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

Hello,

syzbot hit the following crash on upstream commit
a0ec1ded22e6a6bc41981fae22406835b006a66e (Mon Jan 22 20:44:52 2018 +0000)
orangefs: initialize op on loop restart in orangefs_devreq_read

So far this crash happened 3 times on upstream.
C reproducer is attached.
syzkaller reproducer is attached.
Raw console output is attached.
compiler: gcc (GCC) 7.1.1 20170620
.config is attached.
user-space arch: i386

IMPORTANT: if you fix the bug, please add the following tag to the commit:
Reported-by: syzbot+f0ddeb2b032a8e1d9098@syzkaller.appspotmail.com
It will help syzbot understand when the bug is fixed. See footer for  
details.
If you forward the report, please keep this part and the footer.

==================================================================
BUG: KASAN: slab-out-of-bounds in erspan_build_header+0x3bf/0x3d0  
net/ipv4/ip_gre.c:698
Read of size 2 at addr ffff8801d9b9804b by task syzkaller377964/3654

CPU: 0 PID: 3654 Comm: syzkaller377964 Not tainted 4.15.0-rc9+ #185
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
  do_syscall_32_irqs_on arch/x86/entry/common.c:327 [inline]
  do_fast_syscall_32+0x3ee/0xf9d arch/x86/entry/common.c:389
  entry_SYSENTER_compat+0x54/0x63 arch/x86/entry/entry_64_compat.S:129
RIP: 0023:0xf7fcfc79
RSP: 002b:00000000ffc6976c EFLAGS: 00000286 ORIG_RAX: 0000000000000171
RAX: ffffffffffffffda RBX: 0000000000000004 RCX: 0000000020011000
RDX: 0000000000000000 RSI: 0000000000000000 RDI: 0000000020008000
RBP: 000000000000001c R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000000 R12: 0000000000000000
R13: 0000000000000000 R14: 0000000000000000 R15: 0000000000000000

Allocated by task 2203:
  save_stack+0x43/0xd0 mm/kasan/kasan.c:447
  set_track mm/kasan/kasan.c:459 [inline]
  kasan_kmalloc+0xad/0xe0 mm/kasan/kasan.c:551
  kasan_slab_alloc+0x12/0x20 mm/kasan/kasan.c:489
  kmem_cache_alloc+0x12e/0x760 mm/slab.c:3544
  kmem_cache_zalloc include/linux/slab.h:678 [inline]
  copy_signal+0xfe/0xa11 kernel/fork.c:1396
  copy_process.part.38+0x1ed2/0x4b20 kernel/fork.c:1749
  copy_process kernel/fork.c:1565 [inline]
  _do_fork+0x1f7/0xfe0 kernel/fork.c:2044
  SYSC_clone kernel/fork.c:2154 [inline]
  SyS_clone+0x37/0x50 kernel/fork.c:2148
  do_syscall_64+0x273/0x920 arch/x86/entry/common.c:285
  return_from_SYSCALL_64+0x0/0x75

Freed by task 0:
  save_stack+0x43/0xd0 mm/kasan/kasan.c:447
  set_track mm/kasan/kasan.c:459 [inline]
  kasan_slab_free+0x71/0xc0 mm/kasan/kasan.c:524
  __cache_free mm/slab.c:3488 [inline]
  kmem_cache_free+0x83/0x2a0 mm/slab.c:3746
  free_signal_struct kernel/fork.c:403 [inline]
  put_signal_struct kernel/fork.c:409 [inline]
  __put_task_struct+0x33f/0x580 kernel/fork.c:423
  put_task_struct include/linux/sched/task.h:96 [inline]
  delayed_put_task_struct+0xca/0x3d0 kernel/exit.c:180
  __rcu_reclaim kernel/rcu/rcu.h:195 [inline]
  rcu_do_batch kernel/rcu/tree.c:2758 [inline]
  invoke_rcu_callbacks kernel/rcu/tree.c:3012 [inline]
  __rcu_process_callbacks kernel/rcu/tree.c:2979 [inline]
  rcu_process_callbacks+0xd6c/0x17f0 kernel/rcu/tree.c:2996
  __do_softirq+0x2d7/0xb85 kernel/softirq.c:285

The buggy address belongs to the object at ffff8801d9b980c0
  which belongs to the cache signal_cache of size 1296
The buggy address is located 117 bytes to the left of
  1296-byte region [ffff8801d9b980c0, ffff8801d9b985d0)
The buggy address belongs to the page:
page:ffffea000766e600 count:1 mapcount:0 mapping:ffff8801d9b980c0 index:0x0  
compound_mapcount: 0
flags: 0x2fffc0000008100(slab|head)
raw: 02fffc0000008100 ffff8801d9b980c0 0000000000000000 0000000100000005
raw: ffffea000766c420 ffffea000766da20 ffff8801dae27e40 0000000000000000
page dumped because: kasan: bad access detected

Memory state around the buggy address:
  ffff8801d9b97f00: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
  ffff8801d9b97f80: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc

ffff8801d9b98000: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc

                                               ^
  ffff8801d9b98080: fc fc fc fc fc fc fc fc fb fb fb fb fb fb fb fb
  ffff8801d9b98100: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
==================================================================


---
This bug is generated by a dumb bot. It may contain errors.
See https://goo.gl/tpsmEJ for details.
Direct all questions to syzkaller@googlegroups.com.

syzbot will keep track of this bug report.
If you forgot to add the Reported-by tag, once the fix for this bug is  
merged
into any tree, please reply to this email with:
#syz fix: exact-commit-title
If you want to test a patch for this bug, please reply with:
#syz test: git://repo/address.git branch
and provide the patch inline or as an attachment.
To mark this as a duplicate of another syzbot report, please reply with:
#syz dup: exact-subject-of-another-report
If it's a one-off invalid bug report, please reply with:
#syz invalid
Note: if the crash happens again, it will cause creation of a new bug  
report.
Note: all commands must start from beginning of the line in the email body.