On Fri, Nov 9, 2018 at 1:17 PM, Dmitry Vyukov <dvyukov@xxxxxxxxxx> wrote: > On Fri, Nov 9, 2018 at 6:16 AM, Hans Verkuil <hverkuil@xxxxxxxxx> wrote: >> Hi Dmitry, >> >> On 11/02/18 18:15, Dmitry Vyukov wrote: >>> On Wed, Oct 31, 2018 at 10:46 AM, Hans Verkuil <hverkuil@xxxxxxxxx> wrote: >>>> On 10/30/2018 03:02 PM, Dmitry Vyukov wrote: >>>>> Hello Helen and linux-media, >>>>> >>>>> I've attended your talk "Shifting Media App Development into High >>>>> Gear" on OSS Summit last week and approached you with some questions >>>>> if/how this can be used for kernel testing. Thanks, turn out to be a >>>>> very useful talk! >>>>> >>>>> I am working on syzkaller/syzbot, continuous kernel fuzzing system: >>>>> https://github.com/google/syzkaller >>>>> https://github.com/google/syzkaller/blob/master/docs/syzbot.md >>>>> https://syzkaller.appspot.com >>>>> >>>>> After simply enabling CONFIG_VIDEO_VIMC, CONFIG_VIDEO_VIM2M, >>>>> CONFIG_VIDEO_VIVID, CONFIG_VIDEO_VICODEC syzbot has found 8 bugs in >>>>> media subsystem in just 24 hours: >>>>> >>>>> KASAN: use-after-free Read in vb2_mmap >>>>> https://groups.google.com/forum/#!msg/syzkaller-bugs/XGGH69jMWQ0/S8vfxgEmCgAJ >>>>> >>>>> KASAN: use-after-free Write in __vb2_cleanup_fileio >>>>> https://groups.google.com/forum/#!msg/syzkaller-bugs/qKKhsZVPo3o/P6AB2of2CQAJ >>>>> >>>>> WARNING in __vb2_queue_cancel >>>>> https://groups.google.com/forum/#!msg/syzkaller-bugs/S29GU_NtfPY/ZvAz8UDtCQAJ >>>>> >>>>> divide error in vivid_vid_cap_s_dv_timings >>>>> https://groups.google.com/forum/#!msg/syzkaller-bugs/GwF5zGBCfyg/wnuWmW_sCQAJ >>>> >>>> Should be fixed by https://patchwork.linuxtv.org/patch/52641/ >>>> >>>>> >>>>> KASAN: use-after-free Read in wake_up_if_idle >>>>> https://groups.google.com/forum/#!msg/syzkaller-bugs/aBWb_yV1kiI/sWQO63fkCQAJ >>>>> >>>>> KASAN: use-after-free Read in __vb2_perform_fileio >>>>> https://groups.google.com/forum/#!msg/syzkaller-bugs/MdFCZHz0LUQ/qSK_bFbcCQAJ >>>>> >>>>> INFO: task hung in vivid_stop_generating_vid_cap >>>>> https://groups.google.com/forum/#!msg/syzkaller-bugs/F_KFW6PVyTA/wTBeHLfTCQAJ >>>>> >>>>> KASAN: null-ptr-deref Write in kthread_stop >>>>> https://groups.google.com/forum/#!msg/syzkaller-bugs/u0AGnYvSlf4/fUiyfA_TCQAJ >>>> >>>> These last two should be fixed by https://patchwork.linuxtv.org/patch/52640/ >>> >>> This is great! >>> This last one happens super frequently, so harms ability to find other bugs. >>> >>>> Haven't figured out the others yet (hope to get back to that next week). >>> >>> But note that syzbot added few more too! :) >> >> As of now (Fri Nov 9 14:47:31 CET 2018) I fixed all media-related syzbot >> issues with the exception of https://syzkaller.appspot.com/bug?extid=f9966a25169b6d66d61f > > This is great! > I will look into that one, or maybe syzbot will come up with a repro > later. But since it happened only 2 times, it's probably either a > subtle race, or requires interaction between several test processes > (we generally try to keep them isolated, but since this is a global > device collisions are possible). > >> There is no reproducer for that one and I couldn't figure out how it could >> happen. >> >> I've posted patches for all issues except for two that deal with reproducers >> using dup2(). I need to think about my fixes a bit more. dup2() is very nasty :-) >> >> What would be a good improvement is if you add this to the kernel command options: >> "vivid.n_devs=2 vivid.multiplanar=1,2" >> >> This will create two vivid instances, one using the single planar API and one using >> the multiplanar API. That will improve the test coverage. > > Re this and collisions between multiple test processes. We actually > would like to have moar devices and partition them between test > processes. Say if we need need devices for 8 test processes, will it > work to specify something like "vivid.n_devs=16 > vivid.multiplanar=1,2,1,2,1,2,1,2,1,2,1,2,1,2,1,2" and then use > devices 0/1 in the first test process, 2/3 in the second and so on? > > Without giving any flags, I see 8 /dev/video* devices, does > vivid.n_devs defaults to 8? I am a bit lost. vivid.n_devs=16 vivid.multiplanar=1,2,1,2,1,2,1,2,1,2,1,2,1,2,1,2 creates 32 /dev/video* devices. but vivid.n_devs=8 vivid.multiplanar=1,2,1,2,1,2,1,2 creates 24 /dev/video* devices. These parameters also affect /dev/{vbi,radio,swradio} in strange ways Also, by default there is /dev/radio0 and /dev/radio1, are these different types of devices, e.g. "source" and "sink"? Or they are identical? And the same question for other types of devices? How can I create 8 independent partitions of devices? What devices will belong to each partition? >> I also noticed that you appear to test only video devices. But vivid also creates >> vbi, radio and swradio devices. It would be nice to have those tested as well. > > Will do. > FTR, this is these devices: > > # ls -l /dev/{vbi,radio,swradio}* > crw-rw---- 1 root video 81, 14 Nov 9 21:07 /dev/radio0 > crw-rw---- 1 root video 81, 15 Nov 9 21:07 /dev/radio1 > crw-rw---- 1 root video 81, 13 Nov 9 21:07 /dev/swradio0 > crw-rw---- 1 root video 81, 11 Nov 9 21:07 /dev/vbi0 > crw-rw---- 1 root video 81, 12 Nov 9 21:07 /dev/vbi1 > > Why are there 2 radio and vbi? Are they different? Is it possible to > also create more of them? Are there any other useful command line args > for them? > > >>>>> Based on this I think if we put more effort into media fuzzing, it >>>>> will be able to find dozens more. >>>> >>>> Yeah, this is good stuff. Thank you for setting this up. >>>> >>>>> >>>>> syzkaller needs descriptions of kernel interfaces to efficiently cover >>>>> a subsystem. For example, see: >>>>> https://github.com/google/syzkaller/blob/master/sys/linux/uinput.txt >>>>> Hopefully you can read it without much explanation, it basically >>>>> states that there is that node in /dev and here are ioctls and other >>>>> syscalls that are relevant for this device and here are types of >>>>> arguments and layout of involved data structures. >>>>> >>>>> Turned we actually have such descriptions for /dev/video* and /dev/v4l-subdev*: >>>>> https://github.com/google/syzkaller/blob/master/sys/linux/video4linux.txt >>>>> But we don't have anything for /dev/media*, fuzzer merely knows that >>>>> it can open the device: >>>>> https://github.com/google/syzkaller/blob/12b38f22c18c6109a5cc1c0238d015eef121b9b7/sys/linux/sys.txt#L479 >>>>> and then it will just blindly execute completely random workload on >>>>> it, e.g. most likely it won't be able to come up with a proper complex >>>>> structure layout for some ioctls. And I am actually not completely >>>>> sure about completeness and coverage of video4linux.txt descriptions >>>>> too as they were contributed by somebody interested in android >>>>> testing. >>>> >>>> A quick look suggests that it is based on the 4.9 videodev2.h, which ain't >>>> too bad. There are some differences between the 4.20 videodev2.h and the >>>> 4.9, but not too many. >>> >>> >>> Makes sense because the latest android use 4.9. >>> Looking at the diff with 4.9 it seems that the APIs were mostly just >>> extended, so we just need to extend syzkaller descriptions >>> correspondingly. >>> >>>>> I wonder if somebody knowledgeable in /dev/media interface be willing >>>>> to contribute additional descriptions? >>>> >>>> We'll have to wait for 4.20-rc1 to be released since there are important >>>> additions to the media API. >>> >>> Additions are fine. We can extend syzkaller descriptions later too. >>> But