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=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SIGNED_OFF_BY,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 251ACC43387 for ; Sun, 23 Dec 2018 03:59:11 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id E766C2171F for ; Sun, 23 Dec 2018 03:59:10 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2393117AbeLWD7J (ORCPT ); Sat, 22 Dec 2018 22:59:09 -0500 Received: from gateway34.websitewelcome.com ([192.185.150.114]:13959 "EHLO gateway34.websitewelcome.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1731793AbeLWD7J (ORCPT ); Sat, 22 Dec 2018 22:59:09 -0500 Received: from cm11.websitewelcome.com (cm11.websitewelcome.com [100.42.49.5]) by gateway34.websitewelcome.com (Postfix) with ESMTP id ED1C5197509 for ; Sat, 22 Dec 2018 21:37:07 -0600 (CST) Received: from gator4166.hostgator.com ([108.167.133.22]) by cmsmtp with SMTP id auZfg4iF3dnCeauZfgDvvn; Sat, 22 Dec 2018 21:37:07 -0600 X-Authority-Reason: nr=8 Received: from [189.250.106.44] (port=48690 helo=[192.168.1.76]) by gator4166.hostgator.com with esmtpsa (TLSv1.2:ECDHE-RSA-AES128-GCM-SHA256:128) (Exim 4.91) (envelope-from ) id 1gauZd-002kYV-AF; Sat, 22 Dec 2018 21:37:07 -0600 Subject: Re: [PATCH] net: core: Fix Spectre v1 vulnerability To: Alexei Starovoitov Cc: David Miller , ast@kernel.org, daniel@iogearbox.net, netdev@vger.kernel.org, linux-kernel@vger.kernel.org References: <20181221204901.GA30045@embeddedor> <20181222.150722.1493687829239836271.davem@davemloft.net> <20181222235952.keue7a336sg7jfim@ast-mbp.dhcp.thefacebook.com> <20181222.184051.718127928973898182.davem@davemloft.net> <20181223030039.wrpytx7pwfcljdjm@ast-mbp.dhcp.thefacebook.com> From: "Gustavo A. R. Silva" Message-ID: <37df17ba-7fcf-ab04-fe9a-d2a6fc5b6b9c@embeddedor.com> Date: Sat, 22 Dec 2018 21:37:02 -0600 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.2.1 MIME-Version: 1.0 In-Reply-To: <20181223030039.wrpytx7pwfcljdjm@ast-mbp.dhcp.thefacebook.com> 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: 189.250.106.44 X-Source-L: No X-Exim-ID: 1gauZd-002kYV-AF X-Source: X-Source-Args: X-Source-Dir: X-Source-Sender: ([192.168.1.76]) [189.250.106.44]:48690 X-Source-Auth: gustavo@embeddedor.com X-Email-Count: 11 X-Source-Cap: Z3V6aWRpbmU7Z3V6aWRpbmU7Z2F0b3I0MTY2Lmhvc3RnYXRvci5jb20= X-Local-Domain: yes Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 12/22/18 9:00 PM, Alexei Starovoitov wrote: > On Sat, Dec 22, 2018 at 08:53:40PM -0600, Gustavo A. R. Silva wrote: >> Hi, >> >> On 12/22/18 8:40 PM, David Miller wrote: >>> From: Alexei Starovoitov >>> Date: Sat, 22 Dec 2018 15:59:54 -0800 >>> >>>> On Sat, Dec 22, 2018 at 03:07:22PM -0800, David Miller wrote: >>>>> From: "Gustavo A. R. Silva" >>>>> Date: Fri, 21 Dec 2018 14:49:01 -0600 >>>>> >>>>>> flen is indirectly controlled by user-space, hence leading to >>>>>> a potential exploitation of the Spectre variant 1 vulnerability. >>>>>> >>>>>> This issue was detected with the help of Smatch: >>>>>> >>>>>> net/core/filter.c:1101 bpf_check_classic() warn: potential spectre issue 'filter' [w] >>>>>> >>>>>> Fix this by sanitizing flen before using it to index filter at line 1101: >>>>>> >>>>>> switch (filter[flen - 1].code) { >>>>>> >>>>>> and through pc at line 1040: >>>>>> >>>>>> const struct sock_filter *ftest = &filter[pc]; >>>>>> >>>>>> Notice that given that speculation windows are large, the policy is >>>>>> to kill the speculation on the first load and not worry if it can be >>>>>> completed with a dependent