mctp: route: hold key->lock in mctp_flow_prepare_output()

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>
Link: https://patch.msgid.link/20260306031402.857224-1-dg573847474@gmail.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

diff --git a/net/mctp/route.c b/net/mctp/route.c
index 0381377ab76042c5814e0c672b68ad079e12b23c..59ad60b885631c4de4f859284bc8c0f71201f8f8 100644 (file)
--- 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) {}