Re: KASAN: null-ptr-deref Write in event_handler

[Shuah Khan]

On 10/7/20 8:28 AM, Andrey Konovalov wrote:
> On Wed, Oct 7, 2020 at 3:56 PM Shuah Khan <skhan@xxxxxxxxxxxxxxxxxxx> wrote:
> > On 10/5/20 2:44 PM, Shuah Khan wrote:
> > > On 10/5/20 8:04 AM, Andrey Konovalov wrote:
> > > > On Mon, Oct 5, 2020 at 3:59 PM syzbot
> > > > <syzbot+bf1a360e305ee719e364@xxxxxxxxxxxxxxxxxxxxxxxxx> wrote:
> > > > > Hello,
> > > > >
> > > > > syzbot found the following issue on:
> > > > >
> > > > > HEAD commit:    d3d45f82 Merge tag 'pinctrl-v5.9-2' of
> > > > > git://git.kernel.or..
> > > > > git tree:       upstream
> > > > > console output: https://syzkaller.appspot.com/x/log.txt?x=15781d8f900000
> > > > > kernel config:
> > > > > https://syzkaller.appspot.com/x/.config?x=89ab6a0c48f30b49
> > > > > dashboard link:
> > > > > https://syzkaller.appspot.com/bug?extid=bf1a360e305ee719e364
> > > > > compiler:       gcc (GCC) 10.1.0-syz 20200507
> > > > > syz repro:
> > > > > https://syzkaller.appspot.com/x/repro.syz?x=16cbaa7d900000
> > > > > C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=1364f367900000
> > > > >
> > > > > IMPORTANT: if you fix the issue, please add the following tag to the
> > > > > commit:
> > > > > Reported-by: syzbot+bf1a360e305ee719e364@xxxxxxxxxxxxxxxxxxxxxxxxx
> > > > >
> > > > > vhci_hcd: stop threads
> > > > > vhci_hcd: release socket
> > > > > vhci_hcd: disconnect device
> > > > > ==================================================================
> > > > > BUG: KASAN: null-ptr-deref in instrument_atomic_write
> > > > > include/linux/instrumented.h:71 [inline]
> > > > > BUG: KASAN: null-ptr-deref in atomic_fetch_add_relaxed
> > > > > include/asm-generic/atomic-instrumented.h:142 [inline]
> > > > > BUG: KASAN: null-ptr-deref in refcount_add
> > > > > include/linux/refcount.h:201 [inline]
> > > > > BUG: KASAN: null-ptr-deref in refcount_inc
> > > > > include/linux/refcount.h:241 [inline]
> > > > > BUG: KASAN: null-ptr-deref in get_task_struct
> > > > > include/linux/sched/task.h:104 [inline]
> > > > > BUG: KASAN: null-ptr-deref in kthread_stop+0x90/0x7e0
> > > > > kernel/kthread.c:591
> > > > > Write of size 4 at addr 000000000000001c by task kworker/u4:5/2519
> > > > >
> > > > > CPU: 1 PID: 2519 Comm: kworker/u4:5 Not tainted 5.9.0-rc7-syzkaller #0
> > > > > Hardware name: Google Google Compute Engine/Google Compute Engine,
> > > > > BIOS Google 01/01/2011
> > > > > Workqueue: usbip_event event_handler
> > > > > Call Trace:
> > > > >    __dump_stack lib/dump_stack.c:77 [inline]
> > > > >    dump_stack+0x198/0x1fd lib/dump_stack.c:118
> > > > >    __kasan_report mm/kasan/report.c:517 [inline]
> > > > >    kasan_report.cold+0x5/0x37 mm/kasan/report.c:530
> > > > >    check_memory_region_inline mm/kasan/generic.c:186 [inline]
> > > > >    check_memory_region+0x13d/0x180 mm/kasan/generic.c:192
> > > > >    instrument_atomic_write include/linux/instrumented.h:71 [inline]
> > > > >    atomic_fetch_add_relaxed
> > > > > include/asm-generic/atomic-instrumented.h:142 [inline]
> > > > >    refcount_add include/linux/refcount.h:201 [inline]
> > > > >    refcount_inc include/linux/refcount.h:241 [inline]
> > > > >    get_task_struct include/linux/sched/task.h:104 [inline]
> > > > >    kthread_stop+0x90/0x7e0 kernel/kthread.c:591
> > > > >    vhci_shutdown_connection+0x170/0x2a0 drivers/usb/usbip/vhci_hcd.c:1015
> > > > >    event_handler+0x1a5/0x450 drivers/usb/usbip/usbip_event.c:78
> > > > >    process_one_work+0x94c/0x1670 kernel/workqueue.c:2269
> > > > >    worker_thread+0x64c/0x1120 kernel/workqueue.c:2415
> > > > >    kthread+0x3b5/0x4a0 kernel/kthread.c:292
