netdev - Re: [PATCH v4 for-next 05/12] IB/cm: Share listening CM IDs

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Message-ID: <20150519183545.GH18675@obsidianresearch.com>
Date:	Tue, 19 May 2015 12:35:45 -0600
From:	Jason Gunthorpe <jgunthorpe@...idianresearch.com>
To:	Haggai Eran <haggaie@...lanox.com>
Cc:	Doug Ledford <dledford@...hat.com>, linux-rdma@...r.kernel.org,
	netdev@...r.kernel.org, Liran Liss <liranl@...lanox.com>,
	Guy Shapiro <guysh@...lanox.com>,
	Shachar Raindel <raindel@...lanox.com>,
	Yotam Kenneth <yotamke@...lanox.com>
Subject: Re: [PATCH v4 for-next 05/12] IB/cm: Share listening CM IDs

On Sun, May 17, 2015 at 08:51:01AM +0300, Haggai Eran wrote:
> @@ -212,6 +212,8 @@ struct cm_id_private {
>  	spinlock_t lock;	/* Do not acquire inside cm.lock */
>  	struct completion comp;
>  	atomic_t refcount;
> +	/* Number of clients sharing this ib_cm_id. Only valid for listeners. */
> +	atomic_t sharecount;

No need for this atomic, hold the lock

The use of the atomic looks racy:

> +	if (!atomic_dec_and_test(&cm_id_priv->sharecount)) {
> +		/* The id is still shared. */
> +		return;
> +	}

Might race with this:

> +		if (atomic_inc_return(&cm_id_priv->sharecount) == 1) {
> +			/* This ID is already being destroyed */
> +			atomic_dec(&cm_id_priv->sharecount);
> +			goto new_id;
> +		}
> +

Resulting in use-after-free of cm_id_priv->sharecount

Don't try and be clever with atomics, it is almost always wrong.

The share count should be 'listen_sharecount' because it *only* works
for listen.

The above test in cm_destroy_id should only be in the listen branch of
the if.

> + * Create a new listening ib_cm_id and listen on the given service ID.
> + *
> + * If there's an existing ID listening on that same device and service ID,
> + * return it.
> + *

.. Callers should call cm_destroy_id when done with the listen ..

Jason
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