KASAN: use-after-free Read in pppol2tp_release

[syzbot]

Hello,

syzbot found the following crash on:

HEAD commit: 8a174b47 sched/fair: prevent possible infinite loop in sch..
git tree: android-4.9
console output: https://syzkaller.appspot.com/x/log.txt?x=13dcd543800000
kernel config: https://syzkaller.appspot.com/x/.config?x=1951622140a47f94
dashboard link: https://syzkaller.appspot.com/bug?extid=012dd53cccd8afbe7524
compiler: gcc (GCC) 7.1.1 20170620
syz repro: https://syzkaller.appspot.com/x/repro.syz?x=16bd91bd800000
C reproducer: https://syzkaller.appspot.com/x/repro.c?x=170571bd800000

IMPORTANT: if you fix the bug, please add the following tag to the commit:
Reported-by: syzbot+012dd5...@syzkaller.appspotmail.com

==================================================================
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 ffff8801bf7be280 by task syzkaller117096/3812

CPU: 1 PID: 3812 Comm: syzkaller117096 Not tainted 4.9.80-g8a174b47 #31
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS
Google 01/01/2011
ffff8801d70d7d18 ffffffff81d94be9 ffffea0006fdef80 ffff8801bf7be280
0000000000000000 ffff8801bf7be280 ffffffff82ed49f0 ffff8801d70d7d50
ffffffff8153e113 ffff8801bf7be280 0000000000000004 0000000000000000
Call Trace:
[<ffffffff81d94be9>] __dump_stack lib/dump_stack.c:15 [inline]
[<ffffffff81d94be9>] dump_stack+0xc1/0x128 lib/dump_stack.c:51
[<ffffffff8153e113>] print_address_description+0x73/0x280
mm/kasan/report.c:252
[<ffffffff8153e635>] kasan_report_error mm/kasan/report.c:351 [inline]
[<ffffffff8153e635>] kasan_report+0x275/0x360 mm/kasan/report.c:408
[<ffffffff8153e774>] __asan_report_load4_noabort+0x14/0x20
mm/kasan/report.c:428
[<ffffffff8358dc73>] pppol2tp_sock_to_session net/l2tp/l2tp_ppp.c:157
[inline]
[<ffffffff8358dc73>] pppol2tp_release+0x293/0x2e0 net/l2tp/l2tp_ppp.c:487
[<ffffffff82ed489d>] sock_release+0x8d/0x1e0 net/socket.c:599
[<ffffffff82ed4a06>] sock_close+0x16/0x20 net/socket.c:1046
[<ffffffff8157570c>] __fput+0x28c/0x6e0 fs/file_table.c:208
[<ffffffff81575be5>] ____fput+0x15/0x20 fs/file_table.c:244
[<ffffffff811957d5>] 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:160
[<ffffffff81006340>] prepare_exit_to_usermode arch/x86/entry/common.c:190
[inline]
[<ffffffff81006340>] syscall_return_slowpath+0x1a0/0x1e0
arch/x86/entry/common.c:259
[<ffffffff838b35ab>] entry_SYSCALL_64_fastpath+0xe6/0xe8

Allocated by task 3812:
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
entry_SYSCALL_64_fastpath+0x29/0xe8

Freed by task 3813:
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:437
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:160
prepare_exit_to_usermode arch/x86/entry/common.c:190 [inline]
syscall_return_slowpath+0x1a0/0x1e0 arch/x86/entry/common.c:259
entry_SYSCALL_64_fastpath+0xe6/0xe8

The buggy address belongs to the object at ffff8801bf7be280
which belongs to the cache kmalloc-512 of size 512
The buggy address is located 0 bytes inside of
512-byte region [ffff8801bf7be280, ffff8801bf7be480)
The buggy address belongs to the page:
page:ffffea0006fdef80 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:
ffff8801bf7be180: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
ffff8801bf7be200: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
> ffff8801bf7be280: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
^
ffff8801bf7be300: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
ffff8801bf7be380: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
==================================================================


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