Re: [PATCH v5 07/11] x86: use fallback for random_get_entropy() instead of zero

Borislav Petkov
2022-04-19T18:23:35Z

On Tue, Apr 19, 2022 at 01:16:46PM +0200, Jason A. Donenfeld wrote:
> In the event that random_get_entropy() can't access a cycle counter or
> similar, falling back to returning 0 is really not the best we can do.
> Instead, at least calling random_get_entropy_fallback() would be
> preferable, because that always needs to return _something_, even
> falling back to jiffies eventually. It's not as though
> random_get_entropy_fallback() is super high precision or guaranteed to
> be entropic, but basically anything that's not zero all the time is
> better than returning zero all the time.
>
> If CONFIG_X86_TSC=n, then it's possible that we're running on a 486 with
> no RDTSC, so we only need the fallback code for that case.
>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Cc: Arnd Bergmann <arnd@arndb.de>
> Cc: x86@kernel.org
> Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
> ---
> arch/x86/include/asm/timex.h | 10 ++++++++++
> 1 file changed, 10 insertions(+)
>
> diff --git a/arch/x86/include/asm/timex.h b/arch/x86/include/asm/timex.h
> index a4a8b1b16c0c..fac180359693 100644
> --- a/arch/x86/include/asm/timex.h
> +++ b/arch/x86/include/asm/timex.h
> @@ -5,6 +5,16 @@
> #include <asm/processor.h>
> #include <asm/tsc.h>
>
> +static inline unsigned long random_get_entropy(void)
> +{
> +#ifndef CONFIG_X86_TSC
> + if (!boot_cpu_has(X86_FEATURE_TSC))

cpu_feature_enabled() pls.

--
Regards/Gruss,
Boris.