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

[Jeremy Kerr]

Hi Chengfeng,

For v2 (as requested by Jakub), can you correct my email address? I'm
jk@, not jeremy@.

> 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.

Good catch. I don't *think* we can hit this at present, as the key will
be unique over distinct sendmsg() calls, and the re-use of a key is all
sequential (ie., during fragmentation).

However, that may not always be the case, and we're currently violating
the __must_hold(key->lock) on mctp_dev_set_key() through this path. So
the addition of the lock here looks good.

One comment on the implementation:

> diff --git a/net/mctp/route.c b/net/mctp/route.c
> index 0381377ab760..4a1ac55ad31e 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 +367,17 @@ static void mctp_flow_prepare_output(struct sk_buff *skb, struct mctp_dev *dev)
>  
>         key = flow->key;
>  
> +       spin_lock_irqsave(&key->lock, flags);
> +
>         if (key->dev) {
>                 WARN_ON(key->dev != dev);
> -               return;
> +               goto out_unlock;
>         }

You could shift the mctp_dev_set_key() to an else block here, and avoid
the goto. Following that, it may be more readable if you invert the
logic (if (!key->dev) ...), but I will leave that as your call.

Cheers,


Jeremy