mctp: make __mctp_dev_get() take a refcount hold

Previously there was a race that could allow the mctp_dev refcount
to hit zero:

rcu_read_lock();
mdev = __mctp_dev_get(dev);
// mctp_unregister() happens here, mdev->refs hits zero
mctp_dev_hold(dev);
rcu_read_unlock();

Now we make __mctp_dev_get() take the hold itself. It is safe to test
against the zero refcount because __mctp_dev_get() is called holding
rcu_read_lock and mctp_dev uses kfree_rcu().

Reported-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Matt Johnston <matt@codeconstruct.com.au>
Signed-off-by: David S. Miller <davem@davemloft.net>

diff --git a/net/mctp/device.c b/net/mctp/device.c
index 9e097e6..b754c31 100644 (file)
--- a/net/mctp/device.c
+++ b/net/mctp/device.c
@@ -25,12 +25,25 @@ struct mctp_dump_cb {
        size_t a_idx;
 };
 
-/* unlocked: caller must hold rcu_read_lock */
+/* unlocked: caller must hold rcu_read_lock.
+ * Returned mctp_dev has its refcount incremented, or NULL if unset.
+ */
 struct mctp_dev *__mctp_dev_get(const struct net_device *dev)
 {
-       return rcu_dereference(dev->mctp_ptr);
+       struct mctp_dev *mdev = rcu_dereference(dev->mctp_ptr);
+
+       /* RCU guarantees that any mdev is still live.
+        * Zero refcount implies a pending free, return NULL.
+        */
+       if (mdev)
+               if (!refcount_inc_not_zero(&mdev->refs))
+                       return NULL;
+       return mdev;
 }
 
+/* Returned mctp_dev does not have refcount incremented. The returned pointer
+ * remains live while rtnl_lock is held, as that prevents mctp_unregister()
+ */
 struct mctp_dev *mctp_dev_get_rtnl(const struct net_device *dev)
 {
        return rtnl_dereference(dev->mctp_ptr);
@@ -124,6 +137,7 @@ static int mctp_dump_addrinfo(struct sk_buff *skb, struct netlink_callback *cb)
                                if (mdev) {
                                        rc = mctp_dump_dev_addrinfo(mdev,
                                                                    skb, cb);
+                                       mctp_dev_put(mdev);
                                        // Error indicates full buffer, this
                                        // callback will get retried.
                                        if (rc < 0)
@@ -298,7 +312,7 @@ void mctp_dev_hold(struct mctp_dev *mdev)
 
 void mctp_dev_put(struct mctp_dev *mdev)
 {
-       if (refcount_dec_and_test(&mdev->refs)) {
+       if (mdev && refcount_dec_and_test(&mdev->refs)) {
                dev_put(mdev->dev);
                kfree_rcu(mdev, rcu);
        }
@@ -370,6 +384,7 @@ static size_t mctp_get_link_af_size(const struct net_device *dev,
        if (!mdev)
                return 0;
        ret = nla_total_size(4); /* IFLA_MCTP_NET */
+       mctp_dev_put(mdev);
        return ret;
 }
 

diff --git a/net/mctp/route.c b/net/mctp/route.c
index fe6c8bf..6f277e5 100644 (file)
--- a/net/mctp/route.c
+++ b/net/mctp/route.c
@@ -836,7 +836,7 @@ int mctp_local_output(struct sock *sk, struct mctp_route *rt,
 {
        struct mctp_sock *msk = container_of(sk, struct mctp_sock, sk);
        struct mctp_skb_cb *cb = mctp_cb(skb);
-       struct mctp_route tmp_rt;
+       struct mctp_route tmp_rt = {0};
        struct mctp_sk_key *key;
        struct net_device *dev;
        struct mctp_hdr *hdr;
@@ -948,6 +948,7 @@ out_release:
                mctp_route_release(rt);
 
        dev_put(dev);
+       mctp_dev_put(tmp_rt.dev);
 
        return rc;
 
@@ -1124,11 +1125,13 @@ static int mctp_pkttype_receive(struct sk_buff *skb, struct net_device *dev,
 
        rt->output(rt, skb);
        mctp_route_release(rt);
+       mctp_dev_put(mdev);
 
        return NET_RX_SUCCESS;
 
 err_drop:
        kfree_skb(skb);
+       mctp_dev_put(mdev);
        return NET_RX_DROP;
 }
 

diff --git a/net/mctp/test/utils.c b/net/mctp/test/utils.c
index 7b79187..e03ba66 100644 (file)
--- a/net/mctp/test/utils.c
+++ b/net/mctp/test/utils.c
@@ -54,7 +54,6 @@ struct mctp_test_dev *mctp_test_create_dev(void)
 
        rcu_read_lock();
        dev->mdev = __mctp_dev_get(ndev);
-       mctp_dev_hold(dev->mdev);
        rcu_read_unlock();
 
        return dev;