Hi,

I've got the following error report while running the syzkaller fuzzer:

==================================================================
BUG: KASAN: use-after-free in l2tp_ip6_close+0x239/0x2a0 at addr
ffff8800677276d8
Write of size 8 by task a.out/8668
CPU: 0 PID: 8668 Comm: a.out Not tainted 4.9.0-rc4+ #354
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Bochs 01/01/2011
 ffff8800694d7b00 ffffffff81b46a64 ffff88006adb5780 ffff8800677276c0
 ffff880067727c68 ffff8800677276c0 ffff8800694d7b28 ffffffff8150a86c
 ffff8800694d7bb8 ffff88006adb5780 ffff8800e77276d8 ffff8800694d7ba8
Call Trace:
 [<     inline     >] __dump_stack lib/dump_stack.c:15
 [<ffffffff81b46a64>] dump_stack+0xb3/0x10f lib/dump_stack.c:51
 [<ffffffff8150a86c>] kasan_object_err+0x1c/0x70 mm/kasan/report.c:156
 [<     inline     >] print_address_description mm/kasan/report.c:194
 [<ffffffff8150ab07>] kasan_report_error+0x1f7/0x4d0 mm/kasan/report.c:283
 [<     inline     >] kasan_report mm/kasan/report.c:303
 [<ffffffff8150b01e>] __asan_report_store8_noabort+0x3e/0x40
mm/kasan/report.c:329
 [<     inline     >] __write_once_size ./include/linux/compiler.h:272
 [<     inline     >] __hlist_del ./include/linux/list.h:622
 [<     inline     >] hlist_del_init ./include/linux/list.h:637
 [<ffffffff83825f49>] l2tp_ip6_close+0x239/0x2a0 net/l2tp/l2tp_ip6.c:239
 [<ffffffff8316b31f>] inet_release+0xef/0x1c0 net/ipv4/af_inet.c:415
 [<ffffffff832cd4d0>] inet6_release+0x50/0x70 net/ipv6/af_inet6.c:422
 [<ffffffff82b6d89e>] sock_release+0x8e/0x1d0 net/socket.c:570
 [<ffffffff82b6d9f6>] sock_close+0x16/0x20 net/socket.c:1017
 [<ffffffff81524bdd>] __fput+0x29d/0x720 fs/file_table.c:208
 [<ffffffff815250e5>] ____fput+0x15/0x20 fs/file_table.c:244
 [<ffffffff81172928>] task_work_run+0xf8/0x170 kernel/task_work.c:116
 [<     inline     >] exit_task_work ./include/linux/task_work.h:21
 [<ffffffff8111bda3>] do_exit+0x883/0x2ac0 kernel/exit.c:828
 [<ffffffff8112234e>] do_group_exit+0x10e/0x340 kernel/exit.c:931
 [<     inline     >] SYSC_exit_group kernel/exit.c:942
 [<ffffffff8112259d>] SyS_exit_group+0x1d/0x20 kernel/exit.c:940
 [<ffffffff83fc1501>] entry_SYSCALL_64_fastpath+0x1f/0xc2
arch/x86/entry/entry_64.S:209
Object at ffff8800677276c0, in cache L2TP/IPv6 size: 1448
Allocated:
PID = 8692
[<ffffffff8107e236>] save_stack_trace+0x16/0x20 arch/x86/kernel/stacktrace.c:57
[<ffffffff81509bd6>] save_stack+0x46/0xd0 mm/kasan/kasan.c:495
[<     inline     >] set_track mm/kasan/kasan.c:507
[<ffffffff81509e4b>] kasan_kmalloc+0xab/0xe0 mm/kasan/kasan.c:598
[<ffffffff8150a3b2>] kasan_slab_alloc+0x12/0x20 mm/kasan/kasan.c:537
[<     inline     >] slab_post_alloc_hook mm/slab.h:417
[<     inline     >] slab_alloc_node mm/slub.c:2708
[<     inline     >] slab_alloc mm/slub.c:2716
[<ffffffff81505064>] kmem_cache_alloc+0xb4/0x270 mm/slub.c:2721
[<ffffffff82b77ca9>] sk_prot_alloc+0x69/0x2b0 net/core/sock.c:1327
[<ffffffff82b80898>] sk_alloc+0x38/0xaf0 net/core/sock.c:1389
