KASAN: use-after-free Read in rt6_dump_route

Hello,

syzbot hit the following crash on net-next commit
564737f981fb4b4b3266901508bb9b90d9d43de8

Unfortunately, I don't have any reproducer for this crash yet.
Raw console output is attached.
compiler: gcc (GCC) 7.1.1 20170620
.config is attached.

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Reported-by: syzbot+f87d3d3b24e8c0150dd6@syzkaller.appspotmail.com
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==================================================================
BUG: KASAN: use-after-free in rt6_dump_route+0x287/0x2f0  
net/ipv6/route.c:4436
Read of size 4 at addr ffff8801ca95dc14 by task syz-executor2/9561

CPU: 1 PID: 9561 Comm: syz-executor2 Not tainted 4.15.0-rc7+ #189
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_load4_noabort+0x14/0x20 mm/kasan/report.c:429
  rt6_dump_route+0x287/0x2f0 net/ipv6/route.c:4436
  fib6_dump_node+0x100/0x1a0 net/ipv6/ip6_fib.c:431
  fib6_walk_continue+0x46c/0x8a0 net/ipv6/ip6_fib.c:1838
  fib6_walk+0x91/0xf0 net/ipv6/ip6_fib.c:1886
  fib6_dump_table net/ipv6/ip6_fib.c:490 [inline]
  inet6_dump_fib+0x83d/0xe10 net/ipv6/ip6_fib.c:563
  rtnl_dump_all+0x162/0x460 net/core/rtnetlink.c:3114
  netlink_dump+0x492/0xcf0 net/netlink/af_netlink.c:2222
  __netlink_dump_start+0x4f0/0x6d0 net/netlink/af_netlink.c:2319
  netlink_dump_start include/linux/netlink.h:214 [inline]
  rtnetlink_rcv_msg+0x7f0/0xb10 net/core/rtnetlink.c:4493
  netlink_rcv_skb+0x224/0x470 net/netlink/af_netlink.c:2441
  rtnetlink_rcv+0x1c/0x20 net/core/rtnetlink.c:4548
  netlink_unicast_kernel net/netlink/af_netlink.c:1308 [inline]
  netlink_unicast+0x4c4/0x6b0 net/netlink/af_netlink.c:1334
  netlink_sendmsg+0xa4a/0xe60 net/netlink/af_netlink.c:1897
  sock_sendmsg_nosec net/socket.c:630 [inline]
  sock_sendmsg+0xca/0x110 net/socket.c:640
  sock_write_iter+0x31a/0x5d0 net/socket.c:909
  call_write_iter include/linux/fs.h:1772 [inline]
  new_sync_write fs/read_write.c:469 [inline]
  __vfs_write+0x684/0x970 fs/read_write.c:482
  vfs_write+0x189/0x510 fs/read_write.c:544
  SYSC_write fs/read_write.c:589 [inline]
  SyS_write+0xef/0x220 fs/read_write.c:581
  entry_SYSCALL_64_fastpath+0x23/0x9a
RIP: 0033:0x452cf9
RSP: 002b:00007f4663f06c58 EFLAGS: 00000212 ORIG_RAX: 0000000000000001
RAX: ffffffffffffffda RBX: 000000000071bf58 RCX: 0000000000452cf9
RDX: 0000000000000024 RSI: 000000002068cfdc RDI: 0000000000000013
RBP: 0000000000000579 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000212 R12: 00000000006f63f8
R13: 00000000ffffffff R14: 00007f4663f076d4 R15: 0000000000000001

Allocated by task 5943:
  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
  dst_alloc+0x11f/0x1a0 net/core/dst.c:104
  __ip6_dst_alloc+0x35/0x90 net/ipv6/route.c:361
  ip6_dst_alloc+0x29/0xb0 net/ipv6/route.c:376
  ip6_route_info_create+0x4ff/0x2e20 net/ipv6/route.c:2538
  ip6_route_add+0xa2/0x190 net/ipv6/route.c:2779
  ipv6_route_ioctl+0x4db/0x6b0 net/ipv6/route.c:3299
  inet6_ioctl+0xef/0x1e0 net/ipv6/af_inet6.c:520
  sock_do_ioctl+0x65/0xb0 net/socket.c:958
  sock_ioctl+0x2c2/0x440 net/socket.c:1055
  vfs_ioctl fs/ioctl.c:46 [inline]
  do_vfs_ioctl+0x1b1/0x1520 fs/ioctl.c:686
  SYSC_ioctl fs/ioctl.c:701 [inline]
  SyS_ioctl+0x8f/0xc0 fs/ioctl.c:692
  entry_SYSCALL_64_fastpath+0x23/0x9a

Freed by task 8598:
  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
  dst_destroy+0x257/0x370 net/core/dst.c:140
  dst_destroy_rcu+0x16/0x20 net/core/dst.c:153
  __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 ffff8801ca95db40
  which belongs to the cache ip6_dst_cache of size 320
The buggy address is located 212 bytes inside of
  320-byte region [ffff8801ca95db40, ffff8801ca95dc80)
The buggy address belongs to the page:
page:ffffea00072a5740 count:1 mapcount:0 mapping:ffff8801ca95d0c0  
index:0xffff8801ca95d3c0
flags: 0x2fffc0000000100(slab)
raw: 02fffc0000000100 ffff8801ca95d0c0 ffff8801ca95d3c0 0000000100000008
raw: ffffea00074c1ce0 ffffea0007663b60 ffff8801d326a900 0000000000000000
page dumped because: kasan: bad access detected

Memory state around the buggy address:
  ffff8801ca95db00: fc fc fc fc fc fc fc fc fb fb fb fb fb fb fb fb
  ffff8801ca95db80: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb

ffff8801ca95dc00: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb

                          ^
  ffff8801ca95dc80: fc fc fc fc fc fc fc fc 00 00 00 00 00 00 00 00
  ffff8801ca95dd00: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
==================================================================


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