> > > > >    ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:294
> > > > > ==================================================================
> > > > > Kernel panic - not syncing: panic_on_warn set ...
> > > > > CPU: 1 PID: 2519 Comm: kworker/u4:5 Tainted: G    B
> > > > > 5.9.0-rc7-syzkaller #0
> > > > > Hardware name: Google Google Compute Engine/Google Compute Engine,
> > > > > BIOS Google 01/01/2011
> > > > > Workqueue: usbip_event event_handler
> > > > > Call Trace:
> > > > >    __dump_stack lib/dump_stack.c:77 [inline]
> > > > >    dump_stack+0x198/0x1fd lib/dump_stack.c:118
> > > > >    panic+0x382/0x7fb kernel/panic.c:231
> > > > >    end_report+0x4d/0x53 mm/kasan/report.c:104
> > > > >    __kasan_report mm/kasan/report.c:520 [inline]
> > > > >    kasan_report.cold+0xd/0x37 mm/kasan/report.c:530
> > > > >    check_memory_region_inline mm/kasan/generic.c:186 [inline]
> > > > >    check_memory_region+0x13d/0x180 mm/kasan/generic.c:192
> > > > >    instrument_atomic_write include/linux/instrumented.h:71 [inline]
> > > > >    atomic_fetch_add_relaxed
> > > > > include/asm-generic/atomic-instrumented.h:142 [inline]
> > > > >    refcount_add include/linux/refcount.h:201 [inline]
> > > > >    refcount_inc include/linux/refcount.h:241 [inline]
> > > > >    get_task_struct include/linux/sched/task.h:104 [inline]
> > > > >    kthread_stop+0x90/0x7e0 kernel/kthread.c:591
> > > > >    vhci_shutdown_connection+0x170/0x2a0 drivers/usb/usbip/vhci_hcd.c:1015
> > > > >    event_handler+0x1a5/0x450 drivers/usb/usbip/usbip_event.c:78
> > > > >    process_one_work+0x94c/0x1670 kernel/workqueue.c:2269
> > > > >    worker_thread+0x64c/0x1120 kernel/workqueue.c:2415
> > > > >    kthread+0x3b5/0x4a0 kernel/kthread.c:292
> > > > >    ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:294
> > > > > Kernel Offset: disabled
> > > > > Rebooting in 86400 seconds..
> > > >
> > > > Hi Valentina and Shuah,
> > > >
> > > > There appears to be a race condition in the USB/IP vhci_hcd shutdown
> > > > procedure. It happens quite often during fuzzing with syzkaller, and
> > > > prevents us from going deeper into the USB/IP code.
> > > >
> > > > Could you advise us what would be the best fix for this?
> > >
> > > Hi Andrey,
> > >
> > > Reading the comments for this routine, looks like there is an assumption
> > > that context begins cleanup and race conditions aren't considered.
> > >
> > > The right fix is holding vhci->lock and vdev->priv_lock to protect
> > > critical sections in this routine. I will send a patch for this.
> >
> > Hi Andrey,
> >
> > I have been unable to reproduce the problem with the reproducer
> > so far. You mentioned it happens quite often.
> >
> > - matched config with yours
> > - load vhci_hcd module and run the reproducer
>
> Hm, if you matched the config, then the module should be built-in?

Right. I did notice that your config has built-in. This shouldn't
matter, I have a kernel built with it static. I will try it to
see if it makes a difference.

> > I do see the messages during shutdown - stop threads etc.
> >
> > What am I missing?
>
> This appears to be a race that requires precise timings. I failed to
> reproduce it with the C reproducer, but I managed to reproduce it with
> the syzkaller repro program:
>
> https://syzkaller.appspot.com/x/repro.syz?x=16cbaa7d900000
>
> To do that you need to build syzkaller, and copy ./bin/syz-execprog
> and ./bin/syz-executor into your testing environment, and then do:
>
> ./syz-execprog -sandbox=none -repeat=0 -procs=6 ./repro.prog

Thanks for the tips on your environment.

thanks,
-- Shuah