==================================================================
BUG: KASAN: use-after-free in skb_copy_datagram_iter+0x8b9/0x8f0 net/core/datagram.c:442
Read of size 1 at addr ffff8881a3825c42 by task syz-executor.5/4793

CPU: 0 PID: 4793 Comm: syz-executor.5 Not tainted 4.14.123+ #5
Call Trace:
 __dump_stack lib/dump_stack.c:17 [inline]
 dump_stack+0xb9/0x10e lib/dump_stack.c:53
 print_address_description+0x60/0x226 mm/kasan/report.c:252
 kasan_report_error mm/kasan/report.c:351 [inline]
 kasan_report mm/kasan/report.c:409 [inline]
 kasan_report.cold+0xae/0x2d5 mm/kasan/report.c:393

Allocated by task 4842:
 save_stack mm/kasan/kasan.c:447 [inline]
 set_track mm/kasan/kasan.c:459 [inline]
 kasan_kmalloc.part.0+0x4f/0xd0 mm/kasan/kasan.c:551
 slab_post_alloc_hook mm/slab.h:442 [inline]
 slab_alloc_node mm/slub.c:2723 [inline]
 slab_alloc mm/slub.c:2731 [inline]
 __kmalloc_track_caller+0xf1/0x310 mm/slub.c:4288
 __kmalloc_reserve.isra.0+0x2d/0xc0 net/core/skbuff.c:137
 __alloc_skb+0x105/0x550 net/core/skbuff.c:205
 alloc_skb include/linux/skbuff.h:980 [inline]
 sock_wmalloc+0xa5/0xf0 net/core/sock.c:1928
 packet_sendmsg_spkt+0x3bb/0x1210 net/packet/af_packet.c:1962
 sock_sendmsg_nosec net/socket.c:646 [inline]
 sock_sendmsg+0xb7/0x100 net/socket.c:656
 ___sys_sendmsg+0x368/0x890 net/socket.c:2062
 __sys_sendmmsg+0x13c/0x360 net/socket.c:2152
 SYSC_sendmmsg net/socket.c:2183 [inline]
 SyS_sendmmsg+0x2f/0x50 net/socket.c:2178
 do_syscall_64+0x19b/0x510 arch/x86/entry/common.c:292

Freed by task 4842:
 save_stack mm/kasan/kasan.c:447 [inline]
 set_track mm/kasan/kasan.c:459 [inline]
 kasan_slab_free+0xb0/0x190 mm/kasan/kasan.c:524
 slab_free_hook mm/slub.c:1389 [inline]
 slab_free_freelist_hook mm/slub.c:1410 [inline]
 slab_free mm/slub.c:2966 [inline]
 kfree+0xf5/0x310 mm/slub.c:3897
 skb_free_head+0x83/0xa0 net/core/skbuff.c:554
 skb_release_data+0x4ae/0x730 net/core/skbuff.c:574
 skb_release_all+0x46/0x60 net/core/skbuff.c:631
 __kfree_skb net/core/skbuff.c:645 [inline]
 kfree_skb+0xc5/0x350 net/core/skbuff.c:663
 ip6_tnl_start_xmit+0x7ef/0x1b00 net/ipv6/ip6_tunnel.c:1419
 __netdev_start_xmit include/linux/netdevice.h:4033 [inline]
 netdev_start_xmit include/linux/netdevice.h:4042 [inline]
 xmit_one net/core/dev.c:3009 [inline]
 dev_hard_start_xmit+0x18e/0x890 net/core/dev.c:3025
 __dev_queue_xmit+0x11b1/0x1cd0 net/core/dev.c:3525

The buggy address belongs to the object at ffff8881a3825b80
 which belongs to the cache kmalloc-512 of size 512
The buggy address is located 194 bytes inside of
 512-byte region [ffff8881a3825b80, ffff8881a3825d80)
The buggy address belongs to the page:
page:ffffea00068e0900 count:1 mapcount:0 mapping:          (null) index:0x0 compound_mapcount: 0
flags: 0x4000000000010200(slab|head)
raw: 4000000000010200 0000000000000000 0000000000000000 00000001800c000c
raw: 0000000000000000 0000000b00000001 ffff8881da802c00 0000000000000000
page dumped because: kasan: bad access detected

Memory state around the buggy address:
 ffff8881a3825b00: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
 ffff8881a3825b80: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
>ffff8881a3825c00: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
audit: type=1326 audit(2000000318.990:2423): auid=4294967295 uid=0 gid=0 ses=4294967295 subj=unconfined_u:system_r:insmod_t:s0-s0:c0.c1023 pid=4820 comm="syz-executor.4" exe="/root/syz-executor.4" sig=9 arch=c000003e syscall=228 compat=0 ip=0x45c0ba code=0x0
                                           ^
 ffff8881a3825c80: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
 ffff8881a3825d00: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
==================================================================