From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-3.9 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI, SPF_HELO_NONE,SPF_PASS autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 9D146C43461 for ; Thu, 17 Sep 2020 18:07:57 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 1334321D7F for ; Thu, 17 Sep 2020 18:07:57 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=alien8.de header.i=@alien8.de header.b="KIsGzNW5" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726522AbgIQRbw (ORCPT ); Thu, 17 Sep 2020 13:31:52 -0400 Received: from mail.skyhub.de ([5.9.137.197]:45372 "EHLO mail.skyhub.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726610AbgIQR31 (ORCPT ); Thu, 17 Sep 2020 13:29:27 -0400 Received: from zn.tnic (p200300ec2f105300d80be81ad57dea02.dip0.t-ipconnect.de [IPv6:2003:ec:2f10:5300:d80b:e81a:d57d:ea02]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.skyhub.de (SuperMail on ZX Spectrum 128k) with ESMTPSA id B07011EC03D2; Thu, 17 Sep 2020 19:28:42 +0200 (CEST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=alien8.de; s=dkim; t=1600363722; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:in-reply-to:in-reply-to: references:references; bh=DPFZ7R7KMR4GIHVlYtjgerY8lFRwnbkOZgmyXUzJz/8=; b=KIsGzNW5GDbEH/cPxtlMc/bbz/A0UiKdMoc5YwFnhJI8eSiLJbzS/+EpkniTK4FgMaldZ8 LTzJfcs/cJoJgSBmRAH8snOjI13ouHxBeNkoy2TuOLT6oVqvz1waBb2Y9qZON32RJljipD 9VQWx1qx8jeO/V97FNpt9POn51GevXg= Date: Thu, 17 Sep 2020 19:28:40 +0200 From: Borislav Petkov To: Arvind Sankar Cc: x86@kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH] x86/cmdline: Disable instrumentation of cmdline unconditionally Message-ID: <20200917172840.GP31960@zn.tnic> References: <20200906154637.1618112-1-nivedita@alum.mit.edu> <20200917094055.GF31960@zn.tnic> <20200917160548.GA1877352@rani.riverdale.lan> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <20200917160548.GA1877352@rani.riverdale.lan> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Sep 17, 2020 at 12:05:48PM -0400, Arvind Sankar wrote: > I did have a crash and this patch fixed it, but it seems it was on a > branch where I was making changes to cmdline.c, which triggered clang to > use a jump table for cmdline_find_option_bool(). That was the cause of > the crash, and the reason this patch fixed it was because it enabled > -fno-jump-tables, rather than because it disabled instrumentation. I see. > The instrumentation code does write data to random addresses, but > apparently that doesn't necessarily crash the system. This patch would > also be insufficient to fix it, since load_ucode_bsp() itself can have > instrumentation code in it. Yeah, it better not have any. > Eg with GCOV_PROFILE_ALL enabled, the start of the function is: > > c2a7706a : > c2a7706a: 55 push %ebp > c2a7706b: 83 05 c0 4d ba c2 01 addl $0x1,0xc2ba4dc0 > c2a77072: 83 15 c4 4d ba c2 00 adcl $0x0,0xc2ba4dc4 > c2a77079: 89 e5 mov %esp,%ebp > > but when it's called from arch/x86/kernel/head_32.S, paging is disabled > and the code is executing out of physical addresses, so it's going to > read/write data from garbage addresses. Right, so I'm sceptical to all this instrumentation nonsense - it is fine and good if you have it but not everywhere. And certainly not when the thing is loading microcode. microcode/core.c contains all the code, early and late so it might make some little sense to have instrumentation there for whatever reasons, say KASANing (yah, I made a verb :)) the thing for leaks or whatnot, but meh, I can't find it in me to care. So at some point, if it starts getting in the way, we might remove all instrumentation from that thing altogether. > Anyway, please ignore this patch and sorry for the noise. No worries. Thx. -- Regards/Gruss, Boris. https://people.kernel.org/tglx/notes-about-netiquette