net: fix deadlock while clearing neighbor proxy table

When coming from ndisc_netdev_event() in net/ipv6/ndisc.c,
neigh_ifdown() is called with &nd_tbl, locking this while
clearing the proxy neighbor entries when eg. deleting an
interface. Calling the table's pndisc_destructor() with the
lock still held, however, can cause a deadlock: When a
multicast listener is available an IGMP packet of type
ICMPV6_MGM_REDUCTION may be sent out. When reaching
ip6_finish_output2(), if no neighbor entry for the target
address is found, __neigh_create() is called with &nd_tbl,
which it'll want to lock.

Move the elements into their own list, then unlock the table
and perform the destruction.

Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=199289
Fixes: 6fd6ce2056de ("ipv6: Do not depend on rt->n in ip6_finish_output2().")
Signed-off-by: Wolfgang Bumiller <w.bumiller@proxmox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/neighbour.c

@@ -55,7 +55,8 @@ static void neigh_timer_handler(struct timer_list *t);
 static void __neigh_notify(struct neighbour *n, int type, int flags,
 			   u32 pid);
 static void neigh_update_notify(struct neighbour *neigh, u32 nlmsg_pid);
-static int pneigh_ifdown(struct neigh_table *tbl, struct net_device *dev);
+static int pneigh_ifdown_and_unlock(struct neigh_table *tbl,
+				    struct net_device *dev);
 
 #ifdef CONFIG_PROC_FS
 static const struct file_operations neigh_stat_seq_fops;
@@ -291,8 +292,7 @@ int neigh_ifdown(struct neigh_table *tbl, struct net_device *dev)
 {
 	write_lock_bh(&tbl->lock);
 	neigh_flush_dev(tbl, dev);
-	pneigh_ifdown(tbl, dev);
-	write_unlock_bh(&tbl->lock);
+	pneigh_ifdown_and_unlock(tbl, dev);
 
 	del_timer_sync(&tbl->proxy_timer);
 	pneigh_queue_purge(&tbl->proxy_queue);
@@ -681,9 +681,10 @@ int pneigh_delete(struct neigh_table *tbl, struct net *net, const void *pkey,
 	return -ENOENT;
 }
 
-static int pneigh_ifdown(struct neigh_table *tbl, struct net_device *dev)
+static int pneigh_ifdown_and_unlock(struct neigh_table *tbl,
+				    struct net_device *dev)
 {
-	struct pneigh_entry *n, **np;
+	struct pneigh_entry *n, **np, *freelist = NULL;
 	u32 h;
 
 	for (h = 0; h <= PNEIGH_HASHMASK; h++) {
@@ -691,16 +692,23 @@ static int pneigh_ifdown(struct neigh_table *tbl, struct net_device *dev)
 		while ((n = *np) != NULL) {
 			if (!dev || n->dev == dev) {
 				*np = n->next;
-				if (tbl->pdestructor)
-					tbl->pdestructor(n);
-				if (n->dev)
-					dev_put(n->dev);
-				kfree(n);
+				n->next = freelist;
+				freelist = n;
 				continue;
 			}
 			np = &n->next;
 		}
 	}
+	write_unlock_bh(&tbl->lock);
+	while ((n = freelist)) {
+		freelist = n->next;
+		n->next = NULL;
+		if (tbl->pdestructor)
+			tbl->pdestructor(n);
+		if (n->dev)
+			dev_put(n->dev);
+		kfree(n);
+	}
 	return -ENOENT;
 }