syzbot

sch_tbf: burst 19872 is lower than device lo mtu (65550) !
=============================
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
7 locks held by syz.1.49/4507:
 #0: ffff88801a3d2e20 (sk_lock-AF_INET6){+.+.}-{0:0}, at: lock_sock include/net/sock.h:1744 [inline]
 #0: ffff88801a3d2e20 (sk_lock-AF_INET6){+.+.}-{0:0}, at: sctp_setsockopt+0x1ec/0xf80 net/sctp/socket.c:4602
 #1: ffffffff8c31f360 (rcu_read_lock){....}-{1:2}, at: rcu_lock_acquire+0x9/0x30 include/linux/rcupdate.h:313
 #2: ffffffff8c31f360 (rcu_read_lock){....}-{1:2}, at: rcu_lock_acquire+0x9/0x30 include/linux/rcupdate.h:313
 #3: ffffffff8c31f3c0 (rcu_read_lock_bh){....}-{1:2}, at: rcu_lock_acquire+0x9/0x30 include/linux/rcupdate.h:313
 #4: ffffffff8c31f3c0 (rcu_read_lock_bh){....}-{1:2}, at: rcu_lock_acquire+0x9/0x30 include/linux/rcupdate.h:313
 #5: ffff88801f997908 (&sch->q.lock){+...}-{2:2}, at: spin_lock include/linux/spinlock.h:364 [inline]
 #5: ffff88801f997908 (&sch->q.lock){+...}-{2:2}, at: __dev_xmit_skb net/core/dev.c:3924 [inline]
 #5: ffff88801f997908 (&sch->q.lock){+...}-{2:2}, at: __dev_queue_xmit+0xa2a/0x2f80 net/core/dev.c:4266
 #6: ffffffff8c31f360 (rcu_read_lock){....}-{1:2}, at: rcu_lock_acquire+0x5/0x30 include/linux/rcupdate.h:312

stack backtrace:
CPU: 0 PID: 4507 Comm: syz.1.49 Not tainted syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 05/09/2026
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
 cake_enqueue+0x40e4/0x7fd0 net/sched/sch_cake.c:1964
 qdisc_enqueue include/net/sch_generic.h:842 [inline]
 tbf_enqueue+0x224/0x6e0 net/sched/sch_tbf.c:243
 dev_qdisc_enqueue+0x48/0x210 net/core/dev.c:3865
 __dev_xmit_skb net/core/dev.c:3949 [inline]
 __dev_queue_xmit+0xc25/0x2f80 net/core/dev.c:4266
 neigh_hh_output include/net/neighbour.h:493 [inline]
 neigh_output include/net/neighbour.h:507 [inline]
 ip_finish_output2+0xc87/0x10a0 net/ipv4/ip_output.c:228
 NF_HOOK_COND include/linux/netfilter.h:291 [inline]
 ip_output+0x2c0/0x3e0 net/ipv4/ip_output.c:434
 dst_output include/net/dst.h:452 [inline]
 ip_local_out net/ipv4/ip_output.c:126 [inline]
 __ip_queue_xmit+0x11c1/0x1c00 net/ipv4/ip_output.c:538
 sctp_packet_transmit+0x239c/0x2910 net/sctp/output.c:652
 sctp_outq_flush_transports net/sctp/outqueue.c:1170 [inline]
 sctp_outq_flush+0xe62/0x3100 net/sctp/outqueue.c:1218
 sctp_cmd_interpreter net/sctp/sm_sideeffect.c:-1 [inline]
 sctp_side_effects net/sctp/sm_sideeffect.c:1199 [inline]
 sctp_do_sm+0x4ee2/0x55f0 net/sctp/sm_sideeffect.c:1170
 sctp_primitive_REQUESTHEARTBEAT+0x94/0xc0 net/sctp/primitive.c:185
 sctp_apply_peer_addr_params+0xde/0x1870 net/sctp/socket.c:2439
 sctp_setsockopt_peer_addr_params+0x673/0x940 net/sctp/socket.c:2689
 sctp_setsockopt+0x4cd/0xf80 net/sctp/socket.c:4640
 __sys_setsockopt+0x2bf/0x3d0 net/socket.c:2212
 __do_sys_setsockopt net/socket.c:2223 [inline]
 __se_sys_setsockopt net/socket.c:2220 [inline]
 __x64_sys_setsockopt+0xb1/0xc0 net/socket.c:2220
 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: 0033:0x7f3dcc29be59
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:00007f3dca4d4028 EFLAGS: 00000246 ORIG_RAX: 0000000000000036
RAX: ffffffffffffffda RBX: 00007f3dcc515090 RCX: 00007f3dcc29be59
RDX: 0000000000000009 RSI: 0000000000000084 RDI: 0000000000000004
RBP: 00007f3dcc331d6f R08: 000000000000009c R09: 0000000000000000
R10: 0000200000000840 R11: 0000000000000246 R12: 0000000000000000
R13: 00007f3dcc515128 R14: 00007f3dcc515090 R15: 00007fff6f0762d8
 </TASK>