Re: [PATCH] net/sched: Set the flushing flags to false to prevent an infinite loop

[Pedro Tammela <pctammela@mojatatu.com>]

On 06/06/2023 11:45, renmingshuai wrote:
> When a new chain is added by using tc, one soft lockup alarm will be
>   generated after delete the prio 0 filter of the chain. To reproduce
>   the problem, perform the following steps:
> (1) tc qdisc add dev eth0 root handle 1: htb default 1
> (2) tc chain add dev eth0
> (3) tc filter del dev eth0 chain 0 parent 1: prio 0
> (4) tc filter add dev eth0 chain 0 parent 1:

This seems like it could be added to tdc or 3 and 4 must be run in parallel?

> 
> 
> The refcnt of the chain added by step 2 is equal to 1. After step 3,
>   the flushing flag of the chain is set to true in the tcf_chain_flush()
>   called by tc_del_tfilter() because the prio is 0. In this case, if
>   we add a new filter to this chain, it will never succeed and try again
>   and again because the refresh flash is always true and refcnt is 1.
>   A soft lock alarm is generated 20 seconds later.
> The stack is show as below:
> 
> Kernel panic - not syncing: softlockup: hung tasks
> CPU: 2 PID: 3321861 Comm: tc Kdump: loaded Tainted: G
> Hardware name: QEMU Standard PC (i440FX + PIIX, 1996)
> Call Trace:
>   <IRQ>
>   dump_stack+0x57/0x6e
>   panic+0x196/0x3ec
>   watchdog_timer_fn.cold+0x16/0x5c
>   __run_hrtimer+0x5e/0x190
>   __hrtimer_run_queues+0x8a/0xe0
>   hrtimer_interrupt+0x110/0x2c0
>   ? irqtime_account_irq+0x49/0xf0
>   __sysvec_apic_timer_interrupt+0x5f/0xe0
>   asm_call_irq_on_stack+0x12/0x20
>   </IRQ>
>   sysvec_apic_timer_interrupt+0x72/0x80
>   asm_sysvec_apic_timer_interrupt+0x12/0x20
> RIP: 0010:mutex_lock+0x24/0x70
> RSP: 0018:ffffa836004ab9a8 EFLAGS: 00000246
> RAX: 0000000000000000 RBX: ffff95bb02d76700 RCX: 0000000000000000
> RDX: ffff95bb27462100 RSI: 0000000000000000 RDI: ffff95ba5b527000
> RBP: ffff95ba5b527000 R08: 0000000000000001 R09: ffffa836004abbb8
> R10: 000000000000000f R11: 0000000000000000 R12: 0000000000000000
> R13: ffff95ba5b527000 R14: ffffa836004abbb8 R15: 0000000000000001
>   __tcf_chain_put+0x27/0x200
>   tc_new_tfilter+0x5e8/0x810
>   ? tc_setup_cb_add+0x210/0x210
>   rtnetlink_rcv_msg+0x2e3/0x380
>   ? rtnl_calcit.isra.0+0x120/0x120
>   netlink_rcv_skb+0x50/0x100
>   netlink_unicast+0x12d/0x1d0
>   netlink_sendmsg+0x286/0x490
>   sock_sendmsg+0x62/0x70
>   ____sys_sendmsg+0x24c/0x2c0
>   ? import_iovec+0x17/0x20
>   ? sendmsg_copy_msghdr+0x80/0xa0
>   ___sys_sendmsg+0x75/0xc0
>   ? do_fault_around+0x118/0x160
>   ? do_read_fault+0x68/0xf0
>   ? __handle_mm_fault+0x3f9/0x6f0
>   __sys_sendmsg+0x59/0xa0
>   do_syscall_64+0x33/0x40
>   entry_SYSCALL_64_after_hwframe+0x61/0xc6
> RIP: 0033:0x7f96705b8247
> RSP: 002b:00007ffe552e9dc8 EFLAGS: 00000246 ORIG_RAX: 000000000000002e
> RAX: ffffffffffffffda RBX: 0000000000000000 RCX: 00007f96705b8247
> RDX: 0000000000000000 RSI: 00007ffe552e9e40 RDI: 0000000000000003
> RBP: 0000000000000001 R08: 0000000000000001 R09: 0000558113f678b0
> R10: 00007f967069ab00 R11: 0000000000000246 R12: 00000000647ea089
> R13: 00007ffe552e9f30 R14: 0000000000000001 R15: 0000558113175f00
> 
> To avoid this case, set chain->flushing to be false if the chain->refcnt
>   is 1 after flushing the chain when prio is 0.
> 
> Fixes: 726d061286ce ("net: sched: prevent insertion of new classifiers during chain flush")
> Signed-off-by: Ren Mingshuai <renmingshuai@huawei.com>
> ---
>   net/sched/cls_api.c | 7 +++++++
>   1 file changed, 7 insertions(+)
> 
> diff --git a/net/sched/cls_api.c b/net/sched/cls_api.c
> index 2621550bfddc..68be55d75831 100644
> --- a/net/sched/cls_api.c
> +++ b/net/sched/cls_api.c
> @@ -2442,6 +2442,13 @@ static int tc_del_tfilter(struct sk_buff *skb, struct nlmsghdr *n,
>   		tfilter_notify_chain(net, skb, block, q, parent, n,
>   				     chain, RTM_DELTFILTER, extack);
>   		tcf_chain_flush(chain, rtnl_held);
> +		/* Set the flushing flags to false to prevent an infinite loop
> +		 * when a new filter is added.
> +		 */
> +		mutex_lock(&chain->filter_chain_lock);
> +		if (chain->refcnt == 1)
> +			chain->flushing = false;
> +		mutex_unlock(&chain->filter_chain_lock);
>   		err = 0;
>   		goto errout;
>   	}