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BUG: KASAN: use-after-free in erspan_build_header+0x3bf/0x3d0 net/ipv4/ip_gre.c:698
Read of size 2 at addr ffff8801ce42550b by task syz-executor2/12791

CPU: 1 PID: 12791 Comm: syz-executor2 Not tainted 4.15.0-rc9+ #284
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+0x194/0x257 lib/dump_stack.c:53
 print_address_description+0x73/0x250 mm/kasan/report.c:252
 kasan_report_error mm/kasan/report.c:351 [inline]
 kasan_report+0x25b/0x340 mm/kasan/report.c:409
 __asan_report_load_n_noabort+0xf/0x20 mm/kasan/report.c:440
 erspan_build_header+0x3bf/0x3d0 net/ipv4/ip_gre.c:698
 erspan_xmit+0x3b8/0x13b0 net/ipv4/ip_gre.c:740
 __netdev_start_xmit include/linux/netdevice.h:4042 [inline]
 netdev_start_xmit include/linux/netdevice.h:4051 [inline]
 packet_direct_xmit+0x315/0x6b0 net/packet/af_packet.c:266
 packet_snd net/packet/af_packet.c:2943 [inline]
 packet_sendmsg+0x3aed/0x60b0 net/packet/af_packet.c:2968
 sock_sendmsg_nosec net/socket.c:638 [inline]
 sock_sendmsg+0xca/0x110 net/socket.c:648
 SYSC_sendto+0x361/0x5c0 net/socket.c:1729
 SyS_sendto+0x40/0x50 net/socket.c:1697
 entry_SYSCALL_64_fastpath+0x29/0xa0
RIP: 0033:0x453299
RSP: 002b:00007f4035410c58 EFLAGS: 00000212 ORIG_RAX: 000000000000002c
RAX: ffffffffffffffda RBX: 000000000071bea0 RCX: 0000000000453299
RDX: 0000000000000000 RSI: 0000000020003fd9 RDI: 0000000000000013
RBP: 000000000000059d R08: 0000000020008000 R09: 000000000000001c
R10: 000000000000c080 R11: 0000000000000212 R12: 00000000006f7758
R13: 00000000ffffffff R14: 00007f40354116d4 R15: 0000000000000000

The buggy address belongs to the page:
page:ffffea0007390940 count:0 mapcount:-127 mapping:          (null) index:0x0
flags: 0x2fffc0000000000()
raw: 02fffc0000000000 0000000000000000 0000000000000000 00000000ffffff80
raw: ffffea0007401920 ffffea0007354860 0000000000000000 0000000000000000
page dumped because: kasan: bad access detected

Memory state around the buggy address:
 ffff8801ce425400: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
 ffff8801ce425480: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
>ffff8801ce425500: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
                      ^
 ffff8801ce425580: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
 ffff8801ce425600: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
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