neighbor: gc_list changes should be protected by table lock

Adding and removing neighbor entries to / from the gc_list need to be
done while holding the table lock; a couple of places were missed in the
original patch.

Move the list_add_tail in neigh_alloc to ___neigh_create where the lock
is already obtained. Since neighbor entries should rarely be moved
to/from PERMANENT state, add lock/unlock around the gc_list changes in
neigh_change_state rather than extending the lock hold around all
neighbor updates.

Fixes: 5895631 ("neighbor: Improve garbage collection")
Reported-by: Andrei Vagin <avagin@gmail.com>
Reported-by: syzbot+6cc2fd1d3bdd2e007363@syzkaller.appspotmail.com
Reported-by: syzbot+35e87b87c00f386b041f@syzkaller.appspotmail.com
Reported-by: syzbot+b354d1fb59091ea73c37@syzkaller.appspotmail.com
Reported-by: syzbot+3ddead5619658537909b@syzkaller.appspotmail.com
Reported-by: syzbot+424d47d5c456ce8b2bbe@syzkaller.appspotmail.com
Reported-by: syzbot+e4d42eb35f6a27b0a628@syzkaller.appspotmail.com
Signed-off-by: David Ahern <dsahern@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

Author: dsahern

net/core/neighbour.c

@@ -138,11 +138,17 @@ static void neigh_change_state(struct neighbour *n, u8 new)
 	 * add to the gc list if new state is not permanent
 	 */
 	if (new_is_perm && on_gc_list) {
+		write_lock_bh(&n->tbl->lock);
 		list_del_init(&n->gc_list);
+		write_unlock_bh(&n->tbl->lock);
+
 		atomic_dec(&n->tbl->gc_entries);
 	} else if (!new_is_perm && !on_gc_list) {
 		/* add entries to the tail; cleaning removes from the front */
+		write_lock_bh(&n->tbl->lock);
 		list_add_tail(&n->gc_list, &n->tbl->gc_list);
+		write_unlock_bh(&n->tbl->lock);
+
 		atomic_inc(&n->tbl->gc_entries);
 	}
 }
@@ -390,11 +396,7 @@ static struct neighbour *neigh_alloc(struct neigh_table *tbl,
 	n->tbl		  = tbl;
 	refcount_set(&n->refcnt, 1);
 	n->dead		  = 1;
-
-	if (!permanent)
-		list_add_tail(&n->gc_list, &n->tbl->gc_list);
-	else
-		INIT_LIST_HEAD(&n->gc_list);
+	INIT_LIST_HEAD(&n->gc_list);
 
 	atomic_inc(&tbl->entries);
 out:
@@ -616,6 +618,9 @@ static struct neighbour *___neigh_create(struct neigh_table *tbl,
 	}
 
 	n->dead = 0;
+	if (!permanent)
+		list_add_tail(&n->gc_list, &n->tbl->gc_list);
+
 	if (want_ref)
 		neigh_hold(n);
 	rcu_assign_pointer(n->next,