syzbot

============================================
WARNING: possible recursive locking detected
5.15.98-syzkaller #0 Not tainted
--------------------------------------------
syz-executor154/4051 is trying to acquire lock:
ffff0000c26fd120 (sk_lock-AF_PPPOX){+.+.}-{0:0}, at: lock_sock include/net/sock.h:1649 [inline]
ffff0000c26fd120 (sk_lock-AF_PPPOX){+.+.}-{0:0}, at: l2tp_tunnel_register+0x888/0x1974 net/l2tp/l2tp_core.c:1485

but task is already holding lock:
ffff0000c1006120 (sk_lock-AF_PPPOX){+.+.}-{0:0}, at: lock_sock include/net/sock.h:1649 [inline]
ffff0000c1006120 (sk_lock-AF_PPPOX){+.+.}-{0:0}, at: pppol2tp_connect+0x53c/0x1348 net/l2tp/l2tp_ppp.c:676

other info that might help us debug this:
 Possible unsafe locking scenario:

       CPU0
       ----
  lock(sk_lock-AF_PPPOX);
  lock(sk_lock-AF_PPPOX);

 *** DEADLOCK ***

 May be due to missing lock nesting notation

1 lock held by syz-executor154/4051:
 #0: ffff0000c1006120 (sk_lock-AF_PPPOX){+.+.}-{0:0}, at: lock_sock include/net/sock.h:1649 [inline]
 #0: ffff0000c1006120 (sk_lock-AF_PPPOX){+.+.}-{0:0}, at: pppol2tp_connect+0x53c/0x1348 net/l2tp/l2tp_ppp.c:676

stack backtrace:
CPU: 1 PID: 4051 Comm: syz-executor154 Not tainted 5.15.98-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 03/02/2023
Call trace:
 dump_backtrace+0x0/0x530 arch/arm64/kernel/stacktrace.c:152
 show_stack+0x2c/0x3c arch/arm64/kernel/stacktrace.c:216
 __dump_stack lib/dump_stack.c:88 [inline]
 dump_stack_lvl+0x108/0x170 lib/dump_stack.c:106
 dump_stack+0x1c/0x58 lib/dump_stack.c:113
 __lock_acquire+0x62b4/0x7620 kernel/locking/lockdep.c:5011
 lock_acquire+0x2b8/0x894 kernel/locking/lockdep.c:5622
 lock_sock_nested+0xec/0x1ec net/core/sock.c:3207
 lock_sock include/net/sock.h:1649 [inline]
 l2tp_tunnel_register+0x888/0x1974 net/l2tp/l2tp_core.c:1485
 pppol2tp_connect+0x944/0x1348 net/l2tp/l2tp_ppp.c:724
 __sys_connect_file net/socket.c:1899 [inline]
 __sys_connect+0x268/0x290 net/socket.c:1916
 __do_sys_connect net/socket.c:1926 [inline]
 __se_sys_connect net/socket.c:1923 [inline]
 __arm64_sys_connect+0x7c/0x94 net/socket.c:1923
 __invoke_syscall arch/arm64/kernel/syscall.c:38 [inline]
 invoke_syscall+0x98/0x2b8 arch/arm64/kernel/syscall.c:52
 el0_svc_common+0x138/0x258 arch/arm64/kernel/syscall.c:142
 do_el0_svc+0x58/0x14c arch/arm64/kernel/syscall.c:181
 el0_svc+0x7c/0x1f0 arch/arm64/kernel/entry-common.c:596
 el0t_64_sync_handler+0x84/0xe4 arch/arm64/kernel/entry-common.c:614
 el0t_64_sync+0x1a0/0x1a4 <unknown>:584