From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755182AbdBVTw4 (ORCPT ); Wed, 22 Feb 2017 14:52:56 -0500 Received: from galahad.ideasonboard.com ([185.26.127.97]:39074 "EHLO galahad.ideasonboard.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755145AbdBVTwk (ORCPT ); Wed, 22 Feb 2017 14:52:40 -0500 From: Laurent Pinchart To: Sodagudi Prasad Cc: James Morse , mchehab@s-opensource.com, linux-media@vger.kernel.org, shijie.huang@arm.com, catalin.marinas@arm.com, will.deacon@arm.com, mark.rutland@arm.com, akpm@linux-foundation.org, sandeepa.s.prabhu@gmail.com, linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, hans.verkuil@cisco.com, sakari.ailus@linux.intel.com, tiffany.lin@mediatek.com, nick@shmanahar.org, shuah@kernel.org, ricardo.ribalda@gmail.com Subject: Re: Looking more details and reasons for using orig_add_limit. Date: Wed, 22 Feb 2017 21:53:08 +0200 Message-ID: <2944633.ljab0sy3Dg@avalon> User-Agent: KMail/4.14.10 (Linux/4.9.6-gentoo-r1; KDE/4.14.28; x86_64; ; ) In-Reply-To: <568205ddc2e7af6a57a71b8c5cd47d68@codeaurora.org> References: <58A58162.2020101@arm.com> <568205ddc2e7af6a57a71b8c5cd47d68@codeaurora.org> MIME-Version: 1.0 Content-Type: text/plain; charset="utf-8" Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Transfer-Encoding: 8bit X-MIME-Autoconverted: from quoted-printable to 8bit by mail.home.local id v1MJrG5P014027 Hi Prasad, On Tuesday 21 Feb 2017 06:20:58 Sodagudi Prasad wrote: > Hi mchehab/linux-media, > > It is not clear why KERNEL_DS was set explicitly here. In this path > video_usercopy() gets called and it > copies the “struct v4l2_buffer” struct to user space stack memory. > > Can you please share reasons for setting to KERNEL_DS here? It's a bit of historical hack. To implement compat ioctl handling, we copy the ioctl 32-bit argument from userspace, turn it into a native 64-bit ioctl argument, and call the native ioctl code. That code expects the argument to be stored in userspace memory and uses get_user() and put_user() to access it. As the 64-bit argument now lives in kernel memory, my understanding is that we fake things up with KERNEL_DS. The ioctl code should be refactored to get rid of this hack. > static long do_video_ioctl(struct file *file, unsigned int cmd, unsigned > long arg) > { > … > … > > if (compatible_arg) > err = native_ioctl(file, cmd, (unsigned long)up); > else { > mm_segment_t old_fs = get_fs(); > > set_fs(KERNEL_DS); > err = native_ioctl(file, cmd, (unsigned long)&karg); > set_fs(old_fs); > } > … > } > > On 2017-02-16 02:39, James Morse wrote: > > Hi Prasad, > > > > On 15/02/17 21:12, Sodagudi Prasad wrote: > >> On 2017-02-15 04:09, James Morse wrote: > >>> On 15/02/17 05:52, Sodagudi Prasad wrote: > >>>> that driver is calling set_fs(KERNEL_DS) and then copy_to_user() to > >>>> user space > >>>> memory. > >>> > >>> Don't do this, its exactly the case PAN+UAO and the code you pointed > >>> to are > >>> designed to catch. Accessing userspace needs doing carefully, setting > >>> USER_DS > >>> and using the put_user()/copy_to_user() accessors are the required > >>> steps. > >>> > >>> Which driver is doing this? Is it in mainline? > >> > >> Yes. It is mainline driver - > >> drivers/media/v4l2-core/v4l2-compat-ioctl32.c > >> > >> In some v4l2 use-case kernel panic is observed. Below part > >> of the code has set_fs to KERNEL_DS before calling native_ioctl(). > >> > >> static long do_video_ioctl(struct file *file, unsigned int cmd, > >> unsigned long arg) > >> { > >> … > >> … > >> > >> if (compatible_arg) > >> > >> err = native_ioctl(file, cmd, (unsigned