Subject: Re: [PATCH v2] net/sched: Set the flushing flags to false to prevent an infinite loop and add one test to tdc

From: Vlad Buslov <[email-masked]>
Date: Fri, 9 Jun 2023 17:35:02 +0300

On Fri 09 Jun 2023 at 17:46, renmingshuai <[email-masked]> 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:
>
>
> 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.

Hi Mingshuai,

Thanks for investigating and reporting this!

> 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 2 after flushing the chain when prio is 0. I also add one test to tdc.
>
> Fixes: 726d061286ce ("net: sched: prevent insertion of new classifiers during chain flush")
> Signed-off-by: Mingshuai Ren <[email-masked]>
> Reviewed-by: Aichun Li <[email-masked]>
> ---
> V1 -> V2:
>   * Correct the judgment on the value chain->refcnt and add one test to tdc
> ---
>  net/sched/cls_api.c                           |  7 ++++++
>  .../tc-testing/tc-tests/infra/filter.json     | 25 +++++++++++++++++++
>  2 files changed, 32 insertions(+)
>  create mode 100644 tools/testing/selftests/tc-testing/tc-tests/infra/filter.json
>
> diff --git a/net/sched/cls_api.c b/net/sched/cls_api.c
> index 2621550bfddc..3ea054e03fbf 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 == 2)
> +			chain->flushing = false;
> +		mutex_unlock(&chain->filter_chain_lock);

I don't think this check is enough since there can be concurrent filter
ops holding the reference to the chain. This just makes the issue harder
to reproduce.

I'll try to formulate a fix that takes any potential concurrent users
into account and verify it with your test.

>  		err = 0;
>  		goto errout;
>  	}
> diff --git a/tools/testing/selftests/tc-testing/tc-tests/infra/filter.json b/tools/testing/selftests/tc-testing/tc-tests/infra/filter.json
> new file mode 100644
> index 000000000000..db3b42aaa4fa
> --- /dev/null
> +++ b/tools/testing/selftests/tc-testing/tc-tests/infra/filter.json
> @@ -0,0 +1,25 @@
> +[
> +    {
> +        "id": "c2b4",
> +        "name": "Adding a new filter after flushing empty chain doesn't cause an infinite loop",
> +        "category": [
> +            "filter",
> +            "chain"
> +        ],
> +        "setup": [
> +            "$IP link add dev $DUMMY type dummy || /bin/true",
> +            "$TC qdisc add dev $DUMMY root handle 1: htb default 1",
> +            "$TC chain add dev $DUMMY",
> +            "$TC filter del dev $DUMMY chain 0 parent 1: prio 0"
> +        ],
> +        "cmdUnderTest": "$TC filter add dev $DUMMY chain 0 parent 1:",
> +        "expExitCode": "2",
> +        "verifyCmd": "$TC chain ls dev $DUMMY",
> +        "matchPattern": "chain parent 1: chain 0",
> +        "matchCount": "1",
> +        "teardown": [
> +            "$TC qdisc del dev $DUMMY root handle 1: htb default 1",
> +            "$IP link del dev $DUMMY type dummy"
> +        ]
> +    }
> +]