netdev - Re: [PATCH net v2 2/2] lwtunnel: Fix oops on state free after encap module unload

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Date:   Mon, 23 Jan 2017 15:42:48 -0500 (EST)
From:   David Miller <davem@...emloft.net>
To:     rshearma@...cade.com
Cc:     netdev@...r.kernel.org, tom@...bertland.com,
        roopa@...ulusnetworks.com, david.lebrun@...ouvain.be,
        tgraf@...g.ch, dsa@...ulusnetworks.com
Subject: Re: [PATCH net v2 2/2] lwtunnel: Fix oops on state free after
 encap module unload

From: Robert Shearman <rshearma@...cade.com>
Date: Sat, 21 Jan 2017 00:21:26 +0000

> @@ -115,8 +115,12 @@ int lwtunnel_build_state(struct net_device *dev, u16 encap_type,
>  	ret = -EOPNOTSUPP;
>  	rcu_read_lock();

Here 'ret' equals -EOPNOTSUPP

>  	ops = rcu_dereference(lwtun_encaps[encap_type]);
> -	if (likely(ops && ops->build_state))
> -		ret = ops->build_state(dev, encap, family, cfg, lws);
> +	if (likely(ops)) {
> +		if (likely(try_module_get(ops->owner) && ops->build_state))
> +			ret = ops->build_state(dev, encap, family, cfg, lws);
> +		if (ret)
> +			module_put(ops->owner);

If try_module_get() fails, 'ret' will still be -EOPNOTSUPP and we will
module_put() on a module we did not grab a reference to.

I think you need to adjust the logic here.  You only want to 'put' if
try_module_get() succeeds and ->build_state() returns an error.