==================================================================
BUG: KASAN: use-after-free in l2tp_session_queue_purge+0xe8/0x100 net/l2tp/l2tp_core.c:830
Read of size 4 at addr ffff8800b4bf9900 by task syzkaller123327/3859

CPU: 0 PID: 3859 Comm: syzkaller123327 Not tainted 4.4.112-g52c02cf #30
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
 0000000000000000 fc69e61071f95133 ffff8801cf8d77f8 ffffffff81d056fd
 ffffea0002d2fe00 ffff8800b4bf9900 0000000000000000 ffff8800b4bf9900
 ffffffff82dea6e0 ffff8801cf8d7830 ffffffff814fd953 ffff8800b4bf9900
Call Trace:
 [<ffffffff81d056fd>] __dump_stack lib/dump_stack.c:15 [inline]
 [<ffffffff81d056fd>] dump_stack+0xc1/0x124 lib/dump_stack.c:51
 [<ffffffff814fd953>] print_address_description+0x73/0x260 mm/kasan/report.c:252
 [<ffffffff814fde65>] kasan_report_error mm/kasan/report.c:351 [inline]
 [<ffffffff814fde65>] kasan_report+0x285/0x370 mm/kasan/report.c:408
 [<ffffffff814fdfa4>] __asan_report_load4_noabort+0x14/0x20 mm/kasan/report.c:428
 [<ffffffff8345a7c8>] l2tp_session_queue_purge+0xe8/0x100 net/l2tp/l2tp_core.c:830
 [<ffffffff8346785f>] pppol2tp_release+0x1ff/0x310 net/l2tp/l2tp_ppp.c:509
 [<ffffffff82dea58d>] sock_release+0x8d/0x1e0 net/socket.c:586
 [<ffffffff82dea6f6>] sock_close+0x16/0x20 net/socket.c:1037
 [<ffffffff81522f93>] __fput+0x233/0x6d0 fs/file_table.c:208
 [<ffffffff815234b5>] ____fput+0x15/0x20 fs/file_table.c:244
 [<ffffffff8118bb54>] task_work_run+0x104/0x180 kernel/task_work.c:115
 [<ffffffff81132f21>] exit_task_work include/linux/task_work.h:21 [inline]
 [<ffffffff81132f21>] do_exit+0x871/0x2a20 kernel/exit.c:755
 [<ffffffff81139398>] do_group_exit+0x108/0x320 kernel/exit.c:885
 [<ffffffff8115c895>] get_signal+0x565/0x1660 kernel/signal.c:2317
 [<ffffffff8100e7fb>] do_signal+0x8b/0x1d40 arch/x86/kernel/signal.c:712
 [<ffffffff81003602>] exit_to_usermode_loop+0x122/0x170 arch/x86/entry/common.c:247
 [<ffffffff81006373>] prepare_exit_to_usermode+0xe3/0x100 arch/x86/entry/common.c:282
 [<ffffffff83776f88>] retint_user+0x8/0x3c

Allocated by task 3858:
 [<ffffffff81035df6>] save_stack_trace+0x26/0x50 arch/x86/kernel/stacktrace.c:63
 [<ffffffff814fc9c3>] save_stack+0x43/0xd0 mm/kasan/kasan.c:512
 [<ffffffff814fcc8d>] set_track mm/kasan/kasan.c:524 [inline]
 [<ffffffff814fcc8d>] kasan_kmalloc+0xad/0xe0 mm/kasan/kasan.c:616
 [<ffffffff814f8f64>] __kmalloc+0x124/0x320 mm/slub.c:3613
 [<ffffffff834606d9>] kmalloc include/linux/slab.h:481 [inline]
 [<ffffffff834606d9>] kzalloc include/linux/slab.h:620 [inline]
 [<ffffffff834606d9>] l2tp_session_create+0x39/0x10f0 net/l2tp/l2tp_core.c:1744
 [<ffffffff83464d6c>] pppol2tp_connect+0x10fc/0x1930 net/l2tp/l2tp_ppp.c:718
 [<ffffffff82deef46>] SYSC_connect+0x1b6/0x310 net/socket.c:1557
 [<ffffffff82df17b4>] SyS_connect+0x24/0x30 net/socket.c:1538
 [<ffffffff837763d9>] entry_SYSCALL_64_fastpath+0x16/0x92

