[PATCH v3] mctp: route: hold key->lock in mctp_flow_prepare_output()

[Chengfeng Ye <dg573847474@gmail.com>]

mctp_flow_prepare_output() checks key->dev and may call
mctp_dev_set_key(), but it does not hold key->lock while doing so.

mctp_dev_set_key() and mctp_dev_release_key() are annotated with
__must_hold(&key->lock), so key->dev access is intended to be
serialized by key->lock. The mctp_sendmsg() transmit path reaches
mctp_flow_prepare_output() via mctp_local_output() -> mctp_dst_output()
without holding key->lock, so the check-and-set sequence is racy.

Example interleaving:

  CPU0                                  CPU1
  ----                                  ----
  mctp_flow_prepare_output(key, devA)
    if (!key->dev)  // sees NULL
                                        mctp_flow_prepare_output(
                                            key, devB)
                                          if (!key->dev)  // still NULL
                                          mctp_dev_set_key(devB, key)
                                            mctp_dev_hold(devB)
                                            key->dev = devB
    mctp_dev_set_key(devA, key)
      mctp_dev_hold(devA)
      key->dev = devA   // overwrites devB

Now both devA and devB references were acquired, but only the final
key->dev value is tracked for release. One reference can be lost,
causing a resource leak as mctp_dev_release_key() would only decrease
the reference on one dev.

Fix by taking key->lock around the key->dev check and
mctp_dev_set_key() call.

Fixes: 67737c457281 ("mctp: Pass flow data & flow release events to drivers")
Signed-off-by: Chengfeng Ye <dg573847474@gmail.com>
---
v2: Add fix tag
v3: Shift mctp_dev_set_key() to else block and invert the branch logic 
---
 net/mctp/route.c | 11 +++++++----
 1 file changed, 7 insertions(+), 4 deletions(-)

diff --git a/net/mctp/route.c b/net/mctp/route.c
index 0381377ab760..59ad60b88563 100644
--- a/net/mctp/route.c
+++ b/net/mctp/route.c
@@ -359,6 +359,7 @@ static void mctp_flow_prepare_output(struct sk_buff *skb, struct mctp_dev *dev)
 {
 	struct mctp_sk_key *key;
 	struct mctp_flow *flow;
+	unsigned long flags;
 
 	flow = skb_ext_find(skb, SKB_EXT_MCTP);
 	if (!flow)
@@ -366,12 +367,14 @@ static void mctp_flow_prepare_output(struct sk_buff *skb, struct mctp_dev *dev)
 
 	key = flow->key;
 
-	if (key->dev) {
+	spin_lock_irqsave(&key->lock, flags);
+
+	if (!key->dev)
+		mctp_dev_set_key(dev, key);
+	else
 		WARN_ON(key->dev != dev);
-		return;
-	}
 
-	mctp_dev_set_key(dev, key);
+	spin_unlock_irqrestore(&key->lock, flags);
 }
 #else
 static void mctp_skb_set_flow(struct sk_buff *skb, struct mctp_sk_key *key) {}
-- 
2.25.1