KASAN: use-after-free Read in l2tp_nl_tunnel_send

[syzbot]

Hello,

syzbot found the following crash on:

HEAD commit: 12cd844a Linux 4.14.173
git tree: linux-4.14.y
console output: https://syzkaller.appspot.com/x/log.txt?x=10db512de00000
kernel config: https://syzkaller.appspot.com/x/.config?x=8a9d0602a0f7791e
dashboard link: https://syzkaller.appspot.com/bug?extid=4264b9036c985be0654c
compiler: gcc (GCC) 9.0.0 20181231 (experimental)

Unfortunately, I don't have any reproducer for this crash yet.

IMPORTANT: if you fix the bug, please add the following tag to the commit:
Reported-by: syzbot+4264b9...@syzkaller.appspotmail.com

Node 0 hugepages_total=0 hugepages_free=0 hugepages_surp=0 hugepages_size=2048kB
==================================================================
Node 1 hugepages_total=0 hugepages_free=0 hugepages_surp=0 hugepages_size=1048576kB
BUG: KASAN: use-after-free in l2tp_nl_tunnel_send+0xb60/0xcf0 net/l2tp/l2tp_netlink.c:339
Read of size 4 at addr ffff88809604f6a8 by task syz-executor.4/6187

CPU: 1 PID: 6187 Comm: syz-executor.4 Not tainted 4.14.173-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+0x13e/0x194 lib/dump_stack.c:58
print_address_description.cold+0x7c/0x1e2 mm/kasan/report.c:252
kasan_report_error mm/kasan/report.c:351 [inline]
kasan_report mm/kasan/report.c:409 [inline]
kasan_report.cold+0xa9/0x2ae mm/kasan/report.c:393
l2tp_nl_tunnel_send+0xb60/0xcf0 net/l2tp/l2tp_netlink.c:339
Node 1 hugepages_total=0 hugepages_free=0 hugepages_surp=0 hugepages_size=2048kB
l2tp_tunnel_notify.constprop.0+0xab/0x140 net/l2tp/l2tp_netlink.c:121
4086 total pagecache pages
l2tp_nl_cmd_tunnel_create+0x3d6/0x8e0 net/l2tp/l2tp_netlink.c:257
0 pages in swap cache
genl_family_rcv_msg+0x57c/0xb30 net/netlink/genetlink.c:600
Swap cache stats: add 0, delete 0, find 0/0
genl_rcv_msg+0xaf/0x140 net/netlink/genetlink.c:625
netlink_rcv_skb+0x127/0x370 net/netlink/af_netlink.c:2433
Free swap = 0kB
genl_rcv+0x24/0x40 net/netlink/genetlink.c:636
Total swap = 0kB
netlink_unicast_kernel net/netlink/af_netlink.c:1287 [inline]
netlink_unicast+0x437/0x620 net/netlink/af_netlink.c:1313
netlink_sendmsg+0x733/0xbe0 net/netlink/af_netlink.c:1878
sock_sendmsg_nosec net/socket.c:646 [inline]
sock_sendmsg+0xc5/0x100 net/socket.c:656
___sys_sendmsg+0x70a/0x840 net/socket.c:2062
1965979 pages RAM
__sys_sendmsg+0xa3/0x120 net/socket.c:2096
SYSC_sendmsg net/socket.c:2107 [inline]
SyS_sendmsg+0x27/0x40 net/socket.c:2103
do_syscall_64+0x1d5/0x640 arch/x86/entry/common.c:292
entry_SYSCALL_64_after_hwframe+0x42/0xb7
RIP: 0033:0x45c679
0 pages HighMem/MovableOnly
RSP: 002b:00007f3d7c6e2c78 EFLAGS: 00000246 ORIG_RAX: 000000000000002e
RAX: ffffffffffffffda RBX: 00007f3d7c6e36d4 RCX: 000000000045c679
RDX: 0000000000000000 RSI: 0000000020000240 RDI: 0000000000000005
RBP: 000000000076bfa0 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 00000000ffffffff
R13: 0000000000000943 R14: 00000000004d5598 R15: 000000000076bfac

