ipv4: Fix fib_trie rebalancing

While doing trie_rebalance(): resize(), inflate(), halve() RCU free
tnodes before updating their parents. It depends on RCU delaying the
real destruction, but if RCU readers start after call_rcu() and before
parent update they could access freed memory.

It is currently prevented with preempt_disable() on the update side,
but it's not safe, except maybe classic RCU, plus it conflicts with
memory allocations with GFP_KERNEL flag used from these functions.

This patch explicitly delays freeing of tnodes by adding them to the
list, which is flushed after the update is finished.

Reported-by: Yan Zheng <zheng.yan@oracle.com>
Signed-off-by: Jarek Poplawski <jarkao2@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

diff --git a/net/ipv4/fib_trie.c b/net/ipv4/fib_trie.c
index 538d2a9..d1a39b1 100644 (file)
--- a/net/ipv4/fib_trie.c
+++ b/net/ipv4/fib_trie.c
@@ -123,6 +123,7 @@ struct tnode {
        union {
                struct rcu_head rcu;
                struct work_struct work;
+               struct tnode *tnode_free;
        };
        struct node *child[0];
 };
@@ -161,6 +162,8 @@ static void tnode_put_child_reorg(struct tnode *tn, int i, struct node *n,
 static struct node *resize(struct trie *t, struct tnode *tn);
 static struct tnode *inflate(struct trie *t, struct tnode *tn);
 static struct tnode *halve(struct trie *t, struct tnode *tn);
+/* tnodes to free after resize(); protected by RTNL */
+static struct tnode *tnode_free_head;
 
 static struct kmem_cache *fn_alias_kmem __read_mostly;
 static struct kmem_cache *trie_leaf_kmem __read_mostly;
@@ -385,6 +388,29 @@ static inline void tnode_free(struct tnode *tn)
                call_rcu(&tn->rcu, __tnode_free_rcu);
 }
 
+static void tnode_free_safe(struct tnode *tn)
+{
+       BUG_ON(IS_LEAF(tn));
+
+       if (node_parent((struct node *) tn)) {
+               tn->tnode_free = tnode_free_head;
+               tnode_free_head = tn;
+       } else {
+               tnode_free(tn);
+       }
+}
+
+static void tnode_free_flush(void)
+{
+       struct tnode *tn;
+
+       while ((tn = tnode_free_head)) {
+               tnode_free_head = tn->tnode_free;
+               tn->tnode_free = NULL;
+               tnode_free(tn);
+       }
+}
+
 static struct leaf *leaf_new(void)
 {
        struct leaf *l = kmem_cache_alloc(trie_leaf_kmem, GFP_KERNEL);
@@ -495,7 +521,7 @@ static struct node *resize(struct trie *t, struct tnode *tn)
 
        /* No children */
        if (tn->empty_children == tnode_child_length(tn)) {
-               tnode_free(tn);
+               tnode_free_safe(tn);
                return NULL;
        }
        /* One child */
@@ -509,7 +535,7 @@ static struct node *resize(struct trie *t, struct tnode *tn)
 
                        /* compress one level */
                        node_set_parent(n, NULL);
-                       tnode_free(tn);
+                       tnode_free_safe(tn);
                        return n;
                }
        /*
@@ -670,7 +696,7 @@ static struct node *resize(struct trie *t, struct tnode *tn)
                        /* compress one level */
 
                        node_set_parent(n, NULL);
-                       tnode_free(tn);
+                       tnode_free_safe(tn);
                        return n;
                }
 
@@ -756,7 +782,7 @@ static struct tnode *inflate(struct trie *t, struct tnode *tn)
                        put_child(t, tn, 2*i, inode->child[0]);
                        put_child(t, tn, 2*i+1, inode->child[1]);
 
-                       tnode_free(inode);
+                       tnode_free_safe(inode);
                        continue;
                }
 
@@ -801,9 +827,9 @@ static struct tnode *inflate(struct trie *t, struct tnode *tn)
                put_child(t, tn, 2*i, resize(t, left));
                put_child(t, tn, 2*i+1, resize(t, right));
 
-               tnode_free(inode);
+               tnode_free_safe(inode);
        }
-       tnode_free(oldtnode);
+       tnode_free_safe(oldtnode);
        return tn;
 nomem:
        {
@@ -885,7 +911,7 @@ static struct tnode *halve(struct trie *t, struct tnode *tn)
                put_child(t, newBinNode, 1, right);
                put_child(t, tn, i/2, resize(t, newBinNode));
        }
-       tnode_free(oldtnode);
+       tnode_free_safe(oldtnode);
        return tn;
 nomem:
        {
@@ -989,7 +1015,6 @@ static struct node *trie_rebalance(struct trie *t, struct tnode *tn)
        t_key cindex, key;
        struct tnode *tp;
 
-       preempt_disable();
        key = tn->key;
 
        while (tn != NULL && (tp = node_parent((struct node *)tn)) != NULL) {
@@ -1001,16 +1026,18 @@ static struct node *trie_rebalance(struct trie *t, struct tnode *tn)
                                      (struct node *)tn, wasfull);
 
                tp = node_parent((struct node *) tn);
+               tnode_free_flush();
                if (!tp)
                        break;
                tn = tp;
        }
 
        /* Handle last (top) tnode */
-       if (IS_TNODE(tn))
+       if (IS_TNODE(tn)) {
                tn = (struct tnode *)resize(t, (struct tnode *)tn);
+               tnode_free_flush();
+       }
 
-       preempt_enable();
        return (struct node *)tn;
 }