From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932413AbeDWTqv (ORCPT ); Mon, 23 Apr 2018 15:46:51 -0400 Received: from gateway36.websitewelcome.com ([192.185.187.5]:19969 "EHLO gateway36.websitewelcome.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932295AbeDWTqs (ORCPT ); Mon, 23 Apr 2018 15:46:48 -0400 X-Authority-Reason: nr=8 Subject: Re: [PATCH 01/11] media: tm6000: fix potential Spectre variant 1 To: Mauro Carvalho Chehab Cc: Dan Carpenter , linux-media@vger.kernel.org, linux-kernel@vger.kernel.org References: <3d4973141e218fb516422d3d831742d55aaa5c04.1524499368.git.gustavo@embeddedor.com> <20180423152455.363d285c@vento.lan> <3ab9c4c9-0656-a08e-740e-394e2e509ae9@embeddedor.com> <20180423161742.66f939ba@vento.lan> From: "Gustavo A. R. Silva" Message-ID: Date: Mon, 23 Apr 2018 14:22:52 -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: <20180423161742.66f939ba@vento.lan> 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: 189.145.48.65 X-Source-L: No X-Exim-ID: 1fAh3E-001QXi-R8 X-Source: X-Source-Args: X-Source-Dir: X-Source-Sender: ([192.168.1.71]) [189.145.48.65]:60388 X-Source-Auth: gustavo@embeddedor.com X-Email-Count: 24 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 04/23/2018 02:17 PM, Mauro Carvalho Chehab wrote: > Em Mon, 23 Apr 2018 14:11:02 -0500 > "Gustavo A. R. Silva" escreveu: > >> On 04/23/2018 01:24 PM, Mauro Carvalho Chehab wrote: >>> Em Mon, 23 Apr 2018 12:38:03 -0500 >>> "Gustavo A. R. Silva" escreveu: >>> >>>> f->index can be controlled by user-space, hence leading to a >>>> potential exploitation of the Spectre variant 1 vulnerability. >>>> >>>> Smatch warning: >>>> drivers/media/usb/tm6000/tm6000-video.c:879 vidioc_enum_fmt_vid_cap() warn: potential spectre issue 'format' >>>> >>>> Fix this by sanitizing f->index before using it to index >>>> array _format_ >>>> >>>> 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 >>>> >>>> Cc: stable@vger.kernel.org >>>> Reported-by: Dan Carpenter >>>> Signed-off-by: Gustavo A. R. Silva >>>> --- >>>> drivers/media/usb/tm6000/tm6000-video.c | 2 ++ >>>> 1 file changed, 2 insertions(+) >>>> >>>> diff --git a/drivers/media/usb/tm6000/tm6000-video.c b/drivers/media/usb/tm6000/tm6000-video.c >>>> index b2399d4..d701027 100644 >>>> --- a/drivers/media/usb/tm6000/tm6000-video.c >>>> +++ b/drivers/media/usb/tm6000/tm6000-video.c >>>> @@ -26,6 +26,7 @@ >>>> #include >>>> #include >>>> #include >>>> +#include >>>> >>>> #include "tm6000-regs.h" >>>> #include "tm6000.h" >>>> @@ -875,6 +876,7 @@ static int vidioc_enum_fmt_vid_cap(struct file *file, void *priv, >>>> if (f->index >= ARRAY_SIZE(format)) >>>> return -EINVAL; >>>> >>>> + f->index = array_index_nospec(f->index, ARRAY_SIZE(format)); >>> >>> Please enlighten me: how do you think this could be exploited? >>> >>> When an application calls VIDIOC_ENUM_FMT from a /dev/video0 device, >>> it will just enumerate a hardware functionality, with is constant >>> for a given hardware piece. >>> >>> The way it works is that userspace do something like: >>> >>> int ret = 0; >>> >>> for (i = 0; ret == 0; i++) { >>> ret = ioctl(VIDIOC_ENUM_FMT, ...); >>> } >>> >>> in order to read an entire const table. >>> >>> Usually, it doesn't require any special privilege to call this ioctl, >>> but, even if someone changes its permission to 0x400, a simple lsusb >>> output is enough to know what hardware model is there. A lsmod >>> or cat /proc/modules) also tells that the tm6000 module was loaded, >>> with is a very good hint that the tm6000 is there or was there in the >>> past. >>> >>> In the specific case of tm6000, all hardware supports exactly the >>> same formats, as this is usually defined per-driver. So, a quick look >>> at the driver is enough to know exactly what the ioctl would answer. >>> Also, the net is full of other resources that would allow anyone >>> to get the supported formats for a piece of hardware. >>> >>> Even assuming that the OS doesn't have lsusb, that /proc is not >>> mounted, that /dev/video0 require special permissions, that the >>> potential attacker doesn't have physical access to the equipment (in >>> order to see if an USB board is plugged), etc... What possible harm >>> he could do by identifying a hardware feature? >>> >>> Similar notes for the other patches to drivers/media in this >>> series: let's not just start adding bloatware where not needed. >>> >>> Please notice that I'm fine if you want to submit potential >>> Spectre variant 1 fixups, but if you're willing to do so, >>> please provide an explanation about the potential threat scenarios >>> that you're identifying at the code. >>> >>> Dan, >>> >>> It probably makes sense to have somewhere at smatch a place where >>> we could explicitly mark the false-positives, in order to avoid >>> use to receive patches that would just add an extra delay where >>> it is not needed. >>> >> I see I've missed some obvious things that you've pointed out here. I'll >> mark these warnings as False Positives and take your points into account >> for the analysis of the rest of the Spectre issues reported by Smatch. > > Thanks, I 'll mark this series as rejected at patchwork.linuxtv.org. > Please feel free to resubmit any patch if they represent a real > threat, adding a corresponding description about the threat scenario > at the body of the e-mail. > Yeah. I got it. >> Sorry for the noise and thanks for the feedback. > > Anytime. > Much appreciated. :) Thanks -- Gustavo