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BUG: KASAN: use-after-free in l2tp_session_queue_purge+0xf2/0x100 net/l2tp/l2tp_core.c:874
Read of size 4 at addr ffff88806c097580 by task syz-executor.1/15392

CPU: 1 PID: 15392 Comm: syz-executor.1 Not tainted 4.14.150 #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+0x138/0x197 lib/dump_stack.c:53
 print_address_description.cold+0x7c/0x1dc 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+0xa9/0x2af mm/kasan/report.c:393
 __asan_report_load4_noabort+0x14/0x20 mm/kasan/report.c:429
 l2tp_session_queue_purge+0xf2/0x100 net/l2tp/l2tp_core.c:874
 l2tp_tunnel_closeall+0x20c/0x380 net/l2tp/l2tp_core.c:1337
 l2tp_udp_encap_destroy+0x99/0x100 net/l2tp/l2tp_core.c:1366
 udpv6_destroy_sock+0xb3/0xd0 net/ipv6/udp.c:1444
 sk_common_release+0x6b/0x310 net/core/sock.c:2989
 udp_lib_close+0x16/0x20 include/net/udp.h:203
 inet_release+0xec/0x1c0 net/ipv4/af_inet.c:425
 inet6_release+0x53/0x80 net/ipv6/af_inet6.c:450
 __sock_release+0xce/0x2b0 net/socket.c:602
 sock_close+0x1b/0x30 net/socket.c:1139
 __fput+0x275/0x7a0 fs/file_table.c:210
 ____fput+0x16/0x20 fs/file_table.c:244
 task_work_run+0x114/0x190 kernel/task_work.c:113
 tracehook_notify_resume include/linux/tracehook.h:191 [inline]
 exit_to_usermode_loop+0x1da/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+0x4bc/0x640 arch/x86/entry/common.c:297
 entry_SYSCALL_64_after_hwframe+0x42/0xb7
RIP: 0033:0x4139c1
RSP: 002b:00007fff5db0db60 EFLAGS: 00000293 ORIG_RAX: 0000000000000003
RAX: 0000000000000000 RBX: 0000000000000006 RCX: 00000000004139c1
RDX: 0000000000000000 RSI: 0000000000000081 RDI: 0000000000000005
RBP: 0000000000000000 R08: 00000000007616b8 R09: ffffffffffffffff
R10: 00007fff5db0dc30 R11: 0000000000000293 R12: 000000000075bf20
R13: 0000000000000004 R14: 00000000007616c0 R15: 000000000075bf2c

Allocated by task 15409:
 save_stack_trace+0x16/0x20 arch/x86/kernel/stacktrace.c:59
 save_stack+0x45/0xd0 mm/kasan/kasan.c:447
 set_track mm/kasan/kasan.c:459 [inline]
 kasan_kmalloc mm/kasan/kasan.c:551 [inline]
 kasan_kmalloc+0xce/0xf0 mm/kasan/kasan.c:529
 __do_kmalloc mm/slab.c:3720 [inline]
 __kmalloc+0x15d/0x7a0 mm/slab.c:3729
 kmalloc include/linux/slab.h:493 [inline]
 kzalloc include/linux/slab.h:661 [inline]
 l2tp_session_create+0x38/0x1600 net/l2tp/l2tp_core.c:1791
 pppol2tp_connect+0x11bf/0x18b0 net/l2tp/l2tp_ppp.c:721
 SYSC_connect+0x1f6/0x2d0 net/socket.c:1655
 SyS_connect+0x24/0x30 net/socket.c:1636
 do_syscall_64+0x1e8/0x640 arch/x86/entry/common.c:292
 entry_SYSCALL_64_after_hwframe+0x42/0xb7

Freed by task 15409:
 save_stack_trace+0x16/0x20 arch/x86/kernel/stacktrace.c:59
 save_stack+0x45/0xd0 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+0xcc/0x270 mm/slab.c:3815
 l2tp_session_free+0x176/0x210 net/l2tp/l2tp_core.c:1714
 l2tp_session_dec_refcount_1 net/l2tp/l2tp_core.h:300 [inline]
 pppol2tp_session_destruct+0xd8/0x110 net/l2tp/l2tp_ppp.c:460
 __sk_destruct+0x4f/0x580 net/core/sock.c:1556
 sk_destruct+0xa4/0xd0 net/core/sock.c:1596
 __sk_free+0x54/0x230 net/core/sock.c:1604
 sk_free+0x35/0x40 net/core/sock.c:1615
 sock_put include/net/sock.h:1653 [inline]
 pppol2tp_release+0x244/0x300 net/l2tp/l2tp_ppp.c:501
 __sock_release+0xce/0x2b0 net/socket.c:602
 sock_close+0x1b/0x30 net/socket.c:1139
 __fput+0x275/0x7a0 fs/file_table.c:210
 ____fput+0x16/0x20 fs/file_table.c:244
 task_work_run+0x114/0x190 kernel/task_work.c:113
 tracehook_notify_resume include/linux/tracehook.h:191 [inline]
 exit_to_usermode_loop+0x1da/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+0x4bc/0x640 arch/x86/entry/common.c:297
 entry_SYSCALL_64_after_hwframe+0x42/0xb7

The buggy address belongs to the object at ffff88806c097580
 which belongs to the cache kmalloc-512 of size 512
The buggy address is located 0 bytes inside of
 512-byte region [ffff88806c097580, ffff88806c097780)
The buggy address belongs to the page:
page:ffffea0001b025c0 count:1 mapcount:0 mapping:ffff88806c097080 index:0x0
flags: 0x1fffc0000000100(slab)
raw: 01fffc0000000100 ffff88806c097080 0000000000000000 0000000100000006
raw: ffffea00024ec0a0 ffffea0002634c60 ffff8880aa800940 0000000000000000
page dumped because: kasan: bad access detected

Memory state around the buggy address:
 ffff88806c097480: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
 ffff88806c097500: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
>ffff88806c097580: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
                   ^
 ffff88806c097600: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
 ffff88806c097680: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
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