Lock socket before checking the SOCK_ZAPPED flag in l2tp_ip6_bind().
Without lock, a concurrent call could modify the socket flags between
the sock_flag(sk, SOCK_ZAPPED) test and the lock_sock() call. This way,
a socket could be inserted twice in l2tp_ip6_bind_table. Releasing it
would then leave a stale pointer there, generating use-after-free
errors when walking through the list or modifying adjacent entries.

BUG: KASAN: use-after-free in l2tp_ip6_close+0x22e/0x290 at addr ffff8800081b0ed8
Write of size 8 by task syz-executor/10987
CPU: 0 PID: 10987 Comm: syz-executor Not tainted 4.8.0+ #39
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.8.2-0-g33fbe13 by qemu-project.org 04/01/2014
 ffff880031d97838 ffffffff829f835b ffff88001b5a1640 ffff8800081b0ec0
 ffff8800081b15a0 ffff8800081b6d20 ffff880031d97860 ffffffff8174d3cc
 ffff880031d978f0 ffff8800081b0e80 ffff88001b5a1640 ffff880031d978e0
Call Trace:
 [<ffffffff829f835b>] dump_stack+0xb3/0x118 lib/dump_stack.c:15
 [<ffffffff8174d3cc>] kasan_object_err+0x1c/0x70 mm/kasan/report.c:156
 [<     inline     >] print_address_description mm/kasan/report.c:194
 [<ffffffff8174d666>] kasan_report_error+0x1f6/0x4d0 mm/kasan/report.c:283
 [<     inline     >] kasan_report mm/kasan/report.c:303
 [<ffffffff8174db7e>] __asan_report_store8_noabort+0x3e/0x40 mm/kasan/report.c:329
 [<     inline     >] __write_once_size ./include/linux/compiler.h:249
 [<     inline     >] __hlist_del ./include/linux/list.h:622
 [<     inline     >] hlist_del_init ./include/linux/list.h:637
 [<ffffffff8579047e>] l2tp_ip6_close+0x22e/0x290 net/l2tp/l2tp_ip6.c:239
 [<ffffffff850b2dfd>] inet_release+0xed/0x1c0 net/ipv4/af_inet.c:415
 [<ffffffff851dc5a0>] inet6_release+0x50/0x70 net/ipv6/af_inet6.c:422
 [<ffffffff84c4581d>] sock_release+0x8d/0x1d0 net/socket.c:570
 [<ffffffff84c45976>] sock_close+0x16/0x20 net/socket.c:1017
 [<ffffffff817a108c>] __fput+0x28c/0x780 fs/file_table.c:208
 [<ffffffff817a1605>] ____fput+0x15/0x20 fs/file_table.c:244
 [<ffffffff813774f9>] task_work_run+0xf9/0x170
 [<ffffffff81324aae>] do_exit+0x85e/0x2a00
 [<ffffffff81326dc8>] do_group_exit+0x108/0x330
 [<ffffffff81348cf7>] get_signal+0x617/0x17a0 kernel/signal.c:2307
 [<ffffffff811b49af>] do_signal+0x7f/0x18f0
 [<ffffffff810039bf>] exit_to_usermode_loop+0xbf/0x150 arch/x86/entry/common.c:156
 [<     inline     >] prepare_exit_to_usermode arch/x86/entry/common.c:190
 [<ffffffff81006060>] syscall_return_slowpath+0x1a0/0x1e0 arch/x86/entry/common.c:259
 [<ffffffff85e4d726>] entry_SYSCALL_64_fastpath+0xc4/0xc6
Object at ffff8800081b0ec0, in cache L2TP/IPv6 size: 1448
Allocated:
PID = 10987
 [ 1116.897025] [<ffffffff811ddcb6>] save_stack_trace+0x16/0x20
 [ 1116.897025] [<ffffffff8174c736>] save_stack+0x46/0xd0
 [ 1116.897025] [<ffffffff8174c9ad>] kasan_kmalloc+0xad/0xe0
 [ 1116.897025] [<ffffffff8174cee2>] kasan_slab_alloc+0x12/0x20
 [ 1116.897025] [<     inline     >] slab_post_alloc_hook mm/slab.h:417
 [ 1116.897025] [<     inline     >] slab_alloc_node mm/slub.c:2708
 [ 1116.897025] [<     inline     >] slab_alloc mm/slub.c:2716
 [ 1116.897025] [<ffffffff817476a8>] kmem_cache_alloc+0xc8/0x2b0 mm/slub.c:2721
 [ 1116.897025] [<ffffffff84c4f6a9>] sk_prot_alloc+0x69/0x2b0 net/core/sock.c:1326
 [ 1116.897025] [<ffffffff84c58ac8>] sk_alloc+0x38/0xae0 net/core/sock.c:1388
 [ 1116.897025] [<ffffffff851ddf67>] inet6_create+0x2d7/0x1000 net/ipv6/af_inet6.c:182
 [ 1116.897025] [<ffffffff84c4af7b>] __sock_create+0x37b/0x640 net/socket.c:1153
