Subject: [PATCH AUTOSEL 5.17 101/149] mctp: make __mctp_dev_get() take a refcount hold

From: Sasha Levin <[email-masked]>
Date: Fri, 1 Apr 2022 10:24:48 -0400

From: Matt Johnston <[email-masked]>

[ Upstream commit dc121c0084910db985cf1c8ba6fce5d8c307cc02 ]

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 <[email-masked]>
Signed-off-by: Matt Johnston <[email-masked]>
Signed-off-by: David S. Miller <[email-masked]>
Signed-off-by: Sasha Levin <[email-masked]>
---
 net/mctp/device.c     | 21 ++++++++++++++++++---
 net/mctp/route.c      |  5 ++++-
 net/mctp/test/utils.c |  1 -
 3 files changed, 22 insertions(+), 5 deletions(-)

diff --git a/net/mctp/device.c b/net/mctp/device.c
index ef2755f82f87..f86ef6d751bd 100644
--- a/net/mctp/device.c
+++ b/net/mctp/device.c
@@ -24,12 +24,25 @@ 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);
@@ -123,6 +136,7 @@ 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)
@@ -297,7 +311,7 @@ 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);
 	}
@@ -369,6 +383,7 @@ 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 e52cef750500..05fbd318eb98 100644
--- a/net/mctp/route.c
+++ b/net/mctp/route.c
@@ -786,7 +786,7 @@ 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;
@@ -892,6 +892,7 @@ int mctp_local_output(struct sock *sk, struct mctp_route *rt,
 		mctp_route_release(rt);
 
 	dev_put(dev);
+	mctp_dev_put(tmp_rt.dev);
 
 	return rc;
 
@@ -1057,11 +1058,13 @@ 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 7b7918702592..e03ba66bbe18 100644
--- 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;
-- 
2.34.1