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BUG: KASAN: use-after-free in l2tp_session_create+0xb1f/0xcc0 net/l2tp/l2tp_core.c:1809
Read of size 4 at addr ffff8880a1d28428 by task syz-executor.1/12391

CPU: 0 PID: 12391 Comm: syz-executor.1 Not tainted 4.14.275-syzkaller #0
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+0x1b2/0x281 lib/dump_stack.c:58
 print_address_description.cold+0x54/0x1d3 mm/kasan/report.c:252
 kasan_report_error.cold+0x8a/0x191 mm/kasan/report.c:351
 kasan_report mm/kasan/report.c:409 [inline]
 __asan_report_load4_noabort+0x68/0x70 mm/kasan/report.c:429
 l2tp_session_create+0xb1f/0xcc0 net/l2tp/l2tp_core.c:1809
 pppol2tp_connect+0xcf5/0x1370 net/l2tp/l2tp_ppp.c:782
 SYSC_connect net/socket.c:1655 [inline]
 SyS_connect+0x1f4/0x240 net/socket.c:1636
 do_syscall_64+0x1d5/0x640 arch/x86/entry/common.c:292
 entry_SYSCALL_64_after_hwframe+0x46/0xbb
RIP: 0033:0x7f6b8ef0f049
RSP: 002b:00007f6b8d884168 EFLAGS: 00000246 ORIG_RAX: 000000000000002a
RAX: ffffffffffffffda RBX: 00007f6b8f021f60 RCX: 00007f6b8ef0f049
RDX: 000000000000002e RSI: 0000000020000000 RDI: 0000000000000005
RBP: 00007f6b8ef6908d R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
R13: 00007ffcc868a3df R14: 00007f6b8d884300 R15: 0000000000022000

Allocated by task 12391:
 save_stack mm/kasan/kasan.c:447 [inline]
 set_track mm/kasan/kasan.c:459 [inline]
 kasan_kmalloc+0xeb/0x160 mm/kasan/kasan.c:551
 kmem_cache_alloc_trace+0x131/0x3d0 mm/slab.c:3618
 kmalloc include/linux/slab.h:488 [inline]
 kzalloc include/linux/slab.h:661 [inline]
 l2tp_tunnel_create+0x40b/0xd90 net/l2tp/l2tp_core.c:1618
 pppol2tp_connect+0xe05/0x1370 net/l2tp/l2tp_ppp.c:734
 SYSC_connect net/socket.c:1655 [inline]
 SyS_connect+0x1f4/0x240 net/socket.c:1636
 do_syscall_64+0x1d5/0x640 arch/x86/entry/common.c:292
 entry_SYSCALL_64_after_hwframe+0x46/0xbb

Freed by task 12393:
 save_stack mm/kasan/kasan.c:447 [inline]
 set_track mm/kasan/kasan.c:459 [inline]
 kasan_slab_free+0xc3/0x1a0 mm/kasan/kasan.c:524
 __cache_free mm/slab.c:3496 [inline]
 kfree+0xc9/0x250 mm/slab.c:3815
 __rcu_reclaim kernel/rcu/rcu.h:190 [inline]
 rcu_do_batch kernel/rcu/tree.c:2699 [inline]
 invoke_rcu_callbacks kernel/rcu/tree.c:2962 [inline]
 __rcu_process_callbacks kernel/rcu/tree.c:2929 [inline]
 rcu_process_callbacks+0x88b/0x1180 kernel/rcu/tree.c:2946
 __do_softirq+0x24d/0x9ff kernel/softirq.c:288

The buggy address belongs to the object at ffff8880a1d28340
 which belongs to the cache kmalloc-512 of size 512
The buggy address is located 232 bytes inside of
 512-byte region [ffff8880a1d28340, ffff8880a1d28540)
The buggy address belongs to the page:
page:ffffea0002874a00 count:1 mapcount:0 mapping:ffff8880a1d280c0 index:0xffff8880a1d28ac0
flags: 0xfff00000000100(slab)
raw: 00fff00000000100 ffff8880a1d280c0 ffff8880a1d28ac0 0000000100000004
raw: ffffea00027aa120 ffff88813fe64738 ffff88813fe74940 0000000000000000
page dumped because: kasan: bad access detected

Memory state around the buggy address:
 ffff8880a1d28300: fc fc fc fc fc fc fc fc fb fb fb fb fb fb fb fb
 ffff8880a1d28380: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
>ffff8880a1d28400: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
                                  ^
 ffff8880a1d28480: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
 ffff8880a1d28500: fb fb fb fb fb fb fb fb fc fc fc fc fc fc fc fc
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