339049 pages reserved
Allocated by task 6187:
save_stack+0x32/0xa0 mm/kasan/kasan.c:447
set_track mm/kasan/kasan.c:459 [inline]
kasan_kmalloc mm/kasan/kasan.c:551 [inline]
kasan_kmalloc+0xbf/0xe0 mm/kasan/kasan.c:529
kmem_cache_alloc_trace+0x14d/0x7b0 mm/slab.c:3618
kmalloc include/linux/slab.h:488 [inline]
kzalloc include/linux/slab.h:661 [inline]
l2tp_tunnel_create+0x3a3/0xd10 net/l2tp/l2tp_core.c:1612
l2tp_nl_cmd_tunnel_create+0x3b6/0x8e0 net/l2tp/l2tp_netlink.c:251
genl_family_rcv_msg+0x57c/0xb30 net/netlink/genetlink.c:600
0 pages cma reserved
genl_rcv_msg+0xaf/0x140 net/netlink/genetlink.c:625
Out of memory: Kill process 6144 (syz-executor.0) score 1007 or sacrifice child
netlink_rcv_skb+0x127/0x370 net/netlink/af_netlink.c:2433
genl_rcv+0x24/0x40 net/netlink/genetlink.c:636
netlink_unicast_kernel net/netlink/af_netlink.c:1287 [inline]
netlink_unicast+0x437/0x620 net/netlink/af_netlink.c:1313
netlink_sendmsg+0x733/0xbe0 net/netlink/af_netlink.c:1878
sock_sendmsg_nosec net/socket.c:646 [inline]
sock_sendmsg+0xc5/0x100 net/socket.c:656
___sys_sendmsg+0x70a/0x840 net/socket.c:2062
__sys_sendmsg+0xa3/0x120 net/socket.c:2096
SYSC_sendmsg net/socket.c:2107 [inline]
SyS_sendmsg+0x27/0x40 net/socket.c:2103
do_syscall_64+0x1d5/0x640 arch/x86/entry/common.c:292
entry_SYSCALL_64_after_hwframe+0x42/0xb7

Freed by task 6147:
save_stack+0x32/0xa0 mm/kasan/kasan.c:447
set_track mm/kasan/kasan.c:459 [inline]
kasan_slab_free+0x75/0xc0 mm/kasan/kasan.c:524
__cache_free mm/slab.c:3496 [inline]
kfree+0xcb/0x260 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+0x8a3/0x1190 kernel/rcu/tree.c:2946
__do_softirq+0x254/0x9bf kernel/softirq.c:288

The buggy address belongs to the object at ffff88809604f5c0
which belongs to the cache kmalloc-512 of size 512
The buggy address is located 232 bytes inside of
512-byte region [ffff88809604f5c0, ffff88809604f7c0)
The buggy address belongs to the page:
page:ffffea00025813c0 count:1 mapcount:0 mapping:ffff88809604f0c0 index:0x0
flags: 0xfffe0000000100(slab)
raw: 00fffe0000000100 ffff88809604f0c0 0000000000000000 0000000100000006
raw: ffffea00023f9da0 ffffea00022697e0 ffff88812fe56940 0000000000000000
page dumped because: kasan: bad access detected

Memory state around the buggy address:
ffff88809604f580: fc fc fc fc fc fc fc fc fb fb fb fb fb fb fb fb
ffff88809604f600: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
>ffff88809604f680: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
^
ffff88809604f700: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
ffff88809604f780: fb fb fb fb fb fb fb fb fc fc fc fc fc fc fc fc
==================================================================


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[syzbot]

Auto-closing this bug as obsolete.
Crashes did not happen for a while, no reproducer and no activity.