrand() and I'm sure you can use
a single function to test both.

But more importantly, lemme ask around internally whether that is even
a reliable test to detect RDSEED performs properly or not.

Stay tuned...

Thx.

-- 
Regards/Gruss,
    Boris.

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