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SubjectKASAN: use-after-free Read in psi_task_change

syzbot found the following crash on:

HEAD commit: bed38c3e Merge tag 'powerpc-5.3-2' of git://
git tree: upstream
console output:
kernel config:
dashboard link:
compiler: gcc (GCC) 9.0.0 20181231 (experimental)
syz repro:

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

BUG: KASAN: use-after-free in raw_write_seqcount_begin
include/linux/seqlock.h:228 [inline]
BUG: KASAN: use-after-free in write_seqcount_begin_nested
include/linux/seqlock.h:376 [inline]
BUG: KASAN: use-after-free in write_seqcount_begin
include/linux/seqlock.h:382 [inline]
BUG: KASAN: use-after-free in psi_group_change kernel/sched/psi.c:689
BUG: KASAN: use-after-free in psi_task_change+0x89b/0x9d0
Read of size 4 at addr ffff888039cd9460 by task syz-executor.3/9140

CPU: 1 PID: 9140 Comm: syz-executor.3 Not tainted 5.3.0-rc1+ #83
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
print_address_description.cold+0xd4/0x306 mm/kasan/report.c:351
__kasan_report.cold+0x1b/0x36 mm/kasan/report.c:482
kasan_report+0x12/0x17 mm/kasan/common.c:612
__asan_report_load4_noabort+0x14/0x20 mm/kasan/generic_report.c:131
raw_write_seqcount_begin include/linux/seqlock.h:228 [inline]
write_seqcount_begin_nested include/linux/seqlock.h:376 [inline]
write_seqcount_begin include/linux/seqlock.h:382 [inline]
psi_group_change kernel/sched/psi.c:689 [inline]
psi_task_change+0x89b/0x9d0 kernel/sched/psi.c:780
psi_dequeue kernel/sched/stats.h:100 [inline]
dequeue_task kernel/sched/core.c:1191 [inline]
deactivate_task+0x2f3/0x420 kernel/sched/core.c:1215
__schedule+0xecc/0x1580 kernel/sched/core.c:3844
schedule+0xa8/0x270 kernel/sched/core.c:3944
freezable_schedule include/linux/freezer.h:172 [inline]
do_nanosleep+0x201/0x6a0 kernel/time/hrtimer.c:1679
hrtimer_nanosleep+0x2a6/0x570 kernel/time/hrtimer.c:1733
__do_sys_nanosleep kernel/time/hrtimer.c:1767 [inline]
__se_sys_nanosleep kernel/time/hrtimer.c:1754 [inline]
__x64_sys_nanosleep+0x1a6/0x220 kernel/time/hrtimer.c:1754
do_syscall_64+0xfd/0x6a0 arch/x86/entry/common.c:296
RIP: 0033:0x457cd0
Code: c0 5b 5d c3 66 0f 1f 44 00 00 8b 04 24 48 83 c4 18 5b 5d c3 66 0f 1f
44 00 00 83 3d 81 ea 61 00 00 75 14 b8 23 00 00 00 0f 05 <48> 3d 01 f0 ff
ff 0f 83 24 d3 fb ff c3 48 83 ec 08 e8 ea 46 00 00
RSP: 002b:00007fff3ad23fa8 EFLAGS: 00000246 ORIG_RAX: 0000000000000023
RAX: ffffffffffffffda RBX: 000000000001555c RCX: 0000000000457cd0
RDX: 0000000000000000 RSI: 0000000000000000 RDI: 00007fff3ad23fb0
RBP: 0000000000000038 R08: 0000000000000001 R09: 00005555557ca940
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000008
R13: 00007fff3ad24000 R14: 0000000000015503 R15: 00007fff3ad24010

The buggy address belongs to the page:
page:ffffea0000e73640 refcount:0 mapcount:0 mapping:0000000000000000
flags: 0x1fffc0000000000()
raw: 01fffc0000000000 ffffea0000e73648 ffffea0000e73648 0000000000000000
raw: 0000000000000000 0000000000000000 00000000ffffffff 0000000000000000
page dumped because: kasan: bad access detected

Memory state around the buggy address:
ffff888039cd9300: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
ffff888039cd9380: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
> ffff888039cd9400: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
ffff888039cd9480: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
ffff888039cd9500: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff

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