load/store [1]. >>>>>> >>>>>> [1] https://marc.info/?l=linux-kernel&m=152449131114778&w=2 >>>>>> >>>>>> Signed-off-by: Gustavo A. R. Silva >>>>> >>>>> BPF folks, I'll take this directly. >>>>> >>>>> Applied and queued up for -stable, thanks. >>>> >>>> hmm. what was the rush? >>>> I think it is unnecessary change. >>>> Though fp is passed initially from user space >>>> it's copied into kernel struct first. >>>> There is no way user space can force kernel to mispredict >>>> branch in for (pc = 0; pc < flen; pc++) loop. >> The following piece of code is the one that can be mispredicted, not the for >> loop: >> >> 1013 if (flen == 0 || flen > BPF_MAXINSNS) >> 1014 return false; >> >> Instead of calling array_index_nospec() inside bpf_check_basics_ok(), I >> decided to place the call close to the code that could be compromised. This >> is when accessing filter[]. > > Why do you think it can be mispredicted? > Beause fprog->len comes from user space: bpf_prog_create_from_user() -> bpf_check_basics_ok() > I've looked at your other patch for nfc_sock_create() where you're adding: > + proto = array_index_nospec(proto, NFC_SOCKPROTO_MAX); > > 'proto' is the value passed in _register_ into system call. > There is no need to wrap it with array_index_nospec(). > It's not a load from memory and user space cannot make it slow. > Slow load is a necessary attribute to trigger speculative execution > into mispredicted branch. > We might be interpreting the information available about Spectre a bit different, but when the Spectre paper talks about memory vs cpu speed it seems to me that it's just an example to illustrate how the microcode can come into the equation and speculate. So I'm genuinely curious about your last statement: "Slow load is a necessary attribute..." Where did you get that info from? Can't we have the case in which the code can be "trained" to read perfectly valid values for prog->len for quite a while, making the microcode come into place and speculate about: 1013 if (flen == 0 || flen > BPF_MAXINSNS) 1014 return false; and then make flen to be greater than BPF_MAXINSNS? > What tool did you use to analyze this? > Did you analyze all warnings case by case or just trusted the tool > and generated these patches? > I read every case, but I sometimes might be wrong of course. Thanks -- Gustavo From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Gustavo A. R. Silva" Subject: Re: [PATCH] net: core: Fix Spectre v1 vulnerability Date: Sat, 22 Dec 2018 21:37:02 -0600 Message-ID: <37df17ba-7fcf-ab04-fe9a-d2a6fc5b6b9c@embeddedor.com> References: <20181221204901.GA30045@embeddedor> <20181222.150722.1493687829239836271.davem@davemloft.net> <20181222235952.keue7a336sg7jfim@ast-mbp.dhcp.thefacebook.com> <20181222.184051.718127928973898182.davem@davemloft.net> <20181223030039.wrpytx7pwfcljdjm@ast-mbp.dhcp.thefacebook.com> Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 7bit Cc: David Miller , ast@kernel.org, daniel@iogearbox.net, netdev@vger.kernel.org, linux-kernel@vger.kernel.org To: Alexei Starovoitov Return-path: Received: from gateway20.websitewelcome.com ([192.185.60.19]:13521 "EHLO gateway20.websitewelcome.