=============================
WARNING: suspicious RCU usage
5.15.189-syzkaller #0 Not tainted
-----------------------------
net/sched/sch_api.c:304 suspicious rcu_dereference_protected() usage!

other info that might help us debug this:


rcu_scheduler_active = 2, debug_locks = 1
7 locks held by syz.4.3025/14233:
 #0: ffff888075e38e20 (sk_lock-AF_INET){+.+.}-{0:0}, at: lock_sock include/net/sock.h:1694 [inline]
 #0: ffff888075e38e20 (sk_lock-AF_INET){+.+.}-{0:0}, at: l2tp_ip_sendmsg+0x30/0x1570 net/l2tp/l2tp_ip.c:405
 #1: ffffffff8c11c360 (rcu_read_lock){....}-{1:2}, at: rcu_lock_acquire+0x5/0x30 include/linux/rcupdate.h:311
 #2: ffffffff8c11c360 (rcu_read_lock){....}-{1:2}, at: rcu_lock_acquire+0x9/0x30 include/linux/rcupdate.h:312
 #3: ffffffff8c11c3c0 (rcu_read_lock_bh){....}-{1:2}, at: rcu_lock_acquire+0x9/0x30 include/linux/rcupdate.h:312
 #4: ffffffff8c11c3c0 (rcu_read_lock_bh){....}-{1:2}, at: rcu_lock_acquire+0x9/0x30 include/linux/rcupdate.h:312
 #5: ffff888076bcd108 (&sch->q.lock){+.-.}-{2:2}, at: spin_lock include/linux/spinlock.h:363 [inline]
 #5: ffff888076bcd108 (&sch->q.lock){+.-.}-{2:2}, at: __dev_xmit_skb net/core/dev.c:3911 [inline]
 #5: ffff888076bcd108 (&sch->q.lock){+.-.}-{2:2}, at: __dev_queue_xmit+0xb8a/0x2ed0 net/core/dev.c:4253
 #6: ffffffff8c11c360 (rcu_read_lock){....}-{1:2}, at: rcu_lock_acquire+0x5/0x30 include/linux/rcupdate.h:311

stack backtrace:
CPU: 1 PID: 14233 Comm: syz.4.3025 Not tainted 5.15.189-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 07/12/2025
Call Trace:
 <TASK>
 dump_stack_lvl+0x168/0x230 lib/dump_stack.c:106
 qdisc_lookup+0xa6/0x650 net/sched/sch_api.c:304
 qdisc_tree_reduce_backlog+0x190/0x430 net/sched/sch_api.c:793
 sfq_enqueue+0x148e/0x2260 net/sched/sch_sfq.c:-1
 qdisc_enqueue include/net/sch_generic.h:832 [inline]
 tbf_enqueue+0x224/0x6e0 net/sched/sch_tbf.c:243
 dev_qdisc_enqueue+0x48/0x210 net/core/dev.c:3852
 __dev_xmit_skb net/core/dev.c:3936 [inline]
 __dev_queue_xmit+0xd7e/0x2ed0 net/core/dev.c:4253
 neigh_hh_output include/net/neighbour.h:493 [inline]
 neigh_output include/net/neighbour.h:507 [inline]
 ip_finish_output2+0xc69/0x1080 net/ipv4/ip_output.c:228
 dst_output include/net/dst.h:452 [inline]
 ip_local_out net/ipv4/ip_output.c:126 [inline]
 __ip_queue_xmit+0x11c3/0x1c00 net/ipv4/ip_output.c:532
 l2tp_ip_sendmsg+0x5d6/0x1570 net/l2tp/l2tp_ip.c:498
 sock_sendmsg_nosec net/socket.c:704 [inline]
 __sock_sendmsg net/socket.c:716 [inline]
 ____sys_sendmsg+0x5a2/0x8c0 net/socket.c:2436
 ___sys_sendmsg+0x1f0/0x260 net/socket.c:2490
 __sys_sendmmsg+0x27c/0x4a0 net/socket.c:2576
 __do_sys_sendmmsg net/socket.c:2605 [inline]
 __se_sys_sendmmsg net/socket.c:2602 [inline]
 __x64_sys_sendmmsg+0x9c/0xb0 net/socket.c:2602
 do_syscall_x64 arch/x86/entry/common.c:50 [inline]
 do_syscall_64+0x4c/0xa0 arch/x86/entry/common.c:80
 entry_SYSCALL_64_after_hwframe+0x66/0xd0
RIP: 0033:0x7f02f0d54be9
Code: ff ff c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 40 00 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 c7 c1 a8 ff ff ff f7 d8 64 89 01 48
RSP: 002b:00007f02eef7a038 EFLAGS: 00000246 ORIG_RAX: 0000000000000133
RAX: ffffffffffffffda RBX: 00007f02f0f7c180 RCX: 00007f02f0d54be9
RDX: 00000000040000cf RSI: 0000200000000900 RDI: 0000000000000003
RBP: 00007f02f0dd7e19 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
R13: 00007f02f0f7c218 R14: 00007f02f0f7c180 R15: 00007ffdf8cd5b88
 </TASK>