syzbot

============================================
WARNING: possible recursive locking detected
6.1.15-syzkaller #0 Not tainted
--------------------------------------------
syz-executor140/4306 is trying to acquire lock:
ffff0000d4d5e130 (sk_lock-AF_PPPOX){+.+.}-{0:0}, at: lock_sock include/net/sock.h:1721 [inline]
ffff0000d4d5e130 (sk_lock-AF_PPPOX){+.+.}-{0:0}, at: l2tp_tunnel_register+0x888/0x19f8 net/l2tp/l2tp_core.c:1483

but task is already holding lock:
ffff0000cf697130 (sk_lock-AF_PPPOX){+.+.}-{0:0}, at: lock_sock include/net/sock.h:1721 [inline]
ffff0000cf697130 (sk_lock-AF_PPPOX){+.+.}-{0:0}, at: pppol2tp_connect+0x53c/0x1348 net/l2tp/l2tp_ppp.c:675

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-executor140/4306:
 #0: ffff0000cf697130 (sk_lock-AF_PPPOX){+.+.}-{0:0}, at: lock_sock include/net/sock.h:1721 [inline]
 #0: ffff0000cf697130 (sk_lock-AF_PPPOX){+.+.}-{0:0}, at: pppol2tp_connect+0x53c/0x1348 net/l2tp/l2tp_ppp.c:675

stack backtrace:
CPU: 0 PID: 4306 Comm: syz-executor140 Not tainted 6.1.15-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 03/02/2023
Call trace:
 dump_backtrace+0x1c8/0x1f4 arch/arm64/kernel/stacktrace.c:158
 show_stack+0x2c/0x3c arch/arm64/kernel/stacktrace.c:165
 __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+0x6310/0x764c kernel/locking/lockdep.c:5055
 lock_acquire+0x2f8/0x8dc kernel/locking/lockdep.c:5668
 lock_sock_nested+0x78/0x138 net/core/sock.c:3448
 lock_sock include/net/sock.h:1721 [inline]
 l2tp_tunnel_register+0x888/0x19f8 net/l2tp/l2tp_core.c:1483
 pppol2tp_connect+0x944/0x1348 net/l2tp/l2tp_ppp.c:723
 __sys_connect_file net/socket.c:1979 [inline]
 __sys_connect+0x268/0x290 net/socket.c:1996
 __do_sys_connect net/socket.c:2006 [inline]
 __se_sys_connect net/socket.c:2003 [inline]
 __arm64_sys_connect+0x7c/0x94 net/socket.c:2003
 __invoke_syscall arch/arm64/kernel/syscall.c:38 [inline]
 invoke_syscall+0x98/0x2c0 arch/arm64/kernel/syscall.c:52
 el0_svc_common+0x138/0x258 arch/arm64/kernel/syscall.c:142
 do_el0_svc+0x64/0x218 arch/arm64/kernel/syscall.c:206
 el0_svc+0x58/0x168 arch/arm64/kernel/entry-common.c:637
 el0t_64_sync_handler+0x84/0xf0 arch/arm64/kernel/entry-common.c:655
 el0t_64_sync+0x18c/0x190 arch/arm64/kernel/entry.S:581