 [ 1116.897025] [<     inline     >] sock_create net/socket.c:1193
 [ 1116.897025] [<     inline     >] SYSC_socket net/socket.c:1223
 [ 1116.897025] [<ffffffff84c4b46f>] SyS_socket+0xef/0x1b0 net/socket.c:1203
 [ 1116.897025] [<ffffffff85e4d685>] entry_SYSCALL_64_fastpath+0x23/0xc6
Freed:
PID = 10987
 [ 1116.897025] [<ffffffff811ddcb6>] save_stack_trace+0x16/0x20
 [ 1116.897025] [<ffffffff8174c736>] save_stack+0x46/0xd0
 [ 1116.897025] [<ffffffff8174cf61>] kasan_slab_free+0x71/0xb0
 [ 1116.897025] [<     inline     >] slab_free_hook mm/slub.c:1352
 [ 1116.897025] [<     inline     >] slab_free_freelist_hook mm/slub.c:1374
 [ 1116.897025] [<     inline     >] slab_free mm/slub.c:2951
 [ 1116.897025] [<ffffffff81748b28>] kmem_cache_free+0xc8/0x330 mm/slub.c:2973
 [ 1116.897025] [<     inline     >] sk_prot_free net/core/sock.c:1369
 [ 1116.897025] [<ffffffff84c541eb>] __sk_destruct+0x32b/0x4f0 net/core/sock.c:1444
 [ 1116.897025] [<ffffffff84c5aca4>] sk_destruct+0x44/0x80 net/core/sock.c:1452
 [ 1116.897025] [<ffffffff84c5ad33>] __sk_free+0x53/0x220 net/core/sock.c:1460
 [ 1116.897025] [<ffffffff84c5af23>] sk_free+0x23/0x30 net/core/sock.c:1471
 [ 1116.897025] [<ffffffff84c5cb6c>] sk_common_release+0x28c/0x3e0 ./include/net/sock.h:1589
 [ 1116.897025] [<ffffffff8579044e>] l2tp_ip6_close+0x1fe/0x290 net/l2tp/l2tp_ip6.c:243
 [ 1116.897025] [<ffffffff850b2dfd>] inet_release+0xed/0x1c0 net/ipv4/af_inet.c:415
 [ 1116.897025] [<ffffffff851dc5a0>] inet6_release+0x50/0x70 net/ipv6/af_inet6.c:422
 [ 1116.897025] [<ffffffff84c4581d>] sock_release+0x8d/0x1d0 net/socket.c:570
 [ 1116.897025] [<ffffffff84c45976>] sock_close+0x16/0x20 net/socket.c:1017
 [ 1116.897025] [<ffffffff817a108c>] __fput+0x28c/0x780 fs/file_table.c:208
 [ 1116.897025] [<ffffffff817a1605>] ____fput+0x15/0x20 fs/file_table.c:244
 [ 1116.897025] [<ffffffff813774f9>] task_work_run+0xf9/0x170
 [ 1116.897025] [<ffffffff81324aae>] do_exit+0x85e/0x2a00
 [ 1116.897025] [<ffffffff81326dc8>] do_group_exit+0x108/0x330
 [ 1116.897025] [<ffffffff81348cf7>] get_signal+0x617/0x17a0 kernel/signal.c:2307
 [ 1116.897025] [<ffffffff811b49af>] do_signal+0x7f/0x18f0
 [ 1116.897025] [<ffffffff810039bf>] exit_to_usermode_loop+0xbf/0x150 arch/x86/entry/common.c:156
 [ 1116.897025] [<     inline     >] prepare_exit_to_usermode arch/x86/entry/common.c:190
 [ 1116.897025] [<ffffffff81006060>] syscall_return_slowpath+0x1a0/0x1e0 arch/x86/entry/common.c:259
 [ 1116.897025] [<ffffffff85e4d726>] entry_SYSCALL_64_fastpath+0xc4/0xc6
Memory state around the buggy address:
 ffff8800081b0d80: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
 ffff8800081b0e00: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
>ffff8800081b0e80: fc fc fc fc fc fc fc fc fb fb fb fb fb fb fb fb
                                                    ^
 ffff8800081b0f00: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
 ffff8800081b0f80: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb

==================================================================

The same issue exists with l2tp_ip_bind() and l2tp_ip_bind_table.

Fixes: c51ce497

 ("l2tp: fix oops in L2TP IP sockets for connect() AF_UNSPEC case")
Reported-by: Baozeng Ding <sploving1@gmail.com>
Reported-by: Andrey Konovalov <andreyknvl@google.com>
Tested-by: Baozeng Ding <sploving1@gmail.com>
Signed-off-by: Guillaume Nault <g.nault@alphalink.fr>
Signed-off-by: David S. Miller <davem@davemloft.net>