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>

1 files changed, 4 insertions, 1 deletions

diff --git a/net/mctp/route.c b/net/mctp/route.c
index fe6c8bf1ec2c..6f277e56b168 100644
--- 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;
 }