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S2389248AbeLWD5m (ORCPT ); Sat, 22 Dec 2018 22:57:42 -0500 Received: from cm10.websitewelcome.com (cm10.websitewelcome.com [100.42.49.4]) by gateway20.websitewelcome.com (Postfix) with ESMTP id F37F6400C4CFD for ; Sat, 22 Dec 2018 21:37:07 -0600 (CST) In-Reply-To: <20181223030039.wrpytx7pwfcljdjm@ast-mbp.dhcp.thefacebook.com> Content-Language: en-US Sender: netdev-owner@vger.kernel.org List-ID: On 12/22/18 9:00 PM, Alexei Starovoitov wrote: > On Sat, Dec 22, 2018 at 08:53:40PM -0600, Gustavo A. R. Silva wrote: >> Hi, >> >> On 12/22/18 8:40 PM, David Miller wrote: >>> From: Alexei Starovoitov >>> Date: Sat, 22 Dec 2018 15:59:54 -0800 >>> >>>> On Sat, Dec 22, 2018 at 03:07:22PM -0800, David Miller wrote: >>>>> From: "Gustavo A. R. Silva" >>>>> Date: Fri, 21 Dec 2018 14:49:01 -0600 >>>>> >>>>>> flen is indirectly controlled by user-space, hence leading to >>>>>> a potential exploitation of the Spectre variant 1 vulnerability. >>>>>> >>>>>> This issue was detected with the help of Smatch: >>>>>> >>>>>> net/core/filter.c:1101 bpf_check_classic() warn: potential spectre issue 'filter' [w] >>>>>> >>>>>> Fix this by sanitizing flen before using it to index filter at line 1101: >>>>>> >>>>>> switch (filter[flen - 1].code) { >>>>>> >>>>>> and through pc at line 1040: >>>>>> >>>>>> const struct sock_filter *ftest = &filter[pc]; >>>>>> >>>>>> Notice that given that speculation windows are large, the policy is >>>>>> to kill the speculation on the first load and not worry if it can be >>>>>> completed with a dependent load/store [1]. >>>>>> >>>>>> [1] https://marc.info/?l=linux-kernel&m=152449131114778&w=2 >>>>>> >>>>>> Signed-off-by: Gustavo A. R. Silva >>>>> >>>>> BPF folks, I'll take this directly. >>>>> >>>>> Applied and queued up for -stable, thanks. >>>> >>>> hmm. what was the rush? >>>> I think it is unnecessary change. >>>> Though fp is passed initially from user space >>>> it's copied into kernel struct first. >>>> There is no way user space can force kernel to mispredict >>>> branch in for (pc = 0; pc < flen; pc++) loop. >> The following piece of code is the one that can be mispredicted, not the for >> loop: >> >> 1013 if (flen == 0 || flen > BPF_MAXINSNS) >> 1014 return false; >> >> Instead of calling array_index_nospec() inside bpf_check_basics_ok(), I >> decided to place the call close to the code that could be compromised. This >> is when accessing filter[]. > > Why do you think it can be mispredicted? > Beause fprog->len comes from user space: bpf_prog_create_from_user() -> bpf_check_basics_ok() > I've looked at your other patch for nfc_sock_create() where you're adding: > + proto = array_index_nospec(proto, NFC_SOCKPROTO_MAX); > > 'proto' is the value passed in _register_ into system call. > There is no need to wrap it with array_index_nospec(). > It's not a load from memory and user space cannot make it slow. > Slow load is a necessary attribute to trigger speculative execution > into mispredicted branch. > We might be interpreting the information available about Spectre a bit different, but when the Spectre paper talks about memory vs cpu speed it seems to me that it's just an example to illustrate how the microcode can come into the equation and speculate. So I'm genuinely curious about your last statement: "Slow load is a necessary attribute..." Where did you get that info from? Can't we have the case in which the code can be "trained" to read perfectly valid values for prog->len for quite a while, making the microcode come into place and speculate about: 1013 if (flen == 0 || flen > BPF_MAXINSNS) 1014 return false; and then make flen to be greater than BPF_MAXINSNS? > What tool did you use to analyze this? > Did you analyze all warnings case by case or just trusted the tool > and generated these patches? > I read every case, but I sometimes might be wrong of course. Thanks