From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Date: Tue, 19 Apr 2022 20:16:32 +0200 From: Borislav Petkov Subject: Re: [PATCH v5 07/11] x86: use fallback for random_get_entropy() instead of zero Message-ID: References: <20220419111650.1582274-1-Jason@zx2c4.com> <20220419111650.1582274-8-Jason@zx2c4.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <20220419111650.1582274-8-Jason@zx2c4.com> To: "Jason A. Donenfeld" Cc: linux-kernel@vger.kernel.org, linux-crypto@vger.kernel.org, tglx@linutronix.de, arnd@arndb.de, Theodore Ts'o , Dominik Brodowski , Russell King , Catalin Marinas , Will Deacon , Geert Uytterhoeven , Thomas Bogendoerfer , Paul Walmsley , Palmer Dabbelt , Albert Ou , "David S . Miller" , Richard Weinberger , Anton Ivanov , Johannes Berg , Ingo Molnar , Dave Hansen , "H . Peter Anvin" , Chris Zankel , Max Filippov , Stephen Boyd , Dinh Nguyen , linux-arm-kernel@lists.infradead.org, linux-m68k@lists.linux-m68k.org, linux-mips@vger.kernel.org, linux-riscv@lists.infradead.org, sparclinux@vger.kernel.org, linux-um@lists.infradead.org, x86@kernel.org, linux-xtensa@linux-xtensa.org List-ID: 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 > Cc: Arnd Bergmann > Cc: x86@kernel.org > Signed-off-by: Jason A. Donenfeld > --- > 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 > #include > > +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. https://people.kernel.org/tglx/notes-about-netiquette