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SubjectKASAN: use-after-free Read in chaoskey_disconnect

syzbot found the following crash on:

HEAD commit: ff6409a6 usb-fuzzer: main usb gadget fuzzer driver
git tree: usb-fuzzer
console output:
kernel config:
dashboard link:
compiler: gcc (GCC) 9.0.0 20181231 (experimental)
syz repro:
C reproducer:

IMPORTANT: if you fix the bug, please add the following tag to the commit:

usb 3-1: string descriptor 0 read error: -71
usb 3-1: USB disconnect, device number 2
BUG: KASAN: use-after-free in atomic_read
include/asm-generic/atomic-instrumented.h:26 [inline]
BUG: KASAN: use-after-free in refcount_inc_not_zero_checked+0x72/0x1e0
Read of size 4 at addr ffff8881cf679820 by task kworker/1:1/1730

CPU: 1 PID: 1730 Comm: kworker/1:1 Not tainted 5.4.0-rc5+ #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS
Google 01/01/2011
Workqueue: usb_hub_wq hub_event
Call Trace:
__dump_stack lib/dump_stack.c:77 [inline]
dump_stack+0xca/0x13e lib/dump_stack.c:113
print_address_description.constprop.0+0x36/0x50 mm/kasan/report.c:374
__kasan_report.cold+0x1a/0x33 mm/kasan/report.c:506
kasan_report+0xe/0x20 mm/kasan/common.c:634
check_memory_region_inline mm/kasan/generic.c:185 [inline]
check_memory_region+0x128/0x190 mm/kasan/generic.c:192
atomic_read include/asm-generic/atomic-instrumented.h:26 [inline]
refcount_inc_not_zero_checked+0x72/0x1e0 lib/refcount.c:123
refcount_inc_checked+0x12/0x60 lib/refcount.c:156
get_task_struct include/linux/sched/task.h:110 [inline]
kthread_stop+0x6c/0x610 kernel/kthread.c:554
hwrng_unregister+0x190/0x210 drivers/char/hw_random/core.c:538
chaoskey_disconnect+0x1b2/0x200 drivers/usb/misc/chaoskey.c:232
usb_unbind_interface+0x1bd/0x8a0 drivers/usb/core/driver.c:423
__device_release_driver drivers/base/dd.c:1134 [inline]
device_release_driver_internal+0x42f/0x500 drivers/base/dd.c:1165
bus_remove_device+0x2dc/0x4a0 drivers/base/bus.c:532
device_del+0x420/0xb20 drivers/base/core.c:2376
usb_disable_device+0x211/0x690 drivers/usb/core/message.c:1237
usb_disconnect+0x284/0x8d0 drivers/usb/core/hub.c:2200
hub_port_connect drivers/usb/core/hub.c:5035 [inline]
hub_port_connect_change drivers/usb/core/hub.c:5324 [inline]
port_event drivers/usb/core/hub.c:5470 [inline]
hub_event+0x16f2/0x3800 drivers/usb/core/hub.c:5552
process_one_work+0x92b/0x1530 kernel/workqueue.c:2269
process_scheduled_works kernel/workqueue.c:2331 [inline]
worker_thread+0x7ab/0xe20 kernel/workqueue.c:2417
kthread+0x318/0x420 kernel/kthread.c:255
ret_from_fork+0x24/0x30 arch/x86/entry/entry_64.S:352

Allocated by task 2:
save_stack+0x1b/0x80 mm/kasan/common.c:69
set_track mm/kasan/common.c:77 [inline]
__kasan_kmalloc mm/kasan/common.c:510 [inline]
__kasan_kmalloc.constprop.0+0xbf/0xd0 mm/kasan/common.c:483
slab_post_alloc_hook mm/slab.h:584 [inline]
slab_alloc_node mm/slub.c:2779 [inline]
kmem_cache_alloc_node+0xdc/0x310 mm/slub.c:2815
alloc_task_struct_node kernel/fork.c:169 [inline]
dup_task_struct kernel/fork.c:865 [inline]
copy_process+0x4201/0x6470 kernel/fork.c:1851
_do_fork+0x129/0xec0 kernel/fork.c:2366
kernel_thread+0xaa/0xe0 kernel/fork.c:2453
create_kthread kernel/kthread.c:278 [inline]
kthreadd+0x4a2/0x680 kernel/kthread.c:596
ret_from_fork+0x24/0x30 arch/x86/entry/entry_64.S:352

Freed by task 16:
save_stack+0x1b/0x80 mm/kasan/common.c:69
set_track mm/kasan/common.c:77 [inline]
kasan_set_free_info mm/kasan/common.c:332 [inline]
__kasan_slab_free+0x130/0x180 mm/kasan/common.c:471
slab_free_hook mm/slub.c:1424 [inline]
slab_free_freelist_hook mm/slub.c:1475 [inline]
slab_free mm/slub.c:3025 [inline]
kmem_cache_free+0xb9/0x380 mm/slub.c:3041
__put_task_struct+0x1e2/0x4c0 kernel/fork.c:748
put_task_struct include/linux/sched/task.h:119 [inline]
delayed_put_task_struct+0x1b4/0x2c0 kernel/exit.c:182
__rcu_reclaim kernel/rcu/rcu.h:222 [inline]
rcu_do_batch kernel/rcu/tree.c:2157 [inline]
rcu_core+0x630/0x1ca0 kernel/rcu/tree.c:2377
__do_softirq+0x221/0x912 kernel/softirq.c:292

The buggy address belongs to the object at ffff8881cf679800
which belongs to the cache task_struct of size 5888
The buggy address is located 32 bytes inside of
5888-byte region [ffff8881cf679800, ffff8881cf67af00)
The buggy address belongs to the page:
page:ffffea00073d9e00 refcount:1 mapcount:0 mapping:ffff8881da116000
index:0x0 compound_mapcount: 0
flags: 0x200000000010200(slab|head)
raw: 0200000000010200 dead000000000100 dead000000000122 ffff8881da116000
raw: 0000000000000000 0000000080050005 00000001ffffffff 0000000000000000
page dumped because: kasan: bad access detected

Memory state around the buggy address:
ffff8881cf679700: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
ffff8881cf679780: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
> ffff8881cf679800: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
ffff8881cf679880: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
ffff8881cf679900: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb

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