KASAN: null-ptr-deref Write in kthread_stop

[syzbot]

Hello,

syzbot found the following crash on:

HEAD commit:    8c60c36d0b8c Add linux-next specific files for 20181019
git tree:       linux-next
console output: https://syzkaller.appspot.com/x/log.txt?x=16128d7b400000
kernel config:  https://syzkaller.appspot.com/x/.config?x=ddc97ab84fb1ff2a
dashboard link: https://syzkaller.appspot.com/bug?extid=53d5b2df0d9744411e2e
compiler:       gcc (GCC) 8.0.1 20180413 (experimental)
syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=124b1e83400000

IMPORTANT: if you fix the bug, please add the following tag to the commit:
Reported-by: syzbot+53d5b2df0d9744411e2e@xxxxxxxxxxxxxxxxxxxxxxxxx

8021q: adding VLAN 0 to HW filter on device team0
8021q: adding VLAN 0 to HW filter on device team0
8021q: adding VLAN 0 to HW filter on device team0
vivid-000: kernel_thread() failed
==================================================================
BUG: KASAN: null-ptr-deref in atomic_inc  
include/asm-generic/atomic-instrumented.h:109 [inline]
BUG: KASAN: null-ptr-deref in kthread_stop+0x108/0x8f0 kernel/kthread.c:545
Write of size 4 at addr 000000000000001c by task syz-executor3/7274

CPU: 1 PID: 7274 Comm: syz-executor3 Not tainted 4.19.0-rc8-next-20181019+  
#99
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS  
Google 01/01/2011
Call Trace:
 __dump_stack lib/dump_stack.c:77 [inline]
 dump_stack+0x244/0x39d lib/dump_stack.c:113
 kasan_report_error mm/kasan/report.c:352 [inline]
 kasan_report.cold.8+0x6d/0x309 mm/kasan/report.c:412
 check_memory_region_inline mm/kasan/kasan.c:260 [inline]
 check_memory_region+0x13e/0x1b0 mm/kasan/kasan.c:267
 kasan_check_write+0x14/0x20 mm/kasan/kasan.c:278
 atomic_inc include/asm-generic/atomic-instrumented.h:109 [inline]
 kthread_stop+0x108/0x8f0 kernel/kthread.c:545
 vivid_stop_generating_vid_cap+0x2bc/0x93b  
drivers/media/platform/vivid/vivid-kthread-cap.c:919
 vid_cap_stop_streaming+0x8d/0xe0  
drivers/media/platform/vivid/vivid-vid-cap.c:259
 __vb2_queue_cancel+0x171/0xca0  
drivers/media/common/videobuf2/videobuf2-core.c:1668
 vb2_core_streamoff+0x60/0x140  
drivers/media/common/videobuf2/videobuf2-core.c:1804
 __vb2_cleanup_fileio+0x73/0x160  
drivers/media/common/videobuf2/videobuf2-core.c:2325
 vb2_core_queue_release+0x1e/0x80  
drivers/media/common/videobuf2/videobuf2-core.c:2052
 vb2_queue_release drivers/media/common/videobuf2/videobuf2-v4l2.c:672  
[inline]
 _vb2_fop_release+0x1d2/0x2b0  
drivers/media/common/videobuf2/videobuf2-v4l2.c:843
 vb2_fop_release+0x77/0xc0  
drivers/media/common/videobuf2/videobuf2-v4l2.c:857
 vivid_fop_release+0x18e/0x440 drivers/media/platform/vivid/vivid-core.c:474
 v4l2_release+0xfb/0x1a0 drivers/media/v4l2-core/v4l2-dev.c:448
 __fput+0x3bc/0xa70 fs/file_table.c:279
 ____fput+0x15/0x20 fs/file_table.c:312
 task_work_run+0x1e8/0x2a0 kernel/task_work.c:113
 exit_task_work include/linux/task_work.h:22 [inline]
 do_exit+0x1ad1/0x26d0 kernel/exit.c:867
 do_group_exit+0x177/0x440 kernel/exit.c:970
 get_signal+0x8a8/0x1970 kernel/signal.c:2517
 do_signal+0x9c/0x21c0 arch/x86/kernel/signal.c:816
 exit_to_usermode_loop+0x2e5/0x380 arch/x86/entry/common.c:162
 prepare_exit_to_usermode arch/x86/entry/common.c:197 [inline]
 syscall_return_slowpath arch/x86/entry/common.c:268 [inline]
 do_syscall_64+0x6be/0x820 arch/x86/entry/common.c:293
 entry_SYSCALL_64_after_hwframe+0x49/0xbe
RIP: 0033:0x483081
Code: 75 14 b8 23 00 00 00 0f 05 48 3d 01 f0 ff ff 0f 83 d4 f8 f8 ff c3 48  
83 ec 08 e8 ba 70 fd ff 48 89 04 24 b8 23 00 00 00 0f 05 <48> 8b 3c 24 48  
89 c2 e8 03 71 fd ff 48 89 d0 48 83 c4 08 48 3d 01
RSP: 002b:00007ffc69ddaf10 EFLAGS: 00000293 ORIG_RAX: 0000000000000023
RAX: 0000000000000000 RBX: 000000000000fef5 RCX: 0000000000483081
RDX: 0000000000000000 RSI: 0000000000000000 RDI: 00007ffc69ddaf20
RBP: 000000000072bf00 R08: 0000000000000000 R09: 0000000000000000
R10: 00007ffc69ddb000 R11: 0000000000000293 R12: 000000000072c900
R13: 00000000000003e8 R14: 000000000000fc39 R15: 000000000000fc0c
==================================================================


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