CVE-2024-35892: net/sched: fix lockdep splat in qdisc_tree_reduce_backlog()
In the Linux kernel, the following vulnerability has been resolved:

net/sched: fix lockdep splat in qdisc_tree_reduce_backlog()

qdisc_tree_reduce_backlog() is called with the qdisc lock held,
not RTNL.

We must use qdisc_lookup_rcu() instead of qdisc_lookup()

syzbot reported:

WARNING: suspicious RCU usage
6.1.74-syzkaller #0 Not tainted
-----------------------------
net/sched/sch_api.c:305 suspicious rcu_dereference_protected() usage!

other info that might help us debug this:

rcu_scheduler_active = 2, debug_locks = 1
3 locks held by udevd/1142:
  #0: ffffffff87c729a0 (rcu_read_lock){....}-{1:2}, at: rcu_lock_acquire include/linux/rcupdate.h:306 [inline]
  #0: ffffffff87c729a0 (rcu_read_lock){....}-{1:2}, at: rcu_read_lock include/linux/rcupdate.h:747 [inline]
  #0: ffffffff87c729a0 (rcu_read_lock){....}-{1:2}, at: net_tx_action+0x64a/0x970 net/core/dev.c:5282
  #1: ffff888171861108 (&sch->q.lock){+.-.}-{2:2}, at: spin_lock include/linux/spinlock.h:350 [inline]
  #1: ffff888171861108 (&sch->q.lock){+.-.}-{2:2}, at: net_tx_action+0x754/0x970 net/core/dev.c:5297
  #2: ffffffff87c729a0 (rcu_read_lock){....}-{1:2}, at: rcu_lock_acquire include/linux/rcupdate.h:306 [inline]
  #2: ffffffff87c729a0 (rcu_read_lock){....}-{1:2}, at: rcu_read_lock include/linux/rcupdate.h:747 [inline]
  #2: ffffffff87c729a0 (rcu_read_lock){....}-{1:2}, at: qdisc_tree_reduce_backlog+0x84/0x580 net/sched/sch_api.c:792

stack backtrace:
CPU: 1 PID: 1142 Comm: udevd Not tainted 6.1.74-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/25/2024
Call Trace:
 <TASK>
  [<ffffffff85b85f14>] __dump_stack lib/dump_stack.c:88 [inline]
  [<ffffffff85b85f14>] dump_stack_lvl+0x1b1/0x28f lib/dump_stack.c:106
  [<ffffffff85b86007>] dump_stack+0x15/0x1e lib/dump_stack.c:113
  [<ffffffff81802299>] lockdep_rcu_suspicious+0x1b9/0x260 kernel/locking/lockdep.c:6592
  [<ffffffff84f0054c>] qdisc_lookup+0xac/0x6f0 net/sched/sch_api.c:305
  [<ffffffff84f037c3>] qdisc_tree_reduce_backlog+0x243/0x580 net/sched/sch_api.c:811
  [<ffffffff84f5b78c>] pfifo_tail_enqueue+0x32c/0x4b0 net/sched/sch_fifo.c:51
  [<ffffffff84fbcf63>] qdisc_enqueue include/net/sch_generic.h:833 [inline]
  [<ffffffff84fbcf63>] netem_dequeue+0xeb3/0x15d0 net/sched/sch_netem.c:723
  [<ffffffff84eecab9>] dequeue_skb net/sched/sch_generic.c:292 [inline]
  [<ffffffff84eecab9>] qdisc_restart net/sched/sch_generic.c:397 [inline]
  [<ffffffff84eecab9>] __qdisc_run+0x249/0x1e60 net/sched/sch_generic.c:415
  [<ffffffff84d7aa96>] qdisc_run+0xd6/0x260 include/net/pkt_sched.h:125
  [<ffffffff84d85d29>] net_tx_action+0x7c9/0x970 net/core/dev.c:5313
  [<ffffffff85e002bd>] __do_softirq+0x2bd/0x9bd kernel/softirq.c:616
  [<ffffffff81568bca>] invoke_softirq kernel/softirq.c:447 [inline]
  [<ffffffff81568bca>] __irq_exit_rcu+0xca/0x230 kernel/softirq.c:700
  [<ffffffff81568ae9>] irq_exit_rcu+0x9/0x20 kernel/softirq.c:712
  [<ffffffff85b89f52>] sysvec_apic_timer_interrupt+0x42/0x90 arch/x86/kernel/apic/apic.c:1107
  [<ffffffff85c00ccb>] asm_sysvec_apic_timer_interrupt+0x1b/0x20 arch/x86/include/asm/idtentry.h:656
CVSS v3.1: 5.5