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

[renmingshuai <renmingshuai@huawei.com>]

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.
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;
 	}
-- 
2.27.0