From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751909AbeERWL7 (ORCPT ); Fri, 18 May 2018 18:11:59 -0400 Received: from gateway21.websitewelcome.com ([192.185.45.133]:17866 "EHLO gateway21.websitewelcome.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750763AbeERWLy (ORCPT ); Fri, 18 May 2018 18:11:54 -0400 X-Authority-Reason: nr=8 Subject: Re: [PATCH] kernel: sys: fix potential Spectre v1 To: Dan Williams Cc: Thomas Gleixner , Andrew Morton , Linux Kernel Mailing List , Alexei Starovoitov , Peter Zijlstra References: <20180515030038.GA11822@embeddedor.com> <20180515150859.1bccbd8d4543848b30fea859@linux-foundation.org> <50481b83-4c03-f354-bd11-cef7aecdd85f@embeddedor.com> <3d2e5771-c2c9-6e45-3e85-21c0bc86876e@embeddedor.com> From: "Gustavo A. R. Silva" Message-ID: <9521b7db-0ff5-21db-f744-b818cd640783@embeddedor.com> Date: Fri, 18 May 2018 17:11:53 -0500 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.7.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7bit X-AntiAbuse: This header was added to track abuse, please include it with any abuse report X-AntiAbuse: Primary Hostname - gator4166.hostgator.com X-AntiAbuse: Original Domain - vger.kernel.org X-AntiAbuse: Originator/Caller UID/GID - [47 12] / [47 12] X-AntiAbuse: Sender Address Domain - embeddedor.com X-BWhitelist: no X-Source-IP: 187.162.252.93 X-Source-L: No X-Exim-ID: 1fJnbN-002Pz0-RC X-Source: X-Source-Args: X-Source-Dir: X-Source-Sender: 187-162-252-93.static.axtel.net ([192.168.15.106]) [187.162.252.93]:51246 X-Source-Auth: gustavo@embeddedor.com X-Email-Count: 10 X-Source-Cap: Z3V6aWRpbmU7Z3V6aWRpbmU7Z2F0b3I0MTY2Lmhvc3RnYXRvci5jb20= X-Local-Domain: yes Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 05/18/2018 05:08 PM, Dan Williams wrote: >> >> Oh I see now. Just to double check, then something like the following would >> be broken too, because is basically the same as the code above, and well, it >> doesn't make much sense to store the value returned by macro >> array_index_nospec into x, correct?: > > Correct, broken: > >> >> bool foo(int x) >> { >> if(x >= MAX) >> return false; > > Under speculation we may not return here when x is greater than max. > >> x = array_index_nospec(x, MAX); > > x is now sanitized under speculation to zero, but the compiler would > likely just throw this away because nothing consumes it. > >> return true; >> } >> >> int vulnerable(int x) >> { >> if(!foo(x)) >> return -1; > > cpu might speculate that this branch is not taken... > >> >> temp = array[x]; > > ...so x had better be bounded here, otherwise Spectre. > I got it. I appreciate the feedback. Thanks, Dan. -- Gustavo