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

[syzbot]

syzbot has found a reproducer for the following crash on:

HEAD commit:    6201f31a39f8 Add linux-next specific files for 20181030
git tree:       linux-next
console output: https://syzkaller.appspot.com/x/log.txt?x=179da7cb400000
kernel config:  https://syzkaller.appspot.com/x/.config?x=2a22859d870756c1
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=106f6999400000
C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=17267233400000

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

sshd (5629) used greatest stack depth: 15744 bytes left
==================================================================
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 0000000000000020 by task syz-executor789/5655

CPU: 1 PID: 5655 Comm: syz-executor789 Not tainted 4.19.0-next-20181030+  
#101
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
 tracehook_notify_resume include/linux/tracehook.h:188 [inline]
 exit_to_usermode_loop+0x318/0x380 arch/x86/entry/common.c:166
 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: 0033:0x400ef0
Code: 01 f0 ff ff 0f 83 b0 0a 00 00 c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f  
44 00 00 83 3d dd 57 2d 00 00 75 14 b8 03 00 00 00 0f 05 <48> 3d 01 f0 ff  
ff 0f 83 84 0a 00 00 c3 48 83 ec 08 e8 3a 01 00 00
RSP: 002b:00007fff00659738 EFLAGS: 00000246 ORIG_RAX: 0000000000000003
RAX: 0000000000000000 RBX: 0000000000000003 RCX: 0000000000400ef0
RDX: 0000000020000024 RSI: 0000000020000000 RDI: 0000000000000003
RBP: 0000000000000000 R08: 0000000001446880 R09: 00000000004002e0
R10: 000000000000000f R11: 0000000000000246 R12: 0000000000401e00
R13: 0000000000401e90 R14: 0000000000000000 R15: 0000000000000000
==================================================================