netdev - Re: [PATCH net] net/sched: fix lockdep splat in qdisc_tree_reduce

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Message-ID: <CAM0EoMkL4UVOjy1mj-w04kned3e0czuRAiyfq9cWzP4PBWwWYw@mail.gmail.com>
Date: Tue, 2 Apr 2024 12:37:12 -0400
From: Jamal Hadi Salim <jhs@...atatu.com>
To: Eric Dumazet <edumazet@...gle.com>
Cc: "David S . Miller" <davem@...emloft.net>, Jakub Kicinski <kuba@...nel.org>, 
	Paolo Abeni <pabeni@...hat.com>, Cong Wang <xiyou.wangcong@...il.com>, 
	Jiri Pirko <jiri@...nulli.us>, netdev@...r.kernel.org, eric.dumazet@...il.com, 
	syzbot <syzkaller@...glegroups.com>
Subject: Re: [PATCH net] net/sched: fix lockdep splat in qdisc_tree_reduce_backlog()

On Tue, Apr 2, 2024 at 9:41 AM Eric Dumazet <edumazet@...gle.com> wrote:
>
> 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
>
> Fixes: d636fc5dd692 ("net: sched: add rcu annotations around qdisc->qdisc_sleeping")
> Reported-by: syzbot <syzkaller@...glegroups.com>
> Signed-off-by: Eric Dumazet <edumazet@...gle.com>

LGTM.
Acked-by: Jamal Hadi Salim <jhs@...atatu.com>

cheers,
jamal

> ---
>  net/sched/sch_api.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/net/sched/sch_api.c b/net/sched/sch_api.c
> index 65e05b0c98e461953aa8d98020142f0abe3ad8a7..60239378d43fb7adfe3926f927f3883f09673c16 100644
> --- a/net/sched/sch_api.c
> +++ b/net/sched/sch_api.c
> @@ -809,7 +809,7 @@ void qdisc_tree_reduce_backlog(struct Qdisc *sch, int n, int len)
>                 notify = !sch->q.qlen && !WARN_ON_ONCE(!n &&
>                                                        !qdisc_is_offloaded);
>                 /* TODO: perform the search on a per txq basis */
> -               sch = qdisc_lookup(qdisc_dev(sch), TC_H_MAJ(parentid));
> +               sch = qdisc_lookup_rcu(qdisc_dev(sch), TC_H_MAJ(parentid));
>                 if (sch == NULL) {
>                         WARN_ON_ONCE(parentid != TC_H_ROOT);
>                         break;
> --
> 2.44.0.478.gd926399ef9-goog
>