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BUG: KASAN: use-after-free in pppol2tp_session_destruct+0xe9/0x110 net/l2tp/l2tp_ppp.c:459
Read of size 4 at addr ffff8801c6ae1b80 by task syzkaller670880/3804

CPU: 1 PID: 3804 Comm: syzkaller670880 Not tainted 4.9.81-ga25ea24 #44
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
 ffff8801d7fefc18 ffffffff81d94de9 ffffea00071ab800 ffff8801c6ae1b80
 0000000000000000 ffff8801c6ae1b80 ffffffff82ed49f0 ffff8801d7fefc50
 ffffffff8153e173 ffff8801c6ae1b80 0000000000000004 0000000000000000
Call Trace:
 [<ffffffff81d94de9>] __dump_stack lib/dump_stack.c:15 [inline]
 [<ffffffff81d94de9>] dump_stack+0xc1/0x128 lib/dump_stack.c:51
 [<ffffffff8153e173>] print_address_description+0x73/0x280 mm/kasan/report.c:252
 [<ffffffff8153e695>] kasan_report_error mm/kasan/report.c:351 [inline]
 [<ffffffff8153e695>] kasan_report+0x275/0x360 mm/kasan/report.c:408
 [<ffffffff8153e7d4>] __asan_report_load4_noabort+0x14/0x20 mm/kasan/report.c:428
 [<ffffffff8358b219>] pppol2tp_session_destruct+0xe9/0x110 net/l2tp/l2tp_ppp.c:459
 [<ffffffff82ee5053>] __sk_destruct+0x53/0x570 net/core/sock.c:1428
 [<ffffffff82ee97e7>] sk_destruct+0x47/0x80 net/core/sock.c:1463
 [<ffffffff82ee9877>] __sk_free+0x57/0x230 net/core/sock.c:1471
 [<ffffffff82ee9a73>] sk_free+0x23/0x30 net/core/sock.c:1482
 [<ffffffff8358e5ed>] sock_put include/net/sock.h:1588 [inline]
 [<ffffffff8358e5ed>] pppol2tp_release+0x23d/0x2e0 net/l2tp/l2tp_ppp.c:501
 [<ffffffff82ed489d>] sock_release+0x8d/0x1e0 net/socket.c:599
 [<ffffffff82ed4a06>] sock_close+0x16/0x20 net/socket.c:1046
 [<ffffffff8157576c>] __fput+0x28c/0x6e0 fs/file_table.c:208
 [<ffffffff81575c45>] ____fput+0x15/0x20 fs/file_table.c:244
 [<ffffffff81195855>] task_work_run+0x115/0x190 kernel/task_work.c:116
 [<ffffffff81003a4c>] tracehook_notify_resume include/linux/tracehook.h:191 [inline]
 [<ffffffff81003a4c>] exit_to_usermode_loop+0xfc/0x120 arch/x86/entry/common.c:161
 [<ffffffff81007263>] prepare_exit_to_usermode arch/x86/entry/common.c:191 [inline]
 [<ffffffff81007263>] syscall_return_slowpath arch/x86/entry/common.c:260 [inline]
 [<ffffffff81007263>] do_syscall_32_irqs_on arch/x86/entry/common.c:331 [inline]
 [<ffffffff81007263>] do_fast_syscall_32+0x5c3/0x870 arch/x86/entry/common.c:387
 [<ffffffff838b5574>] entry_SYSENTER_compat+0x74/0x83 arch/x86/entry/entry_64_compat.S:127

Allocated by task 3803:
 save_stack_trace+0x16/0x20 arch/x86/kernel/stacktrace.c:57
 save_stack+0x43/0xd0 mm/kasan/kasan.c:505
 set_track mm/kasan/kasan.c:517 [inline]
 kasan_kmalloc+0xad/0xe0 mm/kasan/kasan.c:609
 __kmalloc+0x11d/0x310 mm/slub.c:3741
 kmalloc include/linux/slab.h:495 [inline]
 kzalloc include/linux/slab.h:636 [inline]
 l2tp_session_create+0x38/0x1770 net/l2tp/l2tp_core.c:1839
 pppol2tp_connect+0x10fe/0x18f0 net/l2tp/l2tp_ppp.c:711
 SYSC_connect+0x1b6/0x310 net/socket.c:1562
 SyS_connect+0x24/0x30 net/socket.c:1543
 do_syscall_32_irqs_on arch/x86/entry/common.c:325 [inline]
 do_fast_syscall_32+0x2f7/0x870 arch/x86/entry/common.c:387
 entry_SYSENTER_compat+0x74/0x83 arch/x86/entry/entry_64_compat.S:127

Freed by task 3803:
 save_stack_trace+0x16/0x20 arch/x86/kernel/stacktrace.c:57
 save_stack+0x43/0xd0 mm/kasan/kasan.c:505
 set_track mm/kasan/kasan.c:517 [inline]
 kasan_slab_free+0x72/0xc0 mm/kasan/kasan.c:582
 slab_free_hook mm/slub.c:1355 [inline]
 slab_free_freelist_hook mm/slub.c:1377 [inline]
 slab_free mm/slub.c:2958 [inline]
 kfree+0x103/0x300 mm/slub.c:3878
 l2tp_session_free+0x166/0x200 net/l2tp/l2tp_core.c:1766
 l2tp_session_dec_refcount_1 net/l2tp/l2tp_core.h:297 [inline]
 l2tp_tunnel_closeall+0x26c/0x3a0 net/l2tp/l2tp_core.c:1373
 l2tp_udp_encap_destroy+0x87/0xe0 net/l2tp/l2tp_core.c:1394
 udpv6_destroy_sock+0xb1/0xd0 net/ipv6/udp.c:1336
 sk_common_release+0x6b/0x2f0 net/core/sock.c:2727
 udp_lib_close+0x15/0x20 include/net/udp.h:203
 inet_release+0xfa/0x1d0 net/ipv4/af_inet.c:434
 inet6_release+0x50/0x70 net/ipv6/af_inet6.c:440
 sock_release+0x8d/0x1e0 net/socket.c:599
 sock_close+0x16/0x20 net/socket.c:1046
 __fput+0x28c/0x6e0 fs/file_table.c:208
 ____fput+0x15/0x20 fs/file_table.c:244
 task_work_run+0x115/0x190 kernel/task_work.c:116
 tracehook_notify_resume include/linux/tracehook.h:191 [inline]
 exit_to_usermode_loop+0xfc/0x120 arch/x86/entry/common.c:161
 prepare_exit_to_usermode arch/x86/entry/common.c:191 [inline]
 syscall_return_slowpath arch/x86/entry/common.c:260 [inline]
 do_syscall_32_irqs_on arch/x86/entry/common.c:331 [inline]
 do_fast_syscall_32+0x5c3/0x870 arch/x86/entry/common.c:387
 entry_SYSENTER_compat+0x74/0x83 arch/x86/entry/entry_64_compat.S:127

The buggy address belongs to the object at ffff8801c6ae1b80
 which belongs to the cache kmalloc-512 of size 512
The buggy address is located 0 bytes inside of
 512-byte region [ffff8801c6ae1b80, ffff8801c6ae1d80)
The buggy address belongs to the page:
page:ffffea00071ab800 count:1 mapcount:0 mapping:          (null) index:0x0 compound_mapcount: 0
flags: 0x8000000000004080(slab|head)
page dumped because: kasan: bad access detected

Memory state around the buggy address:
 ffff8801c6ae1a80: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
 ffff8801c6ae1b00: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
>ffff8801c6ae1b80: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
                   ^
 ffff8801c6ae1c00: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
 ffff8801c6ae1c80: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
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