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BUG: KASAN: use-after-free in l2tp_session_queue_purge+0xea/0xf0 net/l2tp/l2tp_core.c:879
Read of size 4 at addr ffff8880977542c0 by task syz-executor.0/8122

CPU: 1 PID: 8122 Comm: syz-executor.0 Not tainted 4.14.176-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+0x13e/0x194 lib/dump_stack.c:58
 print_address_description.cold+0x7c/0x1e2 mm/kasan/report.c:252
 kasan_report_error mm/kasan/report.c:351 [inline]
 kasan_report mm/kasan/report.c:409 [inline]
 kasan_report.cold+0xa9/0x2ae mm/kasan/report.c:393
 l2tp_session_queue_purge+0xea/0xf0 net/l2tp/l2tp_core.c:879
 l2tp_tunnel_closeall+0x1fe/0x370 net/l2tp/l2tp_core.c:1342
 l2tp_udp_encap_destroy+0x8d/0xf0 net/l2tp/l2tp_core.c:1371
 udpv6_destroy_sock+0xa6/0xd0 net/ipv6/udp.c:1444
 sk_common_release+0x64/0x2f0 net/core/sock.c:2992
 inet_release+0xdf/0x1b0 net/ipv4/af_inet.c:425
 inet6_release+0x4c/0x70 net/ipv6/af_inet6.c:450
 __sock_release+0xcd/0x2b0 net/socket.c:602
 sock_close+0x15/0x20 net/socket.c:1139
 __fput+0x25f/0x790 fs/file_table.c:210
 task_work_run+0x113/0x190 kernel/task_work.c:113
 tracehook_notify_resume include/linux/tracehook.h:191 [inline]
 exit_to_usermode_loop+0x1d6/0x220 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+0x4a3/0x640 arch/x86/entry/common.c:297
 entry_SYSCALL_64_after_hwframe+0x42/0xb7
RIP: 0033:0x416421
RSP: 002b:00007ffc27296760 EFLAGS: 00000293 ORIG_RAX: 0000000000000003
RAX: 0000000000000000 RBX: 0000000000000005 RCX: 0000000000416421
RDX: 0000000000000000 RSI: 0000000000000080 RDI: 0000000000000004
RBP: 0000000000000000 R08: 01ffffffffffffff R09: 01ffffffffffffff
R10: 00007ffc27296840 R11: 0000000000000293 R12: 000000000076bfa0
R13: 0000000000770908 R14: 00000000000ff640 R15: 000000000076bfac

Allocated by task 8124:
 save_stack+0x32/0xa0 mm/kasan/kasan.c:447
 set_track mm/kasan/kasan.c:459 [inline]
 kasan_kmalloc mm/kasan/kasan.c:551 [inline]
 kasan_kmalloc+0xbf/0xe0 mm/kasan/kasan.c:529
 __do_kmalloc mm/slab.c:3720 [inline]
 __kmalloc+0x15b/0x7c0 mm/slab.c:3729
 kmalloc include/linux/slab.h:493 [inline]
 kzalloc include/linux/slab.h:661 [inline]
 l2tp_session_create+0x35/0x16f0 net/l2tp/l2tp_core.c:1796
 pppol2tp_connect+0x1154/0x17b0 net/l2tp/l2tp_ppp.c:721
 SYSC_connect+0x1c6/0x250 net/socket.c:1655
 do_syscall_64+0x1d5/0x640 arch/x86/entry/common.c:292
 entry_SYSCALL_64_after_hwframe+0x42/0xb7

Freed by task 8124:
 save_stack+0x32/0xa0 mm/kasan/kasan.c:447
 set_track mm/kasan/kasan.c:459 [inline]
 kasan_slab_free+0x75/0xc0 mm/kasan/kasan.c:524
 __cache_free mm/slab.c:3496 [inline]
 kfree+0xcb/0x260 mm/slab.c:3815
 l2tp_session_dec_refcount_1 net/l2tp/l2tp_core.h:300 [inline]
 pppol2tp_session_destruct+0xcd/0x110 net/l2tp/l2tp_ppp.c:460
 __sk_destruct+0x49/0x640 net/core/sock.c:1556
 sk_destruct+0x97/0xc0 net/core/sock.c:1596
 __sk_free+0x4c/0x220 net/core/sock.c:1604
 sk_free+0x2b/0x40 net/core/sock.c:1615
 sock_put include/net/sock.h:1658 [inline]
 pppol2tp_release+0x247/0x2f0 net/l2tp/l2tp_ppp.c:501
 __sock_release+0xcd/0x2b0 net/socket.c:602
 sock_close+0x15/0x20 net/socket.c:1139
 __fput+0x25f/0x790 fs/file_table.c:210
 task_work_run+0x113/0x190 kernel/task_work.c:113
 tracehook_notify_resume include/linux/tracehook.h:191 [inline]
 exit_to_usermode_loop+0x1d6/0x220 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+0x4a3/0x640 arch/x86/entry/common.c:297
 entry_SYSCALL_64_after_hwframe+0x42/0xb7

The buggy address belongs to the object at ffff8880977542c0
 which belongs to the cache kmalloc-512 of size 512
The buggy address is located 0 bytes inside of
 512-byte region [ffff8880977542c0, ffff8880977544c0)
The buggy address belongs to the page:
page:ffffea00025dd500 count:1 mapcount:0 mapping:ffff888097754040 index:0xffff888097754040
flags: 0xfffe0000000100(slab)
raw: 00fffe0000000100 ffff888097754040 ffff888097754040 0000000100000005
raw: ffffea00025182a0 ffffea00026fe120 ffff88812fe56940 0000000000000000
page dumped because: kasan: bad access detected

Memory state around the buggy address:
 ffff888097754180: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
 ffff888097754200: fb fb fb fb fb fb fb fb fc fc fc fc fc fc fc fc
>ffff888097754280: fc fc fc fc fc fc fc fc fb fb fb fb fb fb fb fb
                                           ^
 ffff888097754300: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
 ffff888097754380: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
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