net/sched: act_mirred: add loop detection

Commit 0f022d3 ("net/sched: Fix mirred deadlock on device recursion")
added code in the fast path, even when act_mirred is not used.

Prepare its revert by implementing loop detection in act_mirred.

Adds an array of device pointers in struct netdev_xmit.

tcf_mirred_is_act_redirect() can detect if the array
already contains the target device.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Kuniyuki Iwashima <kuniyu@google.com>
Reviewed-by: Toke Høiland-Jørgensen <toke@redhat.com>
Tested-by: Jamal Hadi Salim <jhs@mojatatu.com>
Acked-by: Jamal Hadi Salim <jhs@mojatatu.com>
Link: https://patch.msgid.link/20251014171907.3554413-4-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: edumazet

include/linux/netdevice_xmit.h

@@ -2,14 +2,21 @@
 #ifndef _LINUX_NETDEVICE_XMIT_H
 #define _LINUX_NETDEVICE_XMIT_H
 
+#if IS_ENABLED(CONFIG_NET_ACT_MIRRED)
+#define MIRRED_NEST_LIMIT	4
+#endif
+
+struct net_device;
+
 struct netdev_xmit {
 	u16 recursion;
 	u8  more;
 #ifdef CONFIG_NET_EGRESS
 	u8  skip_txqueue;
 #endif
 #if IS_ENABLED(CONFIG_NET_ACT_MIRRED)
-	u8 sched_mirred_nest;
+	u8			sched_mirred_nest;
+	struct net_device	*sched_mirred_dev[MIRRED_NEST_LIMIT];
 #endif
 #if IS_ENABLED(CONFIG_NF_DUP_NETDEV)
 	u8 nf_dup_skb_recursion;

net/sched/act_mirred.c

@@ -29,31 +29,6 @@
 static LIST_HEAD(mirred_list);
 static DEFINE_SPINLOCK(mirred_list_lock);
 
-#define MIRRED_NEST_LIMIT    4
-
-#ifndef CONFIG_PREEMPT_RT
-static u8 tcf_mirred_nest_level_inc_return(void)
-{
-	return __this_cpu_inc_return(softnet_data.xmit.sched_mirred_nest);
-}
-
-static void tcf_mirred_nest_level_dec(void)
-{
-	__this_cpu_dec(softnet_data.xmit.sched_mirred_nest);
-}
-
-#else
-static u8 tcf_mirred_nest_level_inc_return(void)
-{
-	return current->net_xmit.sched_mirred_nest++;
-}
-
-static void tcf_mirred_nest_level_dec(void)
-{
-	current->net_xmit.sched_mirred_nest--;
-}
-#endif
-
 static bool tcf_mirred_is_act_redirect(int action)
 {
 	return action == TCA_EGRESS_REDIR || action == TCA_INGRESS_REDIR;
@@ -439,44 +414,53 @@ TC_INDIRECT_SCOPE int tcf_mirred_act(struct sk_buff *skb,
 {
 	struct tcf_mirred *m = to_mirred(a);
 	int retval = READ_ONCE(m->tcf_action);
-	unsigned int nest_level;
+	struct netdev_xmit *xmit;
 	bool m_mac_header_xmit;
 	struct net_device *dev;
-	int m_eaction;
+	int i, m_eaction;
 	u32 blockid;
 
-	nest_level = tcf_mirred_nest_level_inc_return();
-	if (unlikely(nest_level > MIRRED_NEST_LIMIT)) {
+#ifdef CONFIG_PREEMPT_RT
+	xmit = &current->net_xmit;
+#else
+	xmit = this_cpu_ptr(&softnet_data.xmit);
+#endif
+	if (unlikely(xmit->sched_mirred_nest >= MIRRED_NEST_LIMIT)) {
 		net_warn_ratelimited("Packet exceeded mirred recursion limit on dev %s\n",
 				     netdev_name(skb->dev));
-		retval = TC_ACT_SHOT;
-		goto dec_nest_level;
+		return TC_ACT_SHOT;
 	}
 
 	tcf_lastuse_update(&m->tcf_tm);
 	tcf_action_update_bstats(&m->common, skb);
 
 	blockid = READ_ONCE(m->tcfm_blockid);
-	if (blockid) {
-		retval = tcf_blockcast(skb, m, blockid, res, retval);
-		goto dec_nest_level;
-	}
+	if (blockid)
+		return tcf_blockcast(skb, m, blockid, res, retval);
 
 	dev = rcu_dereference_bh(m->tcfm_dev);
 	if (unlikely(!dev)) {
 		pr_notice_once("tc mirred: target device is gone\n");
 		tcf_action_inc_overlimit_qstats(&m->common);
-		goto dec_nest_level;
+		return retval;
 	}
+	for (i = 0; i < xmit->sched_mirred_nest; i++) {
+		if (xmit->sched_mirred_dev[i] != dev)
+			continue;
+		pr_notice_once("tc mirred: loop on device %s\n",
+			       netdev_name(dev));
+		tcf_action_inc_overlimit_qstats(&m->common);
+		return retval;
+	}
+
+	xmit->sched_mirred_dev[xmit->sched_mirred_nest++] = dev;
 
 	m_mac_header_xmit = READ_ONCE(m->tcfm_mac_header_xmit);
 	m_eaction = READ_ONCE(m->tcfm_eaction);
 
 	retval = tcf_mirred_to_dev(skb, m, dev, m_mac_header_xmit, m_eaction,
 				   retval);
-
-dec_nest_level:
-	tcf_mirred_nest_level_dec();
+	xmit->sched_mirred_nest--;
 
 	return retval;
 }