==================================================================
BUG: KASAN: use-after-free in l2tp_session_queue_purge+0xf4/0x100 net/l2tp/l2tp_core.c:830
Read of size 4 at addr ffff8800b485c000 by task syz-executor090/3935

CPU: 0 PID: 3935 Comm: syz-executor090 Not tainted 4.4.132-g46155cc #42
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
 0000000000000000 b66fa59aa0604ae8 ffff8801cc567b50 ffffffff81e0df8d
 ffffea0002d21700 ffff8800b485c000 0000000000000000 ffff8800b485c000
 ffffffff82f18e30 ffff8801cc567b88 ffffffff8151526c ffff8800b485c000
Call Trace:
 [<ffffffff81e0df8d>] __dump_stack lib/dump_stack.c:15 [inline]
 [<ffffffff81e0df8d>] dump_stack+0xc1/0x124 lib/dump_stack.c:51
 [<ffffffff8151526c>] print_address_description+0x6c/0x216 mm/kasan/report.c:252
 [<ffffffff8151558b>] kasan_report_error mm/kasan/report.c:351 [inline]
 [<ffffffff8151558b>] kasan_report.cold.7+0x175/0x2f7 mm/kasan/report.c:408
 [<ffffffff814f9064>] __asan_report_load4_noabort+0x14/0x20 mm/kasan/report.c:428
 [<ffffffff835988b4>] l2tp_session_queue_purge+0xf4/0x100 net/l2tp/l2tp_core.c:830
 [<ffffffff835a53ef>] pppol2tp_release+0x1ff/0x310 net/l2tp/l2tp_ppp.c:509
 [<ffffffff82f18d06>] sock_release+0x96/0x1c0 net/socket.c:586
 [<ffffffff82f18e46>] sock_close+0x16/0x20 net/socket.c:1037
 [<ffffffff81522655>] __fput+0x235/0x6f0 fs/file_table.c:208
 [<ffffffff81522b95>] ____fput+0x15/0x20 fs/file_table.c:244
 [<ffffffff8118bb8f>] task_work_run+0x10f/0x190 kernel/task_work.c:115
 [<ffffffff81135175>] exit_task_work include/linux/task_work.h:21 [inline]
 [<ffffffff81135175>] do_exit+0x9e5/0x26b0 kernel/exit.c:759
 [<ffffffff8113b0c1>] do_group_exit+0x111/0x330 kernel/exit.c:889
 [<ffffffff8113b2fd>] SYSC_exit_group kernel/exit.c:900 [inline]
 [<ffffffff8113b2fd>] SyS_exit_group+0x1d/0x20 kernel/exit.c:898
 [<ffffffff838c0225>] entry_SYSCALL_64_fastpath+0x22/0x9e

Allocated by task 3933:
 [<ffffffff810341d6>] save_stack_trace+0x26/0x50 arch/x86/kernel/stacktrace.c:63
 [<ffffffff814f8133>] save_stack+0x43/0xd0 mm/kasan/kasan.c:512
 [<ffffffff814f8417>] set_track mm/kasan/kasan.c:524 [inline]
 [<ffffffff814f8417>] kasan_kmalloc+0xc7/0xe0 mm/kasan/kasan.c:616
 [<ffffffff814f4b34>] __kmalloc+0x124/0x310 mm/slub.c:3613
 [<ffffffff8359dcf9>] kmalloc include/linux/slab.h:481 [inline]
 [<ffffffff8359dcf9>] kzalloc include/linux/slab.h:620 [inline]
 [<ffffffff8359dcf9>] l2tp_session_create+0x39/0x1030 net/l2tp/l2tp_core.c:1748
 [<ffffffff835a2a00>] pppol2tp_connect+0x10f0/0x1910 net/l2tp/l2tp_ppp.c:725
 [<ffffffff82f1d728>] SYSC_connect+0x1b8/0x300 net/socket.c:1557
 [<ffffffff82f20064>] SyS_connect+0x24/0x30 net/socket.c:1538
 [<ffffffff838c0225>] entry_SYSCALL_64_fastpath+0x22/0x9e

