From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752159AbeBDTAB (ORCPT ); Sun, 4 Feb 2018 14:00:01 -0500 Received: from aserp2130.oracle.com ([141.146.126.79]:42200 "EHLO aserp2130.oracle.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751577AbeBDS7w (ORCPT ); Sun, 4 Feb 2018 13:59:52 -0500 Subject: Re: [PATCH] xen: hypercall: fix out-of-bounds memcpy To: Arnd Bergmann Cc: Juergen Gross , Nicolas Pitre , Andi Kleen , Dan Carpenter , Jan Beulich , xen-devel , Linux Kernel Mailing List References: <20180202153240.1190361-1-arnd@arndb.de> <1e3424f4-771b-51ad-3630-0faf47e388e0@oracle.com> From: Boris Ostrovsky Message-ID: <87946d71-5c94-1045-2e96-347423820d5c@oracle.com> Date: Sun, 4 Feb 2018 13:55:25 -0500 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.6.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 8bit Content-Language: en-US X-Proofpoint-Virus-Version: vendor=nai engine=5900 definitions=8795 signatures=668662 X-Proofpoint-Spam-Details: rule=notspam policy=default score=0 suspectscore=0 malwarescore=0 phishscore=0 bulkscore=0 spamscore=0 mlxscore=0 mlxlogscore=999 adultscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.0.1-1711220000 definitions=main-1802040253 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 02/04/2018 10:35 AM, Arnd Bergmann wrote: > On Sat, Feb 3, 2018 at 6:08 PM, Boris Ostrovsky > wrote: >> >> On 02/03/2018 10:12 AM, Arnd Bergmann wrote: >>> On Sat, Feb 3, 2018 at 12:33 AM, Boris Ostrovsky >>> wrote: >>>> On 02/02/2018 10:32 AM, Arnd Bergmann wrote: >>>>> The legacy hypercall handlers were originally added with >>>>> a comment explaining that "copying the argument structures in >>>>> HYPERVISOR_event_channel_op() and HYPERVISOR_physdev_op() into the local >>>>> variable is sufficiently safe" and only made sure to not write >>>>> past the end of the argument structure, the checks in linux/string.h >>>>> disagree with that, when link-time optimizations are used: >>>>> >>>>> In function 'memcpy', >>>>> inlined from 'pirq_query_unmask' at drivers/xen/fallback.c:53:2, >>>>> inlined from '__startup_pirq' at >>>>> drivers/xen/events/events_base.c:529:2, >>>>> inlined from 'restore_pirqs' at >>>>> drivers/xen/events/events_base.c:1439:3, >>>>> inlined from 'xen_irq_resume' at >>>>> drivers/xen/events/events_base.c:1581:2: >>>>> include/linux/string.h:350:3: error: call to '__read_overflow2' declared >>>>> with attribute error: detected read beyond size of object passed as 2nd >>>>> parameter >>>>> __read_overflow2(); >>>>> ^ >>>>> make[3]: *** [ccLujFNx.ltrans15.ltrans.o] Error 1 >>>>> make[3]: Target 'all' not remade because of errors. >>>>> lto-wrapper: fatal error: make returned 2 exit status >>>>> compilation terminated. >>>>> ld: error: lto-wrapper failed >>>>> >>>>> This changes the functions so that each argument is accessed with >>>>> exactly the correct length based on the command code. >>>>> >>>>> Fixes: cf47a83fb06e ("xen/hypercall: fix hypercall fallback code for >>>>> very old hypervisors") >>>>> Signed-off-by: Arnd Bergmann >>>>> --- >>>>> drivers/xen/fallback.c | 94 >>>>> ++++++++++++++++++++++++++++---------------------- >>>>> 1 file changed, 53 insertions(+), 41 deletions(-) >>>>> >>>>> default: >>>>> - WARN_ON(rc != -ENOSYS); >>>>> - break; >>>>> + return -ENOSYS; >>>>> } >>>>> >>>>> + memcpy(&op.u, arg, len); >>>>> + rc = _hypercall1(int, event_channel_op_compat, &op); >>>>> + memcpy(arg, &op.u, len); >>>> >>>> >>>> We don't copy back for all commands, only those that are COPY_BACK. >>> >>> Not sure what you mean. Is it harmful to copy back the data for the others >>> in any way? Otherwise I wouldn't micro-optimize this. >> >> >> I should have checked the original commit for fallback.c --- the code that >> it replaced was doing copybacks for all hypercalls and selective copybacks >> is an optimization introduced in that commit. > It was not an optimization but a correctness fix to avoid overflowing > the caller stack on the copy-back operation. What I tried to explain > in my commit message is that the same fix is also needed on > the copy-out before it. It's only a read access beyond the end > of a local variable, but not both the static checks and kasan-stack > get alarmed about it. > Yes, I understand that. I was referring to:     Move the fallback code into out-of-line functions, and handle all of     the operations known by this old a hypervisor individually: *Some don't     require copying back anything at all*, and for the rest use the     individual argument structures' sizes rather than the container's in the original commit. I.e. prior to that commit we *were* copying back for all commands (although possibly with potentially incorrect size). So not copying back for some commands was an optimization. In any case, Reviewed-by: Boris Ostrovsky