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BUG: KASAN: use-after-free in l2tp_session_create+0x9aa/0xb80 net/l2tp/l2tp_core.c:1749
Read of size 4 at addr ffff8801aca6e3e8 by task syz-executor3/6628

CPU: 0 PID: 6628 Comm: syz-executor3 Not tainted 4.16.0-rc1+ #315
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:256
 kasan_report_error mm/kasan/report.c:354 [inline]
 kasan_report+0x23b/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:1749
 pppol2tp_connect+0xed7/0x1dd0 net/l2tp/l2tp_ppp.c:748
 SYSC_connect+0x213/0x4a0 net/socket.c:1639
 SyS_connect+0x24/0x30 net/socket.c:1620
 do_syscall_64+0x280/0x940 arch/x86/entry/common.c:287
 entry_SYSCALL_64_after_hwframe+0x42/0xb7
RIP: 0033:0x453a59
RSP: 002b:00007f3c164a1c68 EFLAGS: 00000246 ORIG_RAX: 000000000000002a
RAX: ffffffffffffffda RBX: 00007f3c164a26d4 RCX: 0000000000453a59
RDX: 000000000000002e RSI: 00000000205fafd2 RDI: 0000000000000017
RBP: 000000000071bea0 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 00000000ffffffff
R13: 000000000000005c R14: 00000000006f0940 R15: 0000000000000000

Allocated by task 6628:
 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+0x5e1/0x17f0 net/l2tp/l2tp_core.c:1545
 pppol2tp_connect+0x14b1/0x1dd0 net/l2tp/l2tp_ppp.c:707
 SYSC_connect+0x213/0x4a0 net/socket.c:1639
 SyS_connect+0x24/0x30 net/socket.c:1620
 do_syscall_64+0x280/0x940 arch/x86/entry/common.c:287
 entry_SYSCALL_64_after_hwframe+0x42/0xb7

Freed by task 4162:
 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 ffff8801aca6e300
 which belongs to the cache kmalloc-512 of size 512
The buggy address is located 232 bytes inside of
 512-byte region [ffff8801aca6e300, ffff8801aca6e500)
The buggy address belongs to the page:
page:ffffea0006b29b80 count:1 mapcount:0 mapping:ffff8801aca6e080 index:0xffff8801aca6ea80
flags: 0x2fffc0000000100(slab)
raw: 02fffc0000000100 ffff8801aca6e080 ffff8801aca6ea80 0000000100000003
raw: ffffea0006b29a60 ffffea00072869a0 ffff8801db000940 0000000000000000
page dumped because: kasan: bad access detected

Memory state around the buggy address:
 ffff8801aca6e280: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
 ffff8801aca6e300: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
>ffff8801aca6e380: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
                                                          ^
 ffff8801aca6e400: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
 ffff8801aca6e480: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
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