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=-0.8 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_PASS,URIBL_BLOCKED autolearn=ham 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 CAB34C4321E for ; Fri, 7 Sep 2018 19:51:35 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 91CE320844 for ; Fri, 7 Sep 2018 19:51:35 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 91CE320844 Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=linux.intel.com Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-kernel-owner@vger.kernel.org Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727328AbeIHAeA (ORCPT ); Fri, 7 Sep 2018 20:34:00 -0400 Received: from mga14.intel.com ([192.55.52.115]:31262 "EHLO mga14.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726187AbeIHAeA (ORCPT ); Fri, 7 Sep 2018 20:34:00 -0400 X-Amp-Result: SKIPPED(no attachment in message) X-Amp-File-Uploaded: False Received: from orsmga002.jf.intel.com ([10.7.209.21]) by fmsmga103.fm.intel.com with ESMTP/TLS/DHE-RSA-AES256-GCM-SHA384; 07 Sep 2018 12:51:33 -0700 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.53,343,1531810800"; d="scan'208";a="89881383" Received: from viggo.jf.intel.com (HELO localhost.localdomain) ([10.54.77.144]) by orsmga002.jf.intel.com with ESMTP; 07 Sep 2018 12:51:31 -0700 Subject: [RFC][PATCH 5/8] x86/mm: fix exception table comments To: linux-kernel@vger.kernel.org Cc: Dave Hansen , sean.j.christopherson@intel.com, peterz@infradead.org, tglx@linutronix.de, x86@kernel.org, luto@kernel.org From: Dave Hansen Date: Fri, 07 Sep 2018 12:49:00 -0700 References: <20180907194852.3C351B82@viggo.jf.intel.com> In-Reply-To: <20180907194852.3C351B82@viggo.jf.intel.com> Message-Id: <20180907194900.DF3B41C0@viggo.jf.intel.com> Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org From: Dave Hansen The comments here are wrong. They are too absolute about where faults can occur when running in the kernel. The comments are also a bit hard to match up with the code. Trim down the comments, and make them more precise. Also add a comment explaining why we are doing the bad_area_nosemaphore() path here. Signed-off-by: Dave Hansen Cc: Sean Christopherson Cc: "Peter Zijlstra (Intel)" Cc: Thomas Gleixner Cc: x86@kernel.org Cc: Andy Lutomirski --- b/arch/x86/mm/fault.c | 27 ++++++++++++++------------- 1 file changed, 14 insertions(+), 13 deletions(-) diff -puN arch/x86/mm/fault.c~pkeys-fault-warnings-03 arch/x86/mm/fault.c --- a/arch/x86/mm/fault.c~pkeys-fault-warnings-03 2018-09-07 11:21:47.696751898 -0700 +++ b/arch/x86/mm/fault.c 2018-09-07 11:21:47.700751898 -0700 @@ -1349,24 +1349,25 @@ void do_user_addr_space_fault(struct pt_ flags |= FAULT_FLAG_INSTRUCTION; /* - * When running in the kernel we expect faults to occur only to - * addresses in user space. All other faults represent errors in - * the kernel and should generate an OOPS. Unfortunately, in the - * case of an erroneous fault occurring in a code path which already - * holds mmap_sem we will deadlock attempting to validate the fault - * against the address space. Luckily the kernel only validly - * references user space from well defined areas of code, which are - * listed in the exceptions table. + * Kernel-mode access to the user address space should only occur + * inside well-defined areas of code listed in the exception + * tables. But, an erroneous kernel fault occurring outside one of + * those areas which also holds mmap_sem might deadlock attempting + * to validate the fault against the address space. * - * As the vast majority of faults will be valid we will only perform - * the source reference check when there is a possibility of a - * deadlock. Attempt to lock the address space, if we cannot we then - * validate the source. If this is invalid we can skip the address - * space check, thus avoiding the deadlock: + * Only do the expensive exception table search when we might be at + * risk of a deadlock: + * 1. We failed to acquire mmap_sem, and + * 2. The access was an explicit kernel-mode access + * (X86_PF_USER=0). */ if (unlikely(!down_read_trylock(&mm->mmap_sem))) { if (!(sw_error_code & X86_PF_USER) && !search_exception_tables(regs->ip)) { + /* + * Fault from code in kernel from + * which we do not expect faults. + */ bad_area_nosemaphore(regs, sw_error_code, address, NULL); return; } _