[<ffffffff832cef05>] inet6_create+0x2e5/0xf60 net/ipv6/af_inet6.c:182
[<ffffffff82b7301f>] __sock_create+0x37f/0x640 net/socket.c:1153
[<     inline     >] sock_create net/socket.c:1193
[<     inline     >] SYSC_socket net/socket.c:1223
[<ffffffff82b73510>] SyS_socket+0xf0/0x1b0 net/socket.c:1203
[<ffffffff83fc1501>] entry_SYSCALL_64_fastpath+0x1f/0xc2
arch/x86/entry/entry_64.S:209
Freed:
PID = 8668
[<ffffffff8107e236>] save_stack_trace+0x16/0x20 arch/x86/kernel/stacktrace.c:57
[<ffffffff81509bd6>] save_stack+0x46/0xd0 mm/kasan/kasan.c:495
[<     inline     >] set_track mm/kasan/kasan.c:507
[<ffffffff8150a433>] kasan_slab_free+0x73/0xc0 mm/kasan/kasan.c:571
[<     inline     >] slab_free_hook mm/slub.c:1352
[<     inline     >] slab_free_freelist_hook mm/slub.c:1374
[<     inline     >] slab_free mm/slub.c:2951
[<ffffffff81506263>] kmem_cache_free+0xb3/0x2c0 mm/slub.c:2973
[<     inline     >] sk_prot_free net/core/sock.c:1370
[<ffffffff82b7c669>] __sk_destruct+0x319/0x480 net/core/sock.c:1445
[<ffffffff82b82b94>] sk_destruct+0x44/0x80 net/core/sock.c:1453
[<ffffffff82b82c24>] __sk_free+0x54/0x230 net/core/sock.c:1461
[<ffffffff82b82e23>] sk_free+0x23/0x30 net/core/sock.c:1472
[<     inline     >] sock_put ./include/net/sock.h:1591
[<ffffffff82b84b04>] sk_common_release+0x294/0x3e0 net/core/sock.c:2745
[<ffffffff83825f19>] l2tp_ip6_close+0x209/0x2a0 net/l2tp/l2tp_ip6.c:243
[<ffffffff8316b31f>] inet_release+0xef/0x1c0 net/ipv4/af_inet.c:415
[<ffffffff832cd4d0>] inet6_release+0x50/0x70 net/ipv6/af_inet6.c:422
[<ffffffff82b6d89e>] sock_release+0x8e/0x1d0 net/socket.c:570
[<ffffffff82b6d9f6>] sock_close+0x16/0x20 net/socket.c:1017
[<ffffffff81524bdd>] __fput+0x29d/0x720 fs/file_table.c:208
[<ffffffff815250e5>] ____fput+0x15/0x20 fs/file_table.c:244
[<ffffffff81172928>] task_work_run+0xf8/0x170 kernel/task_work.c:116
[<     inline     >] exit_task_work ./include/linux/task_work.h:21
[<ffffffff8111bda3>] do_exit+0x883/0x2ac0 kernel/exit.c:828
[<ffffffff8112234e>] do_group_exit+0x10e/0x340 kernel/exit.c:931
[<     inline     >] SYSC_exit_group kernel/exit.c:942
[<ffffffff8112259d>] SyS_exit_group+0x1d/0x20 kernel/exit.c:940
[<ffffffff83fc1501>] entry_SYSCALL_64_fastpath+0x1f/0xc2
arch/x86/entry/entry_64.S:209
Memory state around the buggy address:
 ffff880067727580: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
 ffff880067727600: fb fb fb fb fb fc fc fc fc fc fc fc fc fc fc fc
>ffff880067727680: fc fc fc fc fc fc fc fc fb fb fb fb fb fb fb fb
                                                    ^
 ffff880067727700: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
 ffff880067727780: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
==================================================================

To reproduce run the attached program in a tight parallel loop using
stress (https://godoc.org/golang.org/x/tools/cmd/stress):
$ gcc -lpthread tmp.c
$ ./stress ./a.out

On commit bc33b0ca11e3df467777a4fa7639ba488c9d4911 (Nov 5).

Thanks!