=============================
WARNING: suspicious RCU usage
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
4 locks held by syz.6.9669/355:
 #0: ffff888019fbc120 (sk_lock-AF_PPPOX){+.+.}-{0:0}, at: lock_sock include/net/sock.h:1744 [inline]
 #0: ffff888019fbc120 (sk_lock-AF_PPPOX){+.+.}-{0:0}, at: pppoe_sendmsg+0x4d/0x740 drivers/net/ppp/pppoe.c:849
 #1: ffffffff8c31eb00 (rcu_read_lock_bh){....}-{1:2}, at: rcu_lock_acquire+0x9/0x30 include/linux/rcupdate.h:313
 #2: ffff88801fc84908 (&sch->q.lock){+.-.}-{2:2}, at: spin_lock include/linux/spinlock.h:364 [inline]
 #2: ffff88801fc84908 (&sch->q.lock){+.-.}-{2:2}, at: __dev_xmit_skb net/core/dev.c:3911 [inline]
 #2: ffff88801fc84908 (&sch->q.lock){+.-.}-{2:2}, at: __dev_queue_xmit+0xa2a/0x2fd0 net/core/dev.c:4253
 #3: ffffffff8c31eaa0 (rcu_read_lock){....}-{1:2}, at: rcu_lock_acquire+0x5/0x30 include/linux/rcupdate.h:312

stack backtrace:
CPU: 0 PID: 355 Comm: syz.6.9669 Not tainted syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 10/25/2025
Call Trace:
 <TASK>
 dump_stack_lvl+0x188/0x250 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:783
 sfq_enqueue+0x14ad/0x2280 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+0xc25/0x2fd0 net/core/dev.c:4253
 pppoe_sendmsg+0x4f8/0x740 drivers/net/ppp/pppoe.c:900
 sock_sendmsg_nosec net/socket.c:706 [inline]
 __sock_sendmsg net/socket.c:718 [inline]
 ____sys_sendmsg+0x5b7/0x8f0 net/socket.c:2445
 ___sys_sendmsg+0x236/0x2e0 net/socket.c:2499
 __sys_sendmmsg+0x2ba/0x500 net/socket.c:2585
 __do_sys_sendmmsg net/socket.c:2614 [inline]
 __se_sys_sendmmsg net/socket.c:2611 [inline]
 __x64_sys_sendmmsg+0x9c/0xb0 net/socket.c:2611
 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: 0033:0x7fbc08bb4eb9
Code: ff c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 44 00 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 e8 ff ff ff f7 d8 64 89 01 48
RSP: 002b:00007fbc06e10028 EFLAGS: 00000246 ORIG_RAX: 0000000000000133
RAX: ffffffffffffffda RBX: 00007fbc08e2ffa0 RCX: 00007fbc08bb4eb9
RDX: 0000000000034000 RSI: 00002000000008c0 RDI: 0000000000000005
RBP: 00007fbc08c22c1f R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
R13: 00007fbc08e30038 R14: 00007fbc08e2ffa0 R15: 00007ffede9cb708
 </TASK>