long)up); > >> > >> else { > >> > >> mm_segment_t old_fs = get_fs(); > >> > >> set_fs(KERNEL_DS); ====> KERNEL_DS. > >> err = native_ioctl(file, cmd, (unsigned long)&karg); > >> set_fs(old_fs); > >> > >> } > >> > >> Here is the call stack which is resulting crash, because user space > >> memory has > >> read only permissions. > >> [27249.920041] [] __arch_copy_to_user+0x110/0x180 > >> [27249.920047] [] video_ioctl2+0x38/0x44 > >> [27249.920054] [] v4l2_ioctl+0x78/0xb4 > >> [27249.920059] [] do_video_ioctl+0x91c/0x1160 > >> [27249.920064] [] v4l2_compat_ioctl32+0x60/0xcc > >> [27249.920071] [] compat_SyS_ioctl+0x124/0xd88 > >> [27249.920077] [] el0_svc_naked+0x24/0x2 > > > > It's not totally clear to me what is going on here, but some > > observations: > > the ioctl is trying to copy_to_user() to some read-only memory. This > > would > > normally fail gracefully with -EFAULT, but because KERNEL_DS has been > > set, the > > kernel checks this before calling the fault handler and calls die() on > > your ioctl(). > > > > The ioctl code is doing this deliberately as a compat mechanism, but > > the code > > behind file->f_op->unlocked_ioctl() expects fs==USER_DS when it does > > its work. > > That code needs to be made aware of this compat translation, or a > > compat_ioctl > > call provided. > > > > > > Which v4l driver is this? Which ioctl is being called? Does the driver > > using the > > v4l framework have a compat_ioctl() call? > > Yes. Same kernel crash is seen with both video and camera use cases. > Yes. Driver have compact_ioctl(). > > > What path does this call take through v4l2_compat_ioctl32()? It looks > > like > > compat_ioctl will be skipped in certain cases, v4l2_compat_ioctl32() > > > > has: > >> if (_IOC_TYPE(cmd) == 'V' && _IOC_NR(cmd) < BASE_VIDIOC_PRIVATE) > >> > >> ret = do_video_ioctl(file, cmd, arg); > >> > >> else if (vdev->fops->compat_ioctl32) > >> > >> ret = vdev->fops->compat_ioctl32(file, cmd, arg); > > > > Is your ioctl matched by that top if()? > > Yes. Top if condition in true and do_video_ioctl() getting called. > > >>>> If there is permission fault for user space address the above > >>>> condition > >>>> is leading to kernel crash. Because orig_add_limit is having > >>>> KERNEL_DS as set_fs > >>>> called before copy_to_user(). > >>>> > >>>> 1) So I would like to understand that, is that user space > >>>> pointer leading to > >>>> permission fault not correct(condition_1) in this scenario? > >>> > >>> The correct thing has happened here. To access user space > >>> set_fs(USER_DS) first. > >>> (and set it back to whatever it was afterwards). > >> > >> So, Any clean up needed to above call path similar to what was done in > >> the below > >> commit? > >> commit a7f61e89af73e9bf760826b20dba4e637221fcb9 - compat_ioctl: don't > >> call > >> do_ioctl under set_fs(KERNEL_DS) > > > > That's clever. Is that code doing a conversion, or do you have a > > compat_ioctl() > > in your driver? > > > > It's possible that fs/compat_ioctl.c has done this work, but > > do_video_ioctl() > > un-does it. Someone who knows about v4l and compat-ioctls should take a > > look... > > > > This looks like a case of: > >> The accidental invocation of an unlocked_ioctl handler that > >> unexpectedly > >> calls copy_to_user could be a severe security issue. > > > > that Jann describes in the commit message. Fixing the code behind > > file->f_op->unlocked_ioctl() to consider compat calls from > > do_video_ioctl() is > > one way to solve this. > > > > > > > > Thanks, > > > > James > > -Thanks, Prasad -- Regards, Laurent Pinchart