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SubjectKASAN: use-after-free Read in uprobe_perf_close

syzbot hit the following crash on upstream commit
86bbbebac1933e6e95e8234c4f7d220c5ddd38bc (Mon Apr 2 18:47:07 2018 +0000)
Merge branch 'ras-core-for-linus' of
syzbot dashboard link:

So far this crash happened 4 times on upstream.
C reproducer:
syzkaller reproducer:
Raw console output:
Kernel config:
compiler: gcc (GCC) 7.1.1 20170620

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IPVS: ftp: loaded support on port[0] = 21
BUG: KASAN: use-after-free in uprobe_perf_close+0x3e0/0x570
Read of size 4 at addr ffff8801d955264c by task syzkaller474457/4485

CPU: 0 PID: 4485 Comm: syzkaller474457 Not tainted 4.16.0+ #376
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS
Google 01/01/2011
Call Trace:
__dump_stack lib/dump_stack.c:17 [inline]
dump_stack+0x1a7/0x27d lib/dump_stack.c:53
print_address_description+0x73/0x250 mm/kasan/report.c:256
kasan_report_error mm/kasan/report.c:354 [inline]
kasan_report+0x23c/0x360 mm/kasan/report.c:412
__asan_report_load4_noabort+0x14/0x20 mm/kasan/report.c:432
uprobe_perf_close+0x3e0/0x570 kernel/trace/trace_uprobe.c:1048
trace_uprobe_register+0x4cb/0xc00 kernel/trace/trace_uprobe.c:1210
perf_trace_event_close kernel/trace/trace_event_perf.c:186 [inline]
perf_uprobe_destroy+0x9b/0x130 kernel/trace/trace_event_perf.c:335
_free_event+0x3d7/0x11f0 kernel/events/core.c:4445
put_event+0x35/0x40 kernel/events/core.c:4528
perf_event_release_kernel+0x6e8/0xfe0 kernel/events/core.c:4634
perf_release+0x37/0x50 kernel/events/core.c:4644
__fput+0x327/0x7f0 fs/file_table.c:209
____fput+0x15/0x20 fs/file_table.c:243
task_work_run+0x1ab/0x280 kernel/task_work.c:113
exit_task_work include/linux/task_work.h:22 [inline]
do_exit+0xa75/0x2700 kernel/exit.c:865
do_group_exit+0x149/0x400 kernel/exit.c:968
SYSC_exit_group kernel/exit.c:979 [inline]
SyS_exit_group+0x1d/0x20 kernel/exit.c:977
do_syscall_64+0x281/0x940 arch/x86/entry/common.c:287
RIP: 0033:0x445c89
RSP: 002b:00007ffd332827e8 EFLAGS: 00000206 ORIG_RAX: 00000000000000e7
RAX: ffffffffffffffda RBX: 0000000000000000 RCX: 0000000000445c89
RDX: 0000000000445c89 RSI: 0000000000445c89 RDI: 0000000000000001
RBP: 00000000006da018 R08: 0000000000000000 R09: 0000000000406fd0
R10: 0000000000000000 R11: 0000000000000206 R12: 0000000000406f40
R13: 0000000000406fd0 R14: 0000000000000000 R15: 0000000000000000

Allocated by task 4485:
save_stack+0x43/0xd0 mm/kasan/kasan.c:447
set_track mm/kasan/kasan.c:459 [inline]
kasan_kmalloc+0xad/0xe0 mm/kasan/kasan.c:552
kasan_slab_alloc+0x12/0x20 mm/kasan/kasan.c:489
kmem_cache_alloc_node+0x144/0x760 mm/slab.c:3632
alloc_task_struct_node kernel/fork.c:157 [inline]
dup_task_struct kernel/fork.c:770 [inline]
copy_process.part.38+0x1ab9/0x6140 kernel/fork.c:1631
copy_process kernel/fork.c:1606 [inline]
_do_fork+0x1f7/0xfa0 kernel/fork.c:2087
SYSC_clone kernel/fork.c:2194 [inline]
SyS_clone+0x37/0x50 kernel/fork.c:2188
do_syscall_64+0x281/0x940 arch/x86/entry/common.c:287

Freed by task 0:
save_stack+0x43/0xd0 mm/kasan/kasan.c:447
set_track mm/kasan/kasan.c:459 [inline]
__kasan_slab_free+0x11a/0x170 mm/kasan/kasan.c:520
kasan_slab_free+0xe/0x10 mm/kasan/kasan.c:527
__cache_free mm/slab.c:3486 [inline]
kmem_cache_free+0x83/0x2a0 mm/slab.c:3744
free_task_struct kernel/fork.c:162 [inline]
free_task+0x155/0x1b0 kernel/fork.c:391
__put_task_struct+0x24b/0x3e0 kernel/fork.c:657
put_task_struct include/linux/sched/task.h:96 [inline]
delayed_put_task_struct+0xd8/0x3e0 kernel/exit.c:180
__rcu_reclaim kernel/rcu/rcu.h:178 [inline]
rcu_do_batch kernel/rcu/tree.c:2675 [inline]
invoke_rcu_callbacks kernel/rcu/tree.c:2930 [inline]
__rcu_process_callbacks kernel/rcu/tree.c:2897 [inline]
rcu_process_callbacks+0xd6c/0x17b0 kernel/rcu/tree.c:2914
__do_softirq+0x2d7/0xb85 kernel/softirq.c:285

The buggy address belongs to the object at ffff8801d9552600
which belongs to the cache task_struct of size 6016
The buggy address is located 76 bytes inside of
6016-byte region [ffff8801d9552600, ffff8801d9553d80)
The buggy address belongs to the page:
page:ffffea0007655480 count:1 mapcount:0 mapping:ffff8801d9552600 index:0x0
compound_mapcount: 0
flags: 0x2fffc0000008100(slab|head)
raw: 02fffc0000008100 ffff8801d9552600 0000000000000000 0000000100000001
raw: ffffea0007636ba0 ffff8801dad0c248 ffff8801dad46200 0000000000000000
page dumped because: kasan: bad access detected

Memory state around the buggy address:
ffff8801d9552500: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
ffff8801d9552580: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
> ffff8801d9552600: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
ffff8801d9552680: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
ffff8801d9552700: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb

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