Freed by task 3829:
 [<ffffffff81035df6>] save_stack_trace+0x26/0x50 arch/x86/kernel/stacktrace.c:63
 [<ffffffff814fc9c3>] save_stack+0x43/0xd0 mm/kasan/kasan.c:512
 [<ffffffff814fd2e2>] set_track mm/kasan/kasan.c:524 [inline]
 [<ffffffff814fd2e2>] kasan_slab_free+0x72/0xc0 mm/kasan/kasan.c:589
 [<ffffffff814f9d6c>] slab_free_hook mm/slub.c:1383 [inline]
 [<ffffffff814f9d6c>] slab_free_freelist_hook mm/slub.c:1405 [inline]
 [<ffffffff814f9d6c>] slab_free mm/slub.c:2859 [inline]
 [<ffffffff814f9d6c>] kfree+0xfc/0x300 mm/slub.c:3749
 [<ffffffff8345cc60>] l2tp_session_free+0x170/0x200 net/l2tp/l2tp_core.c:1672
 [<ffffffff8345f511>] l2tp_session_dec_refcount_1 net/l2tp/l2tp_core.h:293 [inline]
 [<ffffffff8345f511>] l2tp_tunnel_closeall+0x2d1/0x3b0 net/l2tp/l2tp_core.c:1279
 [<ffffffff8346007b>] l2tp_udp_encap_destroy+0x8b/0xf0 net/l2tp/l2tp_core.c:1300
 [<ffffffff8336e8c1>] udpv6_destroy_sock+0xb1/0xd0 net/ipv6/udp.c:1421
 [<ffffffff82e016db>] sk_common_release+0x6b/0x300 net/core/sock.c:2680
 [<ffffffff8336d875>] udp_lib_close+0x15/0x20 include/net/udp.h:190
 [<ffffffff831d18ea>] inet_release+0xfa/0x1d0 net/ipv4/af_inet.c:435
 [<ffffffff832f71a0>] inet6_release+0x50/0x70 net/ipv6/af_inet6.c:424
 [<ffffffff82dea58d>] sock_release+0x8d/0x1e0 net/socket.c:586
 [<ffffffff82dea6f6>] sock_close+0x16/0x20 net/socket.c:1037
 [<ffffffff81522f93>] __fput+0x233/0x6d0 fs/file_table.c:208
 [<ffffffff815234b5>] ____fput+0x15/0x20 fs/file_table.c:244
 [<ffffffff8118bb54>] task_work_run+0x104/0x180 kernel/task_work.c:115
 [<ffffffff81132f21>] exit_task_work include/linux/task_work.h:21 [inline]
 [<ffffffff81132f21>] do_exit+0x871/0x2a20 kernel/exit.c:755
 [<ffffffff81139398>] do_group_exit+0x108/0x320 kernel/exit.c:885
 [<ffffffff8115c895>] get_signal+0x565/0x1660 kernel/signal.c:2317
 [<ffffffff8100e7fb>] do_signal+0x8b/0x1d40 arch/x86/kernel/signal.c:712
 [<ffffffff81003602>] exit_to_usermode_loop+0x122/0x170 arch/x86/entry/common.c:247
 [<ffffffff81006373>] prepare_exit_to_usermode+0xe3/0x100 arch/x86/entry/common.c:282
 [<ffffffff83776f88>] retint_user+0x8/0x3c

The buggy address belongs to the object at ffff8800b4bf9900
 which belongs to the cache kmalloc-512 of size 512
The buggy address is located 0 bytes inside of
 512-byte region [ffff8800b4bf9900, ffff8800b4bf9b00)
The buggy address belongs to the page:
PANIC: double fault, error_code: 0x0
CPU: 0 PID: 3859 Comm: syzkaller123327 Not tainted 4.4.112-g52c02cf #30
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
task: ffff8801d0758000 task.stack: ffff8801cf8d0000
RIP: 0010:[<ffffffff8148fd2d>]  [<ffffffff8148fd2d>] dump_page_badflags+0xd/0x250 mm/debug.c:84
RSP: 0018:ffff880100000000  EFLAGS: 00010046
RAX: ffff8801d0758000 RBX: ffffea0002d2fe00 RCX: ffffffff8148fea0
RDX: 0000000000000000 RSI: ffffffff838a8620 RDI: ffffea0002d2fe00
RBP: ffff880100000018 R08: 0000000000000001 R09: 0000000000000000
R10: 0000000000000002 R11: fffffbfff0ad7a1e R12: 0000000000000000
R13: ffffffff838a8620 R14: 0000000000000000 R15: 0000000000000000
FS:  0000000000000000(0000) GS:ffff8801db200000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: ffff8800fffffff8 CR3: 00000000b3cf4000 CR4: 0000000000160670
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Stack:

Call Trace:
 <UNK> 
Code: ff e8 a8 e2 06 00 e9 50 fd ff ff e8 9e e2 06 00 e9 1d fd ff ff 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 11 01 ed ff 48 8d 7b