net: mctp: Ensure keys maintain only one ref to corresponding dev

[ Upstream commit e4f349bd6e58051df698b82f94721f18a02a293d ]

mctp_flow_prepare_output() is called in mctp_route_output(), which
places outbound packets onto a given interface. The packet may represent
a message fragment, in which case we provoke an unbalanced reference
count to the underlying device. This causes trouble if we ever attempt
to remove the interface:

    [   48.702195] usb 1-1: USB disconnect, device number 2
    [   58.883056] unregister_netdevice: waiting for mctpusb0 to become free. Usage count = 2
    [   69.022548] unregister_netdevice: waiting for mctpusb0 to become free. Usage count = 2
    [   79.172568] unregister_netdevice: waiting for mctpusb0 to become free. Usage count = 2
    ...

Predicate the invocation of mctp_dev_set_key() in
mctp_flow_prepare_output() on not already having associated the device
with the key. It's not yet realistic to uphold the property that the key
maintains only one device reference earlier in the transmission sequence
as the route (and therefore the device) may not be known at the time the
key is associated with the socket.

Fixes: 67737c457281 ("mctp: Pass flow data & flow release events to drivers")
Acked-by: Jeremy Kerr <jk@codeconstruct.com.au>
Signed-off-by: Andrew Jeffery <andrew@codeconstruct.com.au>
Link: https://patch.msgid.link/20250508-mctp-dev-refcount-v1-1-d4f965c67bb5@codeconstruct.com.au
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>

diff --git a/net/mctp/route.c b/net/mctp/route.c
index e72cdd4ce588fd5ff2f1452b007693748da8b9b2..62952ad5cb6367a36b3d32871e54341dd7d808f5 100644 (file)
--- a/net/mctp/route.c
+++ b/net/mctp/route.c
@@ -274,8 +274,10 @@ static void mctp_flow_prepare_output(struct sk_buff *skb, struct mctp_dev *dev)
 
        key = flow->key;
 
-       if (WARN_ON(key->dev && key->dev != dev))
+       if (key->dev) {
+               WARN_ON(key->dev != dev);
                return;
+       }
 
        mctp_dev_set_key(dev, key);
 }