vxlan: Add RCU read-side critical sections in the Tx path

JIRA: https://issues.redhat.com/browse/RHEL-115591
Upstream Status: linux.git

commit 804b09b
Author: Ido Schimmel <[email protected]>
Date:   Tue Apr 15 15:11:29 2025 +0300

    vxlan: Add RCU read-side critical sections in the Tx path

    The Tx path does not run from an RCU read-side critical section which
    makes the current lockless accesses to FDB entries invalid. As far as I
    am aware, this has not been a problem in practice, but traces will be
    generated once we transition the FDB lookup to rhashtable_lookup().

    Add rcu_read_{lock,unlock}() around the handling of FDB entries in the
    Tx path. Remove the RCU read-side critical section from vxlan_xmit_nh()
    as now the function is always called from an RCU read-side critical
    section.

    Reviewed-by: Petr Machata <[email protected]>
    Signed-off-by: Ido Schimmel <[email protected]>
    Link: https://patch.msgid.link/[email protected]
    Reviewed-by: Nikolay Aleksandrov <[email protected]>
    Signed-off-by: Paolo Abeni <[email protected]>

Signed-off-by: Guillaume Nault <[email protected]>

Author: Guillaume Nault

drivers/net/vxlan/vxlan_core.c

@@ -1921,12 +1921,15 @@ static int arp_reduce(struct net_device *dev, struct sk_buff *skb, __be32 vni)
 			goto out;
 		}
 
+		rcu_read_lock();
 		f = vxlan_find_mac(vxlan, n->ha, vni);
 		if (f && vxlan_addr_any(&(first_remote_rcu(f)->remote_ip))) {
 			/* bridge-local neighbor */
 			neigh_release(n);
+			rcu_read_unlock();
 			goto out;
 		}
+		rcu_read_unlock();
 
 		reply = arp_create(ARPOP_REPLY, ETH_P_ARP, sip, dev, tip, sha,
 				n->ha, sha);
@@ -2653,14 +2656,10 @@ static void vxlan_xmit_nh(struct sk_buff *skb, struct net_device *dev,
 	memset(&nh_rdst, 0, sizeof(struct vxlan_rdst));
 	hash = skb_get_hash(skb);
 
-	rcu_read_lock();
 	nh = rcu_dereference(f->nh);
-	if (!nh) {
-		rcu_read_unlock();
+	if (!nh)
 		goto drop;
-	}
 	do_xmit = vxlan_fdb_nh_path_select(nh, hash, &nh_rdst);
-	rcu_read_unlock();
 
 	if (likely(do_xmit))
 		vxlan_xmit_one(skb, dev, vni, &nh_rdst, did_rsc);
@@ -2787,6 +2786,7 @@ static netdev_tx_t vxlan_xmit(struct sk_buff *skb, struct net_device *dev)
 	}
 
 	eth = eth_hdr(skb);
+	rcu_read_lock();
 	f = vxlan_find_mac(vxlan, eth->h_dest, vni);
 	did_rsc = false;
 
@@ -2809,7 +2809,7 @@ static netdev_tx_t vxlan_xmit(struct sk_buff *skb, struct net_device *dev)
 			vxlan_vnifilter_count(vxlan, vni, NULL,
 					      VXLAN_VNI_STATS_TX_DROPS, 0);
 			kfree_skb_reason(skb, SKB_DROP_REASON_NO_TX_TARGET);
-			return NETDEV_TX_OK;
+			goto out;
 		}
 	}
 
@@ -2834,6 +2834,8 @@ static netdev_tx_t vxlan_xmit(struct sk_buff *skb, struct net_device *dev)
 			kfree_skb_reason(skb, SKB_DROP_REASON_NO_TX_TARGET);
 	}
 
+out:
+	rcu_read_unlock();
 	return NETDEV_TX_OK;
 }