if they are already in, say, linux-next, then syzbot tests it too. >>> >>> >>>> I can probably come up with something, I'm >>>> just not sure when I get around to it. Ping me in three weeks time if you >>>> haven't heard from me. >>>> >>>>> >>>>> We also have code coverage reports with the coverage fuzzer achieved >>>>> so far. Here in the Cover column: >>>>> https://syzkaller.appspot.com/#managers >>>>> e.g. this one (but note this is a ~80MB html file): >>>>> https://storage.googleapis.com/syzkaller/cover/ci-upstream-kasan-gce-root.html >>>>> This can be used to assess e.g. v4l coverage. But I don't know what's >>>>> coverable in general from syscalls and what's coverable via the stub >>>>> drivers in particular. So some expertise from media developers would >>>>> be helpful too. >>>> >>>> The four virtual drivers should give pretty decent coverage of the core >>>> code. Are you able to test with a 32-bit syzkaller application on a 64-bit >>>> kernel as well? That way the compat32 code is tested. >>> >>> This is coverage from 32-bit instance: >>> https://storage.googleapis.com/syzkaller/cover/ci-upstream-kasan-gce-386.html >>> There is some coverage in drivers/media, but I don't know where to >>> look specifically. E.g. drivers/media/v4l2-core/v4l2-ioctl.c does not >>> mention "compat". >> >> It's in drivers/media/v4l2-core/v4l2-compat-ioctl32.c >> >> BTW, is ci-upstream-kasan-gce-386.html a 64 bit kernel with 32 bit apps? Or >> is it a 32-bit kernel? The latter is not very interesting. > > Yes, it's 64-bit kernel and 32-bit app. Yet to find anybody who would > be interested in 32-bit x86 kernel today :) > > >>>>> Do I understand it correctly that when a process opens /dev/video* or >>>>> /dev/media* it gets a private instance of the device? In particular, >>>>> if several processes test this in parallel, will they collide? Or they >>>>> will stress separate objects? >>>> >>>> It actually depends on the driver. M2M devices will give you a private >>>> instance whenever you open it. Others do not, but you can call most ioctls >>>> in parallel. But after calling REQBUFS or CREATE_BUFS the filehandle that >>>> called those ioctls becomes owner of the device until the buffers are >>>> released. So other filehandles cannot do any streaming operations (EBUSY >>>> will be returned). >>> >>> Are the devices created by VIMC, VIM2M, VIVID, VICODEC M2M or not? >>> I guess VIM2M is M2M, but what about others? >> >> vim2m and vicodec are memory-to-memory devices. But vivid and vimc are not. >> >> Easy to check: v4l2-ctl -D -d /dev/videoX will report Video Memory-to-Memory >> capabilities. >> >>> >>> CREATE_BUFS privatization is somewhat unfortunate, but I guess we can >>> live with it for now. >> >> Sorry, I'm not sure what you mean. > > You said: > >>> But after calling REQBUFS or CREATE_BUFS the filehandle that >>> called those ioctls becomes owner of the device until the buffers are >>> released. So other filehandles cannot do any streaming operations (EBUSY >>> will be returned). > > This semantics are somewhat unfortunate for syzkaller because one test > process will affect/block other test processes, and we try to make > them as independent as possible. E.g. If this can affect syzkaller > ability to create reproducers, because in one run of a test if was > affected by an unrelated test and crashed, but if we try to reproduce > the crash on the same test it won't crash again because now it's not > affected by the unrelated test. > > But if we create more devices and partition them across test > processes, it will resolve this problem? > > >>> I assume that when the process dies it will release everything at >>> least, because fuzzer will sure not pair create with release all the >>> time.
