Messages in this thread | | | Date | Mon, 21 Feb 2022 11:05:00 +0100 | From | Stefano Garzarella <> | Subject | Re: [syzbot] WARNING in vhost_dev_cleanup (2) |
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On Fri, Feb 18, 2022 at 12:23:10PM -0600, Mike Christie wrote: >On 2/18/22 11:53 AM, Mike Christie wrote: >> On 2/17/22 3:48 AM, Stefano Garzarella wrote: >>> >>> On Thu, Feb 17, 2022 at 8:50 AM Michael S. Tsirkin <mst@redhat.com> wrote: >>>> >>>> On Thu, Feb 17, 2022 at 03:39:48PM +0800, Jason Wang wrote: >>>>> On Thu, Feb 17, 2022 at 3:36 PM Michael S. Tsirkin <mst@redhat.com> wrote: >>>>>> >>>>>> On Thu, Feb 17, 2022 at 03:34:13PM +0800, Jason Wang wrote: >>>>>>> On Thu, Feb 17, 2022 at 10:01 AM syzbot >>>>>>> <syzbot+1e3ea63db39f2b4440e0@syzkaller.appspotmail.com> wrote: >>>>>>>> >>>>>>>> Hello, >>>>>>>> >>>>>>>> syzbot found the following issue on: >>>>>>>> >>>>>>>> HEAD commit: c5d9ae265b10 Merge tag 'for-linus' of git://git.kernel.org.. >>>>>>>> git tree: upstream >>>>>>>> console output: https://urldefense.com/v3/__https://syzkaller.appspot.com/x/log.txt?x=132e687c700000__;!!ACWV5N9M2RV99hQ!fLqQTyosTBm7FK50IVmo0ozZhsvUEPFCivEHFDGU3GjlAHDWl07UdOa-t9uf9YisMihn$ >>>>>>>> kernel config: https://urldefense.com/v3/__https://syzkaller.appspot.com/x/.config?x=a78b064590b9f912__;!!ACWV5N9M2RV99hQ!fLqQTyosTBm7FK50IVmo0ozZhsvUEPFCivEHFDGU3GjlAHDWl07UdOa-t9uf9RjOhplp$ >>>>>>>> dashboard link: https://urldefense.com/v3/__https://syzkaller.appspot.com/bug?extid=1e3ea63db39f2b4440e0__;!!ACWV5N9M2RV99hQ!fLqQTyosTBm7FK50IVmo0ozZhsvUEPFCivEHFDGU3GjlAHDWl07UdOa-t9uf9bBf5tv0$ >>>>>>>> compiler: gcc (Debian 10.2.1-6) 10.2.1 20210110, GNU ld (GNU Binutils for Debian) 2.35.2 >>>>>>>> >>>>>>>> Unfortunately, I don't have any reproducer for this issue yet. >>>>>>>> >>>>>>>> IMPORTANT: if you fix the issue, please add the following tag to the commit: >>>>>>>> Reported-by: syzbot+1e3ea63db39f2b4440e0@syzkaller.appspotmail.com >>>>>>>> >>>>>>>> WARNING: CPU: 1 PID: 10828 at drivers/vhost/vhost.c:715 vhost_dev_cleanup+0x8b8/0xbc0 drivers/vhost/vhost.c:715 >>>>>>>> Modules linked in: >>>>>>>> CPU: 0 PID: 10828 Comm: syz-executor.0 Not tainted 5.17.0-rc4-syzkaller-00051-gc5d9ae265b10 #0 >>>>>>>> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011 >>>>>>>> RIP: 0010:vhost_dev_cleanup+0x8b8/0xbc0 drivers/vhost/vhost.c:715 >>>>>>> >>>>>>> Probably a hint that we are missing a flush. >>>>>>> >>>>>>> Looking at vhost_vsock_stop() that is called by vhost_vsock_dev_release(): >>>>>>> >>>>>>> static int vhost_vsock_stop(struct vhost_vsock *vsock) >>>>>>> { >>>>>>> size_t i; >>>>>>> int ret; >>>>>>> >>>>>>> mutex_lock(&vsock->dev.mutex); >>>>>>> >>>>>>> ret = vhost_dev_check_owner(&vsock->dev); >>>>>>> if (ret) >>>>>>> goto err; >>>>>>> >>>>>>> Where it could fail so the device is not actually stopped. >>>>>>> >>>>>>> I wonder if this is something related. >>>>>>> >>>>>>> Thanks >>>>>> >>>>>> >>>>>> But then if that is not the owner then no work should be running, right? >>>>> >>>>> Could it be a buggy user space that passes the fd to another process >>>>> and changes the owner just before the mutex_lock() above? >>>>> >>>>> Thanks >>>> >>>> Maybe, but can you be a bit more explicit? what is the set of >>>> conditions you see that can lead to this? >>> >>> I think the issue could be in the vhost_vsock_stop() as Jason mentioned, >>> but not related to fd passing, but related to the do_exit() function. >>> >>> Looking the stack trace, we are in exit_task_work(), that is called >>> after exit_mm(), so the vhost_dev_check_owner() can fail because >>> current->mm should be NULL at that point. >>> >>> It seems the fput work is queued by fput_many() in a worker queue, and >>> in some cases (maybe a lot of files opened?) the work is still queued >>> when we enter in do_exit(). >> It normally happens if userspace doesn't do a close() when the VM > >Just one clarification. I meant to say it "always" happens when userspace >doesn't do a close. > >It doesn't have anything to do with lots of files or something like that. >We are actually running the vhost device's release function from >do_exit->task_work_run and so all those __fputs are done from something >like qemu's context (current == that process). > >We are *not* hitting the case: > >do_exit->exit_files->put_files_struct->filp_close->fput->fput_many > >and then in there hitting the schedule_delayed_work path. For that >the last __fput would be done from a workqueue thread and so the current >pointer would point to a completely different thread. > > > >> is shutdown and instead let's the kernel's reaper code cleanup. The qemu >> vhost-scsi code doesn't do a close() during shutdown and so this is our >> normal code path. It also happens when something like qemu is not >> gracefully shutdown like during a crash. >> >> So fire up qemu, start IO, then crash it or kill 9 it while IO is still >> running and you can hit it.
Thank you very much for this detailed explanation!
>> >>> >>> That said, I don't know if we can simply remove that check in >>> vhost_vsock_stop(), or check if current->mm is NULL, to understand if >>> the process is exiting. >>> >> >> Should the caller do the vhost_dev_check_owner or tell vhost_vsock_stop >> when to check? >> >> - vhost_vsock_dev_ioctl always wants to check for ownership right? >> >> - For vhost_vsock_dev_release ownership doesn't matter because we >> always want to clean up or it doesn't hurt too much. >> >> For the case where we just do open then close and no ioctls then >> running vhost_vq_set_backend in vhost_vsock_stop is just a minor >> hit of extra work. If we've done ioctls, but are now in >> vhost_vsock_dev_release then we know for the graceful and ungraceful >> case that nothing is going to be accessing this device in the future >> and it's getting completely freed so we must completely clean it up.
Yep, I think the easiest way is to add a parameter to vhost_vsock_stop() to tell when to call vhost_dev_check_owner() or not. This is because dev->mm is protected by dev->mutex, acquired in vhost_vsock_stop().
I will send a patch right away, it would be great if you can review.
Thanks, Stefano
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