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BUG: KASAN: use-after-free in pppol2tp_sock_to_session net/l2tp/l2tp_ppp.c:157 [inline]
BUG: KASAN: use-after-free in pppol2tp_release+0x293/0x2e0 net/l2tp/l2tp_ppp.c:487
Read of size 4 at addr ffff8801b8e93180 by task syzkaller269944/3792

CPU: 0 PID: 3792 Comm: syzkaller269944 Not tainted 4.9.84-ge7f51a5 #45
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
 ffff8801d7a7fce0 ffffffff81d956b9 ffffea0006e3a480 ffff8801b8e93180
 0000000000000000 ffff8801b8e93180 ffffffff82ed59f0 ffff8801d7a7fd18
 ffffffff8153e1a3 ffff8801b8e93180 0000000000000004 0000000000000000
Call Trace:
 [<ffffffff81d956b9>] __dump_stack lib/dump_stack.c:15 [inline]
 [<ffffffff81d956b9>] dump_stack+0xc1/0x128 lib/dump_stack.c:51
 [<ffffffff8153e1a3>] print_address_description+0x73/0x280 mm/kasan/report.c:252
 [<ffffffff8153e6c5>] kasan_report_error mm/kasan/report.c:351 [inline]
 [<ffffffff8153e6c5>] kasan_report+0x275/0x360 mm/kasan/report.c:408
 [<ffffffff8153e804>] __asan_report_load4_noabort+0x14/0x20 mm/kasan/report.c:428
 [<ffffffff8358fbb3>] pppol2tp_sock_to_session net/l2tp/l2tp_ppp.c:157 [inline]
 [<ffffffff8358fbb3>] pppol2tp_release+0x293/0x2e0 net/l2tp/l2tp_ppp.c:487
 [<ffffffff82ed589d>] sock_release+0x8d/0x1e0 net/socket.c:599
 [<ffffffff82ed5a06>] sock_close+0x16/0x20 net/socket.c:1046
 [<ffffffff8157579c>] __fput+0x28c/0x6e0 fs/file_table.c:208
 [<ffffffff81575c75>] ____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
 [<ffffffff810066cf>] prepare_exit_to_usermode arch/x86/entry/common.c:191 [inline]
 [<ffffffff810066cf>] syscall_return_slowpath arch/x86/entry/common.c:260 [inline]
 [<ffffffff810066cf>] do_syscall_64+0x36f/0x490 arch/x86/entry/common.c:287
 [<ffffffff838b53fd>] entry_SYSCALL_64_after_swapgs+0x47/0xc5

Allocated by task 3792:
 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_64+0x1a4/0x490 arch/x86/entry/common.c:282
 entry_SYSCALL_64_after_swapgs+0x47/0xc5

Freed by task 3793:
 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_64+0x36f/0x490 arch/x86/entry/common.c:287
 entry_SYSCALL_64_after_swapgs+0x47/0xc5

The buggy address belongs to the object at ffff8801b8e93180
 which belongs to the cache kmalloc-512 of size 512
The buggy address is located 0 bytes inside of
 512-byte region [ffff8801b8e93180, ffff8801b8e93380)
The buggy address belongs to the page:
page:ffffea0006e3a480 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:
 ffff8801b8e93080: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
 ffff8801b8e93100: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
>ffff8801b8e93180: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
                   ^
 ffff8801b8e93200: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
 ffff8801b8e93280: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
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