=============================
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.5.1535/12362:
 #0: ffffffff8c31eb20 (rcu_read_lock){....}-{1:2}, at: rcu_lock_acquire+0x9/0x30 include/linux/rcupdate.h:313
 #1: ffff888023978148 (dev->qdisc_running_key ?: &qdisc_running_key){+...}-{0:0}, at: net_tx_action+0x6d9/0x880 net/core/dev.c:5154
 #2: ffff888023978108 (&sch->q.lock){+.-.}-{2:2}, at: spin_lock include/linux/spinlock.h:364 [inline]
 #2: ffff888023978108 (&sch->q.lock){+.-.}-{2:2}, at: sch_direct_xmit+0x311/0x4b0 net/sched/sch_generic.c:357
 #3: ffffffff8c31eb20 (rcu_read_lock){....}-{1:2}, at: rcu_lock_acquire+0x5/0x30 include/linux/rcupdate.h:312

stack backtrace:
CPU: 1 PID: 12362 Comm: syz.5.1535 Not tainted syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 04/18/2026
Call Trace:
 <IRQ>
 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
 cake_dequeue+0x1bd6/0x4b10 net/sched/sch_cake.c:2205
 qdisc_peek_dequeued+0x6e/0x1f0 include/net/sch_generic.h:1125
 tbf_dequeue+0x7d/0xce0 net/sched/sch_tbf.c:265
 dequeue_skb net/sched/sch_generic.c:295 [inline]
 qdisc_restart net/sched/sch_generic.c:400 [inline]
 __qdisc_run+0x236/0x1490 net/sched/sch_generic.c:418
 qdisc_run+0x103/0x2f0 include/net/pkt_sched.h:131
 net_tx_action+0x6d9/0x880 net/core/dev.c:5154
 handle_softirqs+0x339/0x830 kernel/softirq.c:576
 __do_softirq kernel/softirq.c:610 [inline]
 invoke_softirq kernel/softirq.c:450 [inline]
 __irq_exit_rcu+0x13b/0x230 kernel/softirq.c:659
 irq_exit_rcu+0x5/0x20 kernel/softirq.c:671
 instr_sysvec_apic_timer_interrupt arch/x86/kernel/apic/apic.c:1108 [inline]
 sysvec_apic_timer_interrupt+0xa0/0xc0 arch/x86/kernel/apic/apic.c:1108
 </IRQ>
 <TASK>
 asm_sysvec_apic_timer_interrupt+0x16/0x20 arch/x86/include/asm/idtentry.h:676
RIP: 0010:__get_user_nocheck_4+0x8/0x10 arch/x86/lib/getuser.S:145
Code: 90 90 0f 01 cb 0f ae e8 0f b6 10 31 c0 0f 01 ca c3 90 0f 01 cb 0f ae e8 0f b7 10 31 c0 0f 01 ca c3 90 0f 01 cb 0f ae e8 8b 10 <31> c0 0f 01 ca c3 90 90 0f 01 cb 0f ae e8 48 8b 10 31 c0 0f 01 ca
RSP: 0000:ffffc900040a7b58 EFLAGS: 00040246
RAX: 00007ffe91ec08c0 RBX: 00007ffe91ec0a00 RCX: 00007ffe91ec08c0
RDX: 0000000000000003 RSI: 0000000000000000 RDI: 00007ffe91ec08c0
RBP: ffffc900040a7cf0 R08: 0000000000000000 R09: 1ffffffff17c6665
R10: dffffc0000000000 R11: fffffbfff17c6666 R12: dffffc0000000000
R13: ffff88802b813b80 R14: 1ffff92000814f7c R15: 0000000000000000
 save_xstate_epilog arch/x86/kernel/fpu/signal.c:116 [inline]
 copy_fpstate_to_sigframe+0x9dc/0xbf0 arch/x86/kernel/fpu/signal.c:217
 get_sigframe arch/x86/kernel/signal.c:295 [inline]
 __setup_rt_frame arch/x86/kernel/signal.c:471 [inline]
 setup_rt_frame arch/x86/kernel/signal.c:780 [inline]
 handle_signal arch/x86/kernel/signal.c:824 [inline]
 arch_do_signal_or_restart+0x679/0x12c0 arch/x86/kernel/signal.c:869
 handle_signal_work kernel/entry/common.c:154 [inline]
 exit_to_user_mode_loop+0x9e/0x130 kernel/entry/common.c:178
 exit_to_user_mode_prepare+0xee/0x180 kernel/entry/common.c:214
 irqentry_exit_to_user_mode+0x5/0x30 kernel/entry/common.c:320
 asm_sysvec_apic_timer_interrupt+0x16/0x20 arch/x86/include/asm/idtentry.h:676
RIP: 0033:0x7f63e3345504
Code: 85 19 01 00 00 be 08 00 00 00 4c 89 ff 83 c5 01 e8 e1 93 fe ff 49 8b 47 40 41 83 47 30 08 48 8d 48 f8 49 89 4f 40 48 89 58 f8 <49> 83 c5 01 45 3b 6c 24 04 0f 82 fd fe ff ff 41 80 7f 60 00 0f 84
RSP: 002b:00007ffe91ec0a90 EFLAGS: 00000202
RAX: 0000000000000001 RBX: ffffffff83e41007 RCX: ffffffff83e41007
RDX: 0000000000001007 RSI: ffffffff83e41007 RDI: 0000000000000004
RBP: 00000000000002aa R08: 00007f63e36d4000 R09: 00007f63e36d6000
R10: 0000000083e4100b R11: 0000000000000004 R12: 00007f63e36ea038
R13: 000000000005b73e R14: ffffffff83e411c1 R15: 00007f63e4219720
 </TASK>
----------------
Code disassembly (best guess):
   0:	90                   	nop
   1:	90                   	nop
   2:	0f 01 cb             	stac
   5:	0f ae e8             	lfence
   8:	0f b6 10             	movzbl (%rax),%edx
   b:	31 c0                	xor    %eax,%eax
   d:	0f 01 ca             	clac
  10:	c3                   	ret
  11:	90                   	nop
  12:	0f 01 cb             	stac
  15:	0f ae e8             	lfence
  18:	0f b7 10             	movzwl (%rax),%edx
  1b:	31 c0                	xor    %eax,%eax
  1d:	0f 01 ca             	clac
  20:	c3                   	ret
  21:	90                   	nop
  22:	0f 01 cb             	stac
  25:	0f ae e8             	lfence
  28:	8b 10                	mov    (%rax),%edx
* 2a:	31 c0                	xor    %eax,%eax <-- trapping instruction
  2c:	0f 01 ca             	clac
  2f:	c3                   	ret
  30:	90                   	nop
  31:	90                   	nop
  32:	0f 01 cb             	stac
  35:	0f ae e8             	lfence
  38:	48 8b 10             	mov    (%rax),%rdx
  3b:	31 c0                	xor    %eax,%eax
  3d:	0f 01 ca             	clac