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SubjectKASAN: slab-out-of-bounds Read in usage_accumulate

syzbot found the following crash on:

HEAD commit: 156c0591 Merge tag 'linux-kselftest-5.2-rc4' of git://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: slab-out-of-bounds in usage_accumulate+0x9e/0xb0
Read of size 8 at addr ffff8880a59cfed0 by task syz-executor.1/9366

CPU: 1 PID: 9366 Comm: syz-executor.1 Not tainted 5.2.0-rc3+ #20
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS
Google 01/01/2011
Call Trace:

Allocated by task 0:
(stack is not available)

Freed by task 0:
(stack is not available)

The buggy address belongs to the object at ffff8880a59ce6c0
which belongs to the cache kmalloc-4k of size 4096
The buggy address is located 2064 bytes to the right of
4096-byte region [ffff8880a59ce6c0, ffff8880a59cf6c0)
The buggy address belongs to the page:
page:ffffea0002967380 refcount:1 mapcount:0 mapping:ffff8880aa400dc0
index:0x0 compound_mapcount: 0
flags: 0x1fffc0000010200(slab|head)
raw: 01fffc0000010200 ffffea000296a008 ffffea000233fe08 ffff8880aa400dc0
raw: 0000000000000000 ffff8880a59ce6c0 0000000100000001 0000000000000000
page dumped because: kasan: bad access detected

Memory state around the buggy address:
ffff8880a59cfd80: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
ffff8880a59cfe00: fc fc fc fc f1 f1 f1 f1 00 f2 f2 f2 00 f2 f2 f2
> ffff8880a59cfe80: 00 f2 f2 f2 00 f2 f2 f2 fc fc fc fc 00 00 00 f2
ffff8880a59cff00: f2 f2 f2 f2 fc fc fc fc 00 00 00 f3 f3 f3 f3 f3
ffff8880a59cff80: 00 00 00 00 00 00 fc fc fc fc fc fc fc fc fc fc

This bug is generated by a bot. It may contain errors.
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