KASAN: slab-out-of-bounds Read in bpf_csum_update

[syzbot]

Hello,

syzbot found the following crash on:

HEAD commit:    0512e0134582 Merge tag 'xfs-4.17-fixes-3' of git://git.ker..
git tree:       upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=17eb2d7b800000
kernel config:  https://syzkaller.appspot.com/x/.config?x=968b0b23c7854c0b
dashboard link: https://syzkaller.appspot.com/bug?extid=efae31b384d5badbd620
compiler:       gcc (GCC) 8.0.1 20180413 (experimental)
syzkaller repro:https://syzkaller.appspot.com/x/repro.syz?x=162c6def800000
C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=14fe3db7800000

IMPORTANT: if you fix the bug, please add the following tag to the commit:
Reported-by: syzbot+efae31b384d5badbd620@xxxxxxxxxxxxxxxxxxxxxxxxx

random: sshd: uninitialized urandom read (32 bytes read)
random: sshd: uninitialized urandom read (32 bytes read)
random: sshd: uninitialized urandom read (32 bytes read)
random: sshd: uninitialized urandom read (32 bytes read)
==================================================================
BUG: KASAN: slab-out-of-bounds in ____bpf_csum_update  
net/core/filter.c:1679 [inline]
BUG: KASAN: slab-out-of-bounds in bpf_csum_update+0xb4/0xc0  
net/core/filter.c:1673
Read of size 1 at addr ffff8801d9235b50 by task syz-executor507/4513

CPU: 0 PID: 4513 Comm: syz-executor507 Not tainted 4.17.0-rc7+ #78
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+0x1b9/0x294 lib/dump_stack.c:113
 print_address_description+0x6c/0x20b mm/kasan/report.c:256
 kasan_report_error mm/kasan/report.c:354 [inline]
 kasan_report.cold.7+0x242/0x2fe mm/kasan/report.c:412
 __asan_report_load1_noabort+0x14/0x20 mm/kasan/report.c:430
 ____bpf_csum_update net/core/filter.c:1679 [inline]
 bpf_csum_update+0xb4/0xc0 net/core/filter.c:1673

Allocated by task 0:
(stack is not available)

Freed by task 0:
(stack is not available)

The buggy address belongs to the object at ffff8801d9235a40
 which belongs to the cache skbuff_head_cache of size 232
The buggy address is located 40 bytes to the right of
 232-byte region [ffff8801d9235a40, ffff8801d9235b28)
The buggy address belongs to the page:
page:ffffea0007648d40 count:1 mapcount:0 mapping:ffff8801d9235040 index:0x0
flags: 0x2fffc0000000100(slab)
raw: 02fffc0000000100 ffff8801d9235040 0000000000000000 000000010000000c
raw: ffffea00074360a0 ffff8801d944d848 ffff8801d9bdd6c0 0000000000000000
page dumped because: kasan: bad access detected

Memory state around the buggy address:
 ffff8801d9235a00: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
 ffff8801d9235a80: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
> ffff8801d9235b00: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
                                                 ^
 ffff8801d9235b80: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
 ffff8801d9235c00: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
==================================================================


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