KASAN: null-ptr-deref Write in io_wq_cancel_all

[syzbot]

Hello,

syzbot found the following crash on:

HEAD commit:    139c2d13 Add linux-next specific files for 20191025
git tree:       linux-next
console output: https://syzkaller.appspot.com/x/log.txt?x=17ab5a70e00000
kernel config:  https://syzkaller.appspot.com/x/.config?x=28fd7a693df38d29
dashboard link: https://syzkaller.appspot.com/bug?extid=d958a65633ea70280b23
compiler:       gcc (GCC) 9.0.0 20181231 (experimental)

Unfortunately, I don't have any reproducer for this crash yet.

IMPORTANT: if you fix the bug, please add the following tag to the commit:
Reported-by: syzbot+d958a65633ea70280b23@xxxxxxxxxxxxxxxxxxxxxxxxx

==================================================================
BUG: KASAN: null-ptr-deref in set_bit  
include/asm-generic/bitops-instrumented.h:28 [inline]
BUG: KASAN: null-ptr-deref in io_wq_cancel_all+0x28/0x2a0 fs/io-wq.c:574
Write of size 8 at addr 0000000000000004 by task syz-executor.5/17477

CPU: 1 PID: 17477 Comm: syz-executor.5 Not tainted 5.4.0-rc4-next-20191025  
#0
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+0x172/0x1f0 lib/dump_stack.c:113
 __kasan_report.cold+0x5/0x41 mm/kasan/report.c:510
 kasan_report+0x12/0x20 mm/kasan/common.c:634
 check_memory_region_inline mm/kasan/generic.c:185 [inline]
 check_memory_region+0x134/0x1a0 mm/kasan/generic.c:192
 __kasan_check_write+0x14/0x20 mm/kasan/common.c:98
 set_bit include/asm-generic/bitops-instrumented.h:28 [inline]
 io_wq_cancel_all+0x28/0x2a0 fs/io-wq.c:574
 io_ring_ctx_wait_and_kill+0x1e2/0x710 fs/io_uring.c:3679
 io_uring_release+0x42/0x50 fs/io_uring.c:3691
 __fput+0x2ff/0x890 fs/file_table.c:280
 ____fput+0x16/0x20 fs/file_table.c:313
 task_work_run+0x145/0x1c0 kernel/task_work.c:113
 exit_task_work include/linux/task_work.h:22 [inline]
 do_exit+0x904/0x2e60 kernel/exit.c:817
 do_group_exit+0x135/0x360 kernel/exit.c:921
 get_signal+0x47c/0x24f0 kernel/signal.c:2734
 do_signal+0x87/0x1700 arch/x86/kernel/signal.c:815
 exit_to_usermode_loop+0x286/0x380 arch/x86/entry/common.c:159
 prepare_exit_to_usermode arch/x86/entry/common.c:194 [inline]
 syscall_return_slowpath arch/x86/entry/common.c:274 [inline]
 do_syscall_64+0x65f/0x760 arch/x86/entry/common.c:300
 entry_SYSCALL_64_after_hwframe+0x49/0xbe
RIP: 0033:0x459ef9
Code: ad b6 fb ff c3 66 2e 0f 1f 84 00 00 00 00 00 66 90 48 89 f8 48 89 f7  
48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff  
ff 0f 83 7b b6 fb ff c3 66 2e 0f 1f 84 00 00 00 00
RSP: 002b:00007f7129716c78 EFLAGS: 00000246 ORIG_RAX: 00000000000001a9
RAX: 0000000000000005 RBX: 0000000000000002 RCX: 0000000000459ef9
RDX: 0000000000000000 RSI: 0000000020000080 RDI: 0000000000000ebf
RBP: 000000000075bf20 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 00007f71297176d4
R13: 00000000004c14ae R14: 00000000004d4c68 R15: 00000000ffffffff
==================================================================


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