linux-kernel - Re: [PATCH v5 07/11] x86: use fallback for random_get

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Date:   Tue, 19 Apr 2022 20:38:41 +0200
From:   "Jason A. Donenfeld" <Jason@...c4.com>
To:     Borislav Petkov <bp@...en8.de>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Linux Crypto Mailing List <linux-crypto@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Arnd Bergmann <arnd@...db.de>, "Theodore Ts'o" <tytso@....edu>,
        Dominik Brodowski <linux@...inikbrodowski.net>,
        Russell King <linux@...linux.org.uk>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        Thomas Bogendoerfer <tsbogend@...ha.franken.de>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Albert Ou <aou@...s.berkeley.edu>,
        "David S . Miller" <davem@...emloft.net>,
        Richard Weinberger <richard@....at>,
        Anton Ivanov <anton.ivanov@...bridgegreys.com>,
        Johannes Berg <johannes@...solutions.net>,
        Ingo Molnar <mingo@...hat.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "H . Peter Anvin" <hpa@...or.com>, Chris Zankel <chris@...kel.net>,
        Max Filippov <jcmvbkbc@...il.com>,
        Stephen Boyd <sboyd@...nel.org>,
        Dinh Nguyen <dinguyen@...nel.org>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
        linux-m68k <linux-m68k@...ts.linux-m68k.org>,
        "open list:BROADCOM NVRAM DRIVER" <linux-mips@...r.kernel.org>,
        linux-riscv <linux-riscv@...ts.infradead.org>,
        sparclinux <sparclinux@...r.kernel.org>,
        linux-um@...ts.infradead.org, X86 ML <x86@...nel.org>,
        linux-xtensa@...ux-xtensa.org
Subject: Re: [PATCH v5 07/11] x86: use fallback for random_get_entropy()
 instead of zero

Hi Borislav,

On Tue, Apr 19, 2022 at 8:16 PM Borislav Petkov <bp@...en8.de> wrote:
> > +static inline unsigned long random_get_entropy(void)
> > +{
> > +#ifndef CONFIG_X86_TSC
> > +     if (!boot_cpu_has(X86_FEATURE_TSC))
>
> cpu_feature_enabled() pls.

This function began as a carbon copy of get_cycles(), which reads:

static inline cycles_t get_cycles(void)
{
#ifndef CONFIG_X86_TSC
       if (!boot_cpu_has(X86_FEATURE_TSC))
               return 0;
#endif

       return rdtsc();
}

As you see, random_get_entropy() is the same function, but with that
zero replaced with the fallback. (Using the fallback in get_cycles()
wouldn't be appropriate.)

So, your options are:
a) We keep this patch as-is, using boot_cpu_has(); or
b) I make an unrelated change inside of this same patch to change
get_cycles() to use cpu_feature_enabled() (in addition to the new
random_get_entropy()).

I think I prefer doing (a), and leaving (b) for another time when you
or another x86 maintainer can do so. But I'll do whichever you say.
Which would you like?

Jason