Freed by task 3900:
 [<ffffffff810341d6>] save_stack_trace+0x26/0x50 arch/x86/kernel/stacktrace.c:63
 [<ffffffff814f8133>] save_stack+0x43/0xd0 mm/kasan/kasan.c:512
 [<ffffffff814f8a62>] set_track mm/kasan/kasan.c:524 [inline]
 [<ffffffff814f8a62>] kasan_slab_free+0x72/0xc0 mm/kasan/kasan.c:589
 [<ffffffff814f5f64>] slab_free_hook mm/slub.c:1383 [inline]
 [<ffffffff814f5f64>] slab_free_freelist_hook mm/slub.c:1405 [inline]
 [<ffffffff814f5f64>] slab_free mm/slub.c:2859 [inline]
 [<ffffffff814f5f64>] kfree+0xf4/0x310 mm/slub.c:3749
 [<ffffffff8359ac60>] l2tp_session_free+0x170/0x200 net/l2tp/l2tp_core.c:1676
 [<ffffffff8359d019>] l2tp_session_dec_refcount_1 net/l2tp/l2tp_core.h:293 [inline]
 [<ffffffff8359d019>] l2tp_tunnel_closeall+0x2b9/0x350 net/l2tp/l2tp_core.c:1279
 [<ffffffff8359db2b>] l2tp_udp_encap_destroy+0x8b/0xf0 net/l2tp/l2tp_core.c:1300
 [<ffffffff83490121>] udpv6_destroy_sock+0xb1/0xd0 net/ipv6/udp.c:1421
 [<ffffffff82f2e5dd>] sk_common_release+0x6d/0x300 net/core/sock.c:2680
 [<ffffffff8348edd5>] udp_lib_close+0x15/0x20 include/net/udp.h:190
 [<ffffffff832f6aaf>] inet_release+0xff/0x1d0 net/ipv4/af_inet.c:435
 [<ffffffff83419640>] inet6_release+0x50/0x70 net/ipv6/af_inet6.c:424
 [<ffffffff82f18d06>] sock_release+0x96/0x1c0 net/socket.c:586
 [<ffffffff82f18e46>] sock_close+0x16/0x20 net/socket.c:1037
 [<ffffffff81522655>] __fput+0x235/0x6f0 fs/file_table.c:208
 [<ffffffff81522b95>] ____fput+0x15/0x20 fs/file_table.c:244
 [<ffffffff8118bb8f>] task_work_run+0x10f/0x190 kernel/task_work.c:115
 [<ffffffff81135175>] exit_task_work include/linux/task_work.h:21 [inline]
 [<ffffffff81135175>] do_exit+0x9e5/0x26b0 kernel/exit.c:759
 [<ffffffff8113b0c1>] do_group_exit+0x111/0x330 kernel/exit.c:889
 [<ffffffff8113b2fd>] SYSC_exit_group kernel/exit.c:900 [inline]
 [<ffffffff8113b2fd>] SyS_exit_group+0x1d/0x20 kernel/exit.c:898
 [<ffffffff838c0225>] entry_SYSCALL_64_fastpath+0x22/0x9e

The buggy address belongs to the object at ffff8800b485c000
 which belongs to the cache kmalloc-512 of size 512
The buggy address is located 0 bytes inside of
 512-byte region [ffff8800b485c000, ffff8800b485c200)
The buggy address belongs to the page:
PANIC: double fault, error_code: 0x0
CPU: 0 PID: 3935 Comm: syz-executor090 Not tainted 4.4.132-g46155cc #42
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
task: ffff8801cdc98000 task.stack: ffff8801cc560000
RIP: 0010:[<ffffffff8148c96a>]  [<ffffffff8148c96a>] dump_page_badflags+0x1a/0x70 mm/debug.c:84
RSP: 0000:ffff880100000000  EFLAGS: 00010086
RAX: ffff8801cdc98000 RBX: ffffea0002d21700 RCX: 0000000000000000
RDX: 0000000000000000 RSI: ffffffff83aa9be0 RDI: ffffea0002d21700
RBP: ffff880100000030 R08: 0000000000000001 R09: 0000000000000000
R10: 0000000000000001 R11: ffffffff858ed134 R12: 0000000000000000
R13: ffffffff83aa9be0 R14: ffff8800b485c000 R15: ffff8800b485c200
FS:  00007f5419132700(0000) GS:ffff8801db200000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: ffff8800fffffff8 CR3: 00000000b1260000 CR4: 00000000001606f0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Stack:

Call Trace:
 <UNK> 
Code: df e8 2b c7 06 00 eb dd 66 0f 1f 84 00 00 00 00 00 55 48 89 e5 41 57 41 56 41 55 49 89 f5 41 54 49 89 d4 53 48 89 fb 48 83 ec 08 <e8> 71 48 ec ff 48 89 da 48 b8 00 00 00 00 00 fc ff df 48 c1 ea