net/xen-netback: prevent UAF in xenvif_flush_hash()

During the list_for_each_entry_rcu iteration call of xenvif_flush_hash,
kfree_rcu does not exist inside the rcu read critical section, so if
kfree_rcu is called when the rcu grace period ends during the iteration,
UAF occurs when accessing head->next after the entry becomes free.

Therefore, to solve this, you need to change it to list_for_each_entry_safe.
Signed-off-by: Jeongjun Park <aha310510@gmail.com>
Link: https://patch.msgid.link/20240822181109.2577354-1-aha310510@gmail.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

drivers/net/xen-netback/hash.c

@@ -95,7 +95,7 @@ static u32 xenvif_new_hash(struct xenvif *vif, const u8 *data,
 
 static void xenvif_flush_hash(struct xenvif *vif)
 {
-	struct xenvif_hash_cache_entry *entry;
+	struct xenvif_hash_cache_entry *entry, *n;
 	unsigned long flags;
 
 	if (xenvif_hash_cache_size == 0)
@@ -103,8 +103,7 @@ static void xenvif_flush_hash(struct xenvif *vif)
 
 	spin_lock_irqsave(&vif->hash.cache.lock, flags);
 
-	list_for_each_entry_rcu(entry, &vif->hash.cache.list, link,
-				lockdep_is_held(&vif->hash.cache.lock)) {
+	list_for_each_entry_safe(entry, n, &vif->hash.cache.list, link) {
 		list_del_rcu(&entry->link);
 		vif->hash.cache.count--;
 		kfree_rcu(entry, rcu);