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BUG: KASAN: use-after-free in l2tp_session_queue_purge+0xf4/0x100 net/l2tp/l2tp_core.c:917
Read of size 4 at addr ffff8801c7e0f400 by task syz-executor694/5094

CPU: 1 PID: 5094 Comm: syz-executor694 Not tainted 4.9.117-g8b21e85 #19
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
 ffff8801d8247cb0 ffffffff81eb4be9 ffffea00071f8380 ffff8801c7e0f400
 0000000000000000 ffff8801c7e0f400 ffffffff83015be0 ffff8801d8247ce8
 ffffffff81567f89 ffff8801c7e0f400 0000000000000004 0000000000000000
Call Trace:
 [<ffffffff81eb4be9>] __dump_stack lib/dump_stack.c:15 [inline]
 [<ffffffff81eb4be9>] dump_stack+0xc1/0x128 lib/dump_stack.c:51
 [<ffffffff81567f89>] print_address_description+0x6c/0x234 mm/kasan/report.c:256
 [<ffffffff81568393>] kasan_report_error mm/kasan/report.c:355 [inline]
 [<ffffffff81568393>] kasan_report.cold.6+0x242/0x2fe mm/kasan/report.c:412
 [<ffffffff8153bef4>] __asan_report_load4_noabort+0x14/0x20 mm/kasan/report.c:432
 [<ffffffff836be844>] l2tp_session_queue_purge+0xf4/0x100 net/l2tp/l2tp_core.c:917
 [<ffffffff836ca4cb>] pppol2tp_release+0x1fb/0x2e0 net/l2tp/l2tp_ppp.c:492
 [<ffffffff83015ab6>] sock_release+0x96/0x1c0 net/socket.c:599
 [<ffffffff83015bf6>] sock_close+0x16/0x20 net/socket.c:1046
 [<ffffffff81578693>] __fput+0x263/0x700 fs/file_table.c:208
 [<ffffffff81578bb5>] ____fput+0x15/0x20 fs/file_table.c:244
 [<ffffffff8119838c>] task_work_run+0x10c/0x180 kernel/task_work.c:116
 [<ffffffff8100559c>] tracehook_notify_resume include/linux/tracehook.h:191 [inline]
 [<ffffffff8100559c>] exit_to_usermode_loop+0xfc/0x120 arch/x86/entry/common.c:161
 [<ffffffff810064d4>] prepare_exit_to_usermode arch/x86/entry/common.c:191 [inline]
 [<ffffffff810064d4>] syscall_return_slowpath arch/x86/entry/common.c:260 [inline]
 [<ffffffff810064d4>] do_syscall_64+0x364/0x490 arch/x86/entry/common.c:287
 [<ffffffff839fcc13>] entry_SYSCALL_64_after_swapgs+0x5d/0xdb

Allocated by task 5095:
 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+0xc7/0xe0 mm/kasan/kasan.c:609
 __kmalloc+0x11d/0x300 mm/slub.c:3741
 kmalloc include/linux/slab.h:495 [inline]
 kzalloc include/linux/slab.h:636 [inline]
 l2tp_session_create+0x38/0x16f0 net/l2tp/l2tp_core.c:1843
 pppol2tp_connect+0x10d7/0x18f0 net/l2tp/l2tp_ppp.c:718
 SYSC_connect+0x1b8/0x300 net/socket.c:1562
 SyS_connect+0x24/0x30 net/socket.c:1543
 do_syscall_64+0x1a6/0x490 arch/x86/entry/common.c:282
 entry_SYSCALL_64_after_swapgs+0x5d/0xdb

Freed by task 5018:
 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+0xfb/0x310 mm/slub.c:3878
 l2tp_session_free+0x166/0x200 net/l2tp/l2tp_core.c:1770
 l2tp_session_dec_refcount_1 net/l2tp/l2tp_core.h:297 [inline]
 l2tp_tunnel_closeall+0x284/0x350 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+0x6d/0x300 net/core/sock.c:2727
 udp_lib_close+0x15/0x20 include/net/udp.h:203
 inet_release+0xff/0x1d0 net/ipv4/af_inet.c:434
 inet6_release+0x50/0x70 net/ipv6/af_inet6.c:440
 sock_release+0x96/0x1c0 net/socket.c:599
 sock_close+0x16/0x20 net/socket.c:1046
 __fput+0x263/0x700 fs/file_table.c:208
 ____fput+0x15/0x20 fs/file_table.c:244
 task_work_run+0x10c/0x180 kernel/task_work.c:116
 exit_task_work include/linux/task_work.h:21 [inline]
 do_exit+0x9e1/0x27b0 kernel/exit.c:833
 do_group_exit+0x111/0x340 kernel/exit.c:937
 get_signal+0x4cf/0x1450 kernel/signal.c:2321
 do_signal+0x87/0x19f0 arch/x86/kernel/signal.c:807
 exit_to_usermode_loop+0xe1/0x120 arch/x86/entry/common.c:157
 prepare_exit_to_usermode+0xbb/0xe0 arch/x86/entry/common.c:191
 retint_user+0x8/0x3c

The buggy address belongs to the object at ffff8801c7e0f400
 which belongs to the cache kmalloc-512 of size 512
The buggy address is located 0 bytes inside of
 512-byte region [ffff8801c7e0f400, ffff8801c7e0f600)
The buggy address belongs to the page:
page:ffffea00071f8380 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:
 ffff8801c7e0f300: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
 ffff8801c7e0f380: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
>ffff8801c7e0f400: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
                   ^
 ffff8801c7e0f480: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
 ffff8801c7e0f500: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
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