==================================================================
BUG: KASAN: use-after-free in l2tp_session_queue_purge+0xb0/0xc0 net/l2tp/l2tp_core.c:874
Read of size 4 at addr ffff888083e88d40 by task syz-executor.0/32564

CPU: 0 PID: 32564 Comm: syz-executor.0 Not tainted 4.14.165-syzkaller #0
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+0xf7/0x13b lib/dump_stack.c:58
 print_address_description.cold.7+0x9/0x1c9 mm/kasan/report.c:252
 kasan_report_error mm/kasan/report.c:351 [inline]
 kasan_report.cold.8+0x11a/0x2d3 mm/kasan/report.c:409
 __asan_report_load4_noabort+0x14/0x20 mm/kasan/report.c:429
 l2tp_session_queue_purge+0xb0/0xc0 net/l2tp/l2tp_core.c:874
 l2tp_tunnel_closeall+0x1b6/0x2f0 net/l2tp/l2tp_core.c:1337
 l2tp_udp_encap_destroy+0x7d/0xc0 net/l2tp/l2tp_core.c:1366
 udpv6_destroy_sock+0x89/0xb0 net/ipv6/udp.c:1444
 sk_common_release+0x5e/0x2c0 net/core/sock.c:2989
 udp_lib_close+0x9/0x10 include/net/udp.h:203
 inet_release+0xd9/0x1c0 net/ipv4/af_inet.c:425
 inet6_release+0x46/0x60 net/ipv6/af_inet6.c:450
 __sock_release+0xc2/0x2a0 net/socket.c:602
 sock_close+0x10/0x20 net/socket.c:1139
 __fput+0x232/0x750 fs/file_table.c:210
 ____fput+0x9/0x10 fs/file_table.c:244
 task_work_run+0xe5/0x170 kernel/task_work.c:113
 tracehook_notify_resume include/linux/tracehook.h:191 [inline]
 exit_to_usermode_loop+0x16a/0x1b0 arch/x86/entry/common.c:164
 prepare_exit_to_usermode arch/x86/entry/common.c:199 [inline]
 syscall_return_slowpath arch/x86/entry/common.c:270 [inline]
 do_syscall_64+0x416/0x5b0 arch/x86/entry/common.c:297
 entry_SYSCALL_64_after_hwframe+0x42/0xb7
RIP: 0033:0x413ae1
RSP: 002b:00007ffec4617ca0 EFLAGS: 00000293 ORIG_RAX: 0000000000000003
RAX: 0000000000000000 RBX: 0000000000000006 RCX: 0000000000413ae1
RDX: 0000000000000000 RSI: 0000000000000081 RDI: 0000000000000005
RBP: 0000000000000000 R08: 0000000000760760 R09: ffffffffffffffff
R10: 00007ffec4617d70 R11: 0000000000000293 R12: 000000000075bf20
R13: 0000000000000003 R14: 0000000000760768 R15: 000000000075bf2c

Allocated by task 32565:
 save_stack_trace+0x16/0x20 arch/x86/kernel/stacktrace.c:59
 save_stack+0x43/0xd0 mm/kasan/kasan.c:447
 set_track mm/kasan/kasan.c:459 [inline]
 kasan_kmalloc+0xc7/0xe0 mm/kasan/kasan.c:551
 __do_kmalloc mm/slab.c:3720 [inline]
 __kmalloc+0x15b/0x7b0 mm/slab.c:3729
 kmalloc include/linux/slab.h:493 [inline]
 kzalloc include/linux/slab.h:661 [inline]
 l2tp_session_create+0x31/0x1630 net/l2tp/l2tp_core.c:1791
 pppol2tp_connect+0x10c1/0x1900 net/l2tp/l2tp_ppp.c:721
 SYSC_connect+0x1e3/0x2a0 net/socket.c:1655
 SyS_connect+0x9/0x10 net/socket.c:1636
 do_syscall_64+0x1c7/0x5b0 arch/x86/entry/common.c:292
 entry_SYSCALL_64_after_hwframe+0x42/0xb7

Freed by task 32565:
 save_stack_trace+0x16/0x20 arch/x86/kernel/stacktrace.c:59
 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:3496 [inline]
 kfree+0xcc/0x270 mm/slab.c:3815
 l2tp_session_free+0x14f/0x1d0 net/l2tp/l2tp_core.c:1714
 l2tp_session_dec_refcount_1 net/l2tp/l2tp_core.h:300 [inline]
 pppol2tp_session_destruct+0xbb/0xf0 net/l2tp/l2tp_ppp.c:460
 __sk_destruct+0x46/0x4e0 net/core/sock.c:1556
 sk_destruct+0x83/0xb0 net/core/sock.c:1596
 __sk_free+0x47/0x1f0 net/core/sock.c:1604
 sk_free+0x23/0x30 net/core/sock.c:1615
 sock_put include/net/sock.h:1658 [inline]
 pppol2tp_release+0x209/0x290 net/l2tp/l2tp_ppp.c:501
 __sock_release+0xc2/0x2a0 net/socket.c:602
 sock_close+0x10/0x20 net/socket.c:1139
 __fput+0x232/0x750 fs/file_table.c:210
 ____fput+0x9/0x10 fs/file_table.c:244
 task_work_run+0xe5/0x170 kernel/task_work.c:113
 tracehook_notify_resume include/linux/tracehook.h:191 [inline]
 exit_to_usermode_loop+0x16a/0x1b0 arch/x86/entry/common.c:164
 prepare_exit_to_usermode arch/x86/entry/common.c:199 [inline]
 syscall_return_slowpath arch/x86/entry/common.c:270 [inline]
 do_syscall_64+0x416/0x5b0 arch/x86/entry/common.c:297
 entry_SYSCALL_64_after_hwframe+0x42/0xb7

The buggy address belongs to the object at ffff888083e88d40
 which belongs to the cache kmalloc-512 of size 512
The buggy address is located 0 bytes inside of
 512-byte region [ffff888083e88d40, ffff888083e88f40)
The buggy address belongs to the page:
page:ffffea00020fa200 count:1 mapcount:0 mapping:ffff888083e880c0 index:0x0
flags: 0x1fffc0000000100(slab)
raw: 01fffc0000000100 ffff888083e880c0 0000000000000000 0000000100000006
raw: ffffea00025af2a0 ffffea0002575f60 ffff8880aa800940 0000000000000000
page dumped because: kasan: bad access detected

Memory state around the buggy address:
 ffff888083e88c00: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
 ffff888083e88c80: fb fb fb fb fb fb fb fb fc fc fc fc fc fc fc fc
>ffff888083e88d00: fc fc fc fc fc fc fc fc fb fb fb fb fb fb fb fb
                                           ^
 ffff888083e88d80: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
 ffff888083e88e00: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
==================================================================