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BUG: KASAN: use-after-free in l2tp_session_create+0x9aa/0xb80 net/l2tp/l2tp_core.c:1693
Read of size 4 at addr ffff8801ca77cda8 by task syz-executor1/9999

CPU: 1 PID: 9999 Comm: syz-executor1 Not tainted 4.16.0-rc4+ #259
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+0x194/0x24d lib/dump_stack.c:53
 print_address_description+0x73/0x250 mm/kasan/report.c:256
 kasan_report_error mm/kasan/report.c:354 [inline]
 kasan_report+0x23c/0x360 mm/kasan/report.c:412
 __asan_report_load4_noabort+0x14/0x20 mm/kasan/report.c:432
 l2tp_session_create+0x9aa/0xb80 net/l2tp/l2tp_core.c:1693
 pppol2tp_connect+0xede/0x1dd0 net/l2tp/l2tp_ppp.c:739
 SYSC_connect+0x213/0x4a0 net/socket.c:1639
 SyS_connect+0x24/0x30 net/socket.c:1620
 do_syscall_32_irqs_on arch/x86/entry/common.c:330 [inline]
 do_fast_syscall_32+0x3ec/0xf9f arch/x86/entry/common.c:392
 entry_SYSENTER_compat+0x70/0x7f arch/x86/entry/entry_64_compat.S:139
RIP: 0023:0xf7ff4c99
RSP: 002b:00000000f77f009c EFLAGS: 00000286 ORIG_RAX: 000000000000016a
RAX: ffffffffffffffda RBX: 0000000000000016 RCX: 0000000020e92000
RDX: 000000000000002e RSI: 0000000000000000 RDI: 0000000000000000
RBP: 0000000000000000 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000000 R12: 0000000000000000
R13: 0000000000000000 R14: 0000000000000000 R15: 0000000000000000

Allocated by task 9999:
 save_stack+0x43/0xd0 mm/kasan/kasan.c:447
 set_track mm/kasan/kasan.c:459 [inline]
 kasan_kmalloc+0xad/0xe0 mm/kasan/kasan.c:552
 kmem_cache_alloc_trace+0x136/0x740 mm/slab.c:3607
 kmalloc include/linux/slab.h:512 [inline]
 kzalloc include/linux/slab.h:701 [inline]
 l2tp_tunnel_create+0x5e1/0x1800 net/l2tp/l2tp_core.c:1487
 pppol2tp_connect+0x14b8/0x1dd0 net/l2tp/l2tp_ppp.c:698
 SYSC_connect+0x213/0x4a0 net/socket.c:1639
 SyS_connect+0x24/0x30 net/socket.c:1620
 do_syscall_32_irqs_on arch/x86/entry/common.c:330 [inline]
 do_fast_syscall_32+0x3ec/0xf9f arch/x86/entry/common.c:392
 entry_SYSENTER_compat+0x70/0x7f arch/x86/entry/entry_64_compat.S:139

Freed by task 4290:
 save_stack+0x43/0xd0 mm/kasan/kasan.c:447
 set_track mm/kasan/kasan.c:459 [inline]
 __kasan_slab_free+0x11a/0x170 mm/kasan/kasan.c:520
 kasan_slab_free+0xe/0x10 mm/kasan/kasan.c:527
 __cache_free mm/slab.c:3485 [inline]
 kfree+0xd9/0x260 mm/slab.c:3800
 __rcu_reclaim kernel/rcu/rcu.h:167 [inline]
 rcu_do_batch kernel/rcu/tree.c:2674 [inline]
 invoke_rcu_callbacks kernel/rcu/tree.c:2933 [inline]
 __rcu_process_callbacks kernel/rcu/tree.c:2900 [inline]
 rcu_process_callbacks+0xe94/0x17f0 kernel/rcu/tree.c:2917
 __do_softirq+0x2d7/0xb85 kernel/softirq.c:285

The buggy address belongs to the object at ffff8801ca77ccc0
 which belongs to the cache kmalloc-512 of size 512
The buggy address is located 232 bytes inside of
 512-byte region [ffff8801ca77ccc0, ffff8801ca77cec0)
The buggy address belongs to the page:
page:ffffea000729df00 count:1 mapcount:0 mapping:ffff8801ca77c040 index:0x0
flags: 0x2fffc0000000100(slab)
raw: 02fffc0000000100 ffff8801ca77c040 0000000000000000 0000000100000006
raw: ffffea0006adf1e0 ffffea0006adf2a0 ffff8801dac00940 0000000000000000
page dumped because: kasan: bad access detected

Memory state around the buggy address:
 ffff8801ca77cc80: fc fc fc fc fc fc fc fc fb fb fb fb fb fb fb fb
 ffff8801ca77cd00: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
>ffff8801ca77cd80: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
                                  ^
 ffff8801ca77ce00: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
 ffff8801ca77ce80: fb fb fb fb fb fb fb fb fc fc fc fc fc fc fc fc
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