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SubjectKASAN: use-after-free Write in percpu_ref_switch_to_percpu

syzbot found the following crash on:

HEAD commit: 754beeec Merge tag 'char-misc-5.6-rc1-2' of git://git.kern..
git tree: upstream
console output:
kernel config:
dashboard link:
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:

BUG: KASAN: use-after-free in percpu_ref_switch_to_percpu+0x69/0x70 lib/percpu-refcount.c:314
Write of size 1 at addr ffff888095e3b430 by task kworker/1:31/2772

CPU: 1 PID: 2772 Comm: kworker/1:31 Not tainted 5.5.0-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
Workqueue: events io_ring_file_ref_switch
Call Trace:
__dump_stack lib/dump_stack.c:77 [inline]
dump_stack+0x197/0x210 lib/dump_stack.c:118
print_address_description.constprop.0.cold+0xd4/0x30b mm/kasan/report.c:374
__kasan_report.cold+0x1b/0x32 mm/kasan/report.c:506
kasan_report+0x12/0x20 mm/kasan/common.c:641
__asan_report_store1_noabort+0x17/0x20 mm/kasan/generic_report.c:137
percpu_ref_switch_to_percpu+0x69/0x70 lib/percpu-refcount.c:314
io_ring_file_ref_switch+0x791/0xac0 fs/io_uring.c:5499
process_one_work+0xa05/0x17a0 kernel/workqueue.c:2264
worker_thread+0x98/0xe40 kernel/workqueue.c:2410
kthread+0x361/0x430 kernel/kthread.c:255
ret_from_fork+0x24/0x30 arch/x86/entry/entry_64.S:352

Allocated by task 26829:
save_stack+0x23/0x90 mm/kasan/common.c:72
set_track mm/kasan/common.c:80 [inline]
__kasan_kmalloc mm/kasan/common.c:515 [inline]
__kasan_kmalloc.constprop.0+0xcf/0xe0 mm/kasan/common.c:488
kasan_kmalloc+0x9/0x10 mm/kasan/common.c:529
__do_kmalloc mm/slab.c:3656 [inline]
__kmalloc+0x163/0x770 mm/slab.c:3665
kmalloc include/linux/slab.h:561 [inline]
kzalloc.constprop.0+0x1b/0x20 include/linux/slab.h:670
io_sqe_files_register fs/io_uring.c:5528 [inline]
__io_uring_register+0xad7/0x2f40 fs/io_uring.c:6875
__do_sys_io_uring_register fs/io_uring.c:6955 [inline]
__se_sys_io_uring_register fs/io_uring.c:6937 [inline]
__x64_sys_io_uring_register+0x1a1/0x560 fs/io_uring.c:6937
do_syscall_64+0xfa/0x790 arch/x86/entry/common.c:294

Freed by task 26826:
save_stack+0x23/0x90 mm/kasan/common.c:72
set_track mm/kasan/common.c:80 [inline]
kasan_set_free_info mm/kasan/common.c:337 [inline]
__kasan_slab_free+0x102/0x150 mm/kasan/common.c:476
kasan_slab_free+0xe/0x10 mm/kasan/common.c:485
__cache_free mm/slab.c:3426 [inline]
kfree+0x10a/0x2c0 mm/slab.c:3757
io_sqe_files_unregister+0x2e2/0x4d0 fs/io_uring.c:5250
io_ring_ctx_free fs/io_uring.c:6229 [inline]
io_ring_ctx_wait_and_kill+0x447/0x9b0 fs/io_uring.c:6310
io_uring_release+0x42/0x50 fs/io_uring.c:6318
__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
tracehook_notify_resume include/linux/tracehook.h:188 [inline]
exit_to_usermode_loop+0x316/0x380 arch/x86/entry/common.c:164
prepare_exit_to_usermode arch/x86/entry/common.c:195 [inline]
syscall_return_slowpath arch/x86/entry/common.c:278 [inline]
do_syscall_64+0x676/0x790 arch/x86/entry/common.c:304

The buggy address belongs to the object at ffff888095e3b400
which belongs to the cache kmalloc-256 of size 256
The buggy address is located 48 bytes inside of
256-byte region [ffff888095e3b400, ffff888095e3b500)
The buggy address belongs to the page:
page:ffffea0002578ec0 refcount:1 mapcount:0 mapping:ffff8880aa4008c0 index:0x0
flags: 0xfffe0000000200(slab)
raw: 00fffe0000000200 ffffea00025c09c8 ffffea00027f8308 ffff8880aa4008c0
raw: 0000000000000000 ffff888095e3b000 0000000100000008 0000000000000000
page dumped because: kasan: bad access detected

Memory state around the buggy address:
ffff888095e3b300: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
ffff888095e3b380: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
>ffff888095e3b400: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
ffff888095e3b480: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
ffff888095e3b500: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc

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