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BUG: KASAN: use-after-free in l2tp_session_create+0xa6d/0xc60 net/l2tp/l2tp_core.c:1757
Read of size 4 at addr ffff8801cd54dd68 by task syz-executor3/6642

CPU: 1 PID: 6642 Comm: syz-executor3 Not tainted 4.15.0-rc9+ #284
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+0x194/0x257 lib/dump_stack.c:53
 print_address_description+0x73/0x250 mm/kasan/report.c:252
 kasan_report_error mm/kasan/report.c:351 [inline]
 kasan_report+0x25b/0x340 mm/kasan/report.c:409
 __asan_report_load4_noabort+0x14/0x20 mm/kasan/report.c:429
 l2tp_session_create+0xa6d/0xc60 net/l2tp/l2tp_core.c:1757
 pppol2tp_connect+0xed7/0x1dd0 net/l2tp/l2tp_ppp.c:748
 SYSC_connect+0x213/0x4a0 net/socket.c:1621
 SyS_connect+0x24/0x30 net/socket.c:1602
 entry_SYSCALL_64_fastpath+0x29/0xa0
RIP: 0033:0x453299
RSP: 002b:00007f8a00aebc58 EFLAGS: 00000212 ORIG_RAX: 000000000000002a
RAX: ffffffffffffffda RBX: 000000000071bec8 RCX: 0000000000453299
RDX: 000000000000002e RSI: 0000000020e92000 RDI: 0000000000000017
RBP: 000000000071bec8 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000212 R12: 0000000000000000
R13: 0000000000a2f33f R14: 00007f8a00aec9c0 R15: 0000000000000000

Allocated by task 6642:
 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:551
 kmem_cache_alloc_trace+0x136/0x750 mm/slab.c:3610
 kmalloc include/linux/slab.h:499 [inline]
 kzalloc include/linux/slab.h:688 [inline]
 l2tp_tunnel_create+0x5e1/0x17f0 net/l2tp/l2tp_core.c:1554
 pppol2tp_connect+0x14b7/0x1dd0 net/l2tp/l2tp_ppp.c:707
 SYSC_connect+0x213/0x4a0 net/socket.c:1621
 SyS_connect+0x24/0x30 net/socket.c:1602
 entry_SYSCALL_64_fastpath+0x29/0xa0

Freed by task 6643:
 save_stack+0x43/0xd0 mm/kasan/kasan.c:447
 set_track mm/kasan/kasan.c:459 [inline]
 kasan_slab_free+0x71/0xc0 mm/kasan/kasan.c:524
 __cache_free mm/slab.c:3488 [inline]
 kfree+0xd6/0x260 mm/slab.c:3803
 __rcu_reclaim kernel/rcu/rcu.h:190 [inline]
 rcu_do_batch kernel/rcu/tree.c:2758 [inline]
 invoke_rcu_callbacks kernel/rcu/tree.c:3012 [inline]
 __rcu_process_callbacks kernel/rcu/tree.c:2979 [inline]
 rcu_process_callbacks+0xe94/0x17f0 kernel/rcu/tree.c:2996
 __do_softirq+0x2d7/0xb85 kernel/softirq.c:285

The buggy address belongs to the object at ffff8801cd54dc80
 which belongs to the cache kmalloc-512 of size 512
The buggy address is located 232 bytes inside of
 512-byte region [ffff8801cd54dc80, ffff8801cd54de80)
The buggy address belongs to the page:
page:ffffea0007355340 count:1 mapcount:0 mapping:ffff8801cd54d000 index:0x0
flags: 0x2fffc0000000100(slab)
raw: 02fffc0000000100 ffff8801cd54d000 0000000000000000 0000000100000006
raw: ffffea000745daa0 ffffea0006eb15a0 ffff8801dac00940 0000000000000000
page dumped because: kasan: bad access detected

Memory state around the buggy address:
 ffff8801cd54dc00: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
 ffff8801cd54dc80: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
>ffff8801cd54dd00: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
                                                          ^
 ffff8801cd54dd80: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
 ffff8801cd54de00: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
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