KASAN: use-after-free Read in bpf_csum_update

[syzbot]

Hello,

syzbot found the following crash on:

HEAD commit:    bcece5dc40b9 bpf: Change bpf_fib_lookup to return -EAFNOSU..
git tree:       bpf-next
console output: https://syzkaller.appspot.com/x/log.txt?x=161e2c6f800000
kernel config:  https://syzkaller.appspot.com/x/.config?x=e4078980b886800c
dashboard link: https://syzkaller.appspot.com/bug?extid=3d0b2441dbb71751615e
compiler:       gcc (GCC) 8.0.1 20180413 (experimental)
syzkaller repro:https://syzkaller.appspot.com/x/repro.syz?x=17cb5adf800000
C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=17ebf19f800000

IMPORTANT: if you fix the bug, please add the following tag to the commit:
Reported-by: syzbot+3d0b2441dbb71751615e@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: use-after-free in ____bpf_csum_update net/core/filter.c:1907  
[inline]
BUG: KASAN: use-after-free in bpf_csum_update+0xb4/0xc0  
net/core/filter.c:1901
Read of size 1 at addr ffff8801ad062f10 by task syz-executor354/4488

CPU: 1 PID: 4488 Comm: syz-executor354 Not tainted 4.17.0-rc6+ #29
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:1907 [inline]
 bpf_csum_update+0xb4/0xc0 net/core/filter.c:1901

The buggy address belongs to the page:
page:ffffea0006b41880 count:0 mapcount:0 mapping:0000000000000000 index:0x0
flags: 0x2fffc0000000000()
raw: 02fffc0000000000 0000000000000000 0000000000000000 00000000ffffffff
raw: ffffea0006b41820 ffffea0006b671a0 0000000000000000 0000000000000000
page dumped because: kasan: bad access detected

Memory state around the buggy address:
 ffff8801ad062e00: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
 ffff8801ad062e80: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
> ffff8801ad062f00: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
                         ^
 ffff8801ad062f80: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
 ffff8801ad063000: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
==================================================================


---
This bug is generated by a bot. It may contain errors.
See https://goo.gl/tpsmEJ for more information about syzbot.
syzbot engineers can be reached at syzkaller@xxxxxxxxxxxxxxxxx

syzbot will keep track of this bug report. See:
https://goo.gl/tpsmEJ#bug-status-tracking for how to communicate with  
syzbot.
syzbot can test patches for this bug, for details see:
https://goo.gl/tpsmEJ#testing-patches