Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753175AbcKSXhi (ORCPT ); Sat, 19 Nov 2016 18:37:38 -0500 Received: from mail.kernel.org ([198.145.29.136]:43812 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752968AbcKSXhg (ORCPT ); Sat, 19 Nov 2016 18:37:36 -0500 From: Andy Lutomirski To: x86@kernel.org Cc: Borislav Petkov , linux-kernel@vger.kernel.org, Brian Gerst , Matthew Whitehead , Andy Lutomirski Subject: [PATCH] x86/boot: Fail the boot if !M486 and CPUID is missing Date: Sat, 19 Nov 2016 15:37:30 -0800 Message-Id: <70eac6639f23df8be5fe03fa1984aedd5d40077a.1479598603.git.luto@kernel.org> X-Mailer: git-send-email 2.7.4 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1632 Lines: 43 Linux will have all kinds of sporadic problems on systems that don't have the CPUID instruction unless CONFIG_M486=y. In particular, sync_core() will explode. I believe that these kernels had a better chance of working before commit 05fb3c199bb0 ("x86/boot: Initialize FPU and X86_FEATURE_ALWAYS even if we don't have CPUID"). That commit inadvertently fixed a serious bug: we used to fail to detect the FPU if CPUID wasn't present. Because we also used to forget to set X86_FEATURE_ALWAYS, we end up with no cpu feature bits set at all. This meant that alternative patching didn't do anything and, if paravirt was disabled, we could plausibly finish the entire boot process without calling sync_core(). Rather than trying to work around these issues, just have the kernel fail loudly if it's running on a CPUID-less 486, doesn't have CPUID, and doesn't have CONFIG_M486 set. Reported-by: Matthew Whitehead Signed-off-by: Andy Lutomirski --- arch/x86/boot/cpu.c | 6 ++++++ 1 file changed, 6 insertions(+) diff --git a/arch/x86/boot/cpu.c b/arch/x86/boot/cpu.c index 26240dde081e..4224ede43b4e 100644 --- a/arch/x86/boot/cpu.c +++ b/arch/x86/boot/cpu.c @@ -87,6 +87,12 @@ int validate_cpu(void) return -1; } + if (CONFIG_X86_MINIMUM_CPU_FAMILY <= 4 && !IS_ENABLED(CONFIG_M486) && + !has_eflag(X86_EFLAGS_ID)) { + printf("This kernel requires a CPU with the CPUID instruction. Build with CONFIG_M486=y to run on this CPU.\n"); + return -1; + } + if (err_flags) { puts("This kernel requires the following features " "not present on the CPU:\n"); -- 2.7.4