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

[syzbot]

syzbot has found a reproducer for 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=12888a00e00000
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)
syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=160573c0e00000

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.2/9365

CPU: 1 PID: 9365 Comm: syz-executor.2 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+0x47a/0x530 arch/x86/entry/common.c:274
 ret_from_fork+0x15/0x30 arch/x86/entry/entry_64.S:344
RIP: 0033:0x45c909
Code: ff 48 85 f6 0f 84 d7 8c fb ff 48 83 ee 10 48 89 4e 08 48 89 3e 48 89  
d7 4c 89 c2 4d 89 c8 4c 8b 54 24 08 b8 38 00 00 00 0f 05 <48> 85 c0 0f 8c  
ae 8c fb ff 74 01 c3 31 ed 48 f7 c7 00 00 01 00 75
RSP: 002b:00007fe3de137db0 EFLAGS: 00000202 ORIG_RAX: 0000000000000038
RAX: 0000000000000000 RBX: 00007fe3de138700 RCX: 000000000045c909
RDX: 00007fe3de1389d0 RSI: 00007fe3de137db0 RDI: 00000000003d0f00
RBP: 00007ffff688f6a0 R08: 00007fe3de138700 R09: 00007fe3de138700
R10: 00007fe3de1389d0 R11: 0000000000000202 R12: 0000000000000000
R13: 00007ffff688f53f R14: 00007fe3de1389c0 R15: 000000000075bfd4
==================================================================