Re: [PATCH net] net: dsa: mt7530: fix .get_stats64 sleeping in atomic context

[Jakub Kicinski]

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---
Subject: net: dsa: mt7530: fix .get_stats64 sleeping in atomic context

This commit addresses a sleep-in-atomic bug when reading MDIO-connected
switch port statistics by delegating the register reads to a delayed
workqueue. The get_stats64 callback now returns cached values protected
by a spinlock instead of synchronously reading from the MDIO bus.
MMIO-connected switches are unaffected and continue to read counters
directly.

> diff --git a/drivers/net/dsa/mt7530.c b/drivers/net/dsa/mt7530.c
> index b9423389c2ef0..786d3a8492bcb 100644
> --- a/drivers/net/dsa/mt7530.c
> +++ b/drivers/net/dsa/mt7530.c

[ ... ]

> @@ -951,6 +952,43 @@ static void mt7530_get_stats64(struct dsa_switch *ds, int port,
>  			       &storage->rx_crc_errors);
>  }
>  
> +static void mt7530_stats_poll(struct work_struct *work)
> +{
> +	struct mt7530_priv *priv = container_of(work, struct mt7530_priv,
> +						stats_work.work);
> +	struct rtnl_link_stats64 stats = {};
> +	struct dsa_port *dp;
> +	int port;
> +
> +	dsa_switch_for_each_user_port(dp, priv->ds) {
> +		port = dp->index;
> +
> +		mt7530_read_port_stats64(priv, port, &stats);
> +
> +		spin_lock(&priv->stats_lock);

Are we at risk of a deadlock here?

The get_stats64 callback can be called by dev_get_stats(), which runs in
softirq context.

If this delayed work runs in process context and is preempted by a softirq
that attempts to acquire the lock via dev_get_stats(), a deadlock could
occur. Both paths should likely use spin_lock_bh() and spin_unlock_bh().

> +		priv->ports[port].stats = stats;
> +		spin_unlock(&priv->stats_lock);
> +	}
> +
> +	schedule_delayed_work(&priv->stats_work,
> +			      MT7530_STATS_POLL_INTERVAL);
> +}
> +
> +static void mt7530_get_stats64(struct dsa_switch *ds, int port,
> +			       struct rtnl_link_stats64 *storage)
> +{
> +	struct mt7530_priv *priv = ds->priv;
> +
> +	if (priv->bus) {
> +		spin_lock(&priv->stats_lock);
> +		*storage = priv->ports[port].stats;
> +		spin_unlock(&priv->stats_lock);
> +		mod_delayed_work(system_wq, &priv->stats_work, 0);

Could this unconditional background refresh cause a denial of service on
the MDIO bus?

If an unprivileged user continually reads network statistics in a tight
loop, mt7530_stats_poll() will execute continuously, potentially starving
other MDIO tasks like PHY link-state monitoring.

Additionally, is it possible for this call to re-arm the workqueue during
teardown?

In mt7530_remove_common(), cancel_delayed_work_sync() is called before
dsa_unregister_switch(). If get_stats64() is called during or after
dsa_unregister_switch(), it would re-arm the workqueue after it was
explicitly cancelled, potentially causing a use-after-free when priv is
freed.

> +	} else {
> +		mt7530_read_port_stats64(priv, port, storage);
> +	}
> +}

[ ... ]

> @@ -3137,6 +3175,13 @@ mt753x_setup(struct dsa_switch *ds)
>  	if (ret && priv->irq_domain)
>  		mt7530_free_mdio_irq(priv);
>  
> +	if (!ret && priv->bus) {
> +		spin_lock_init(&priv->stats_lock);
> +		INIT_DELAYED_WORK(&priv->stats_work, mt7530_stats_poll);
> +		schedule_delayed_work(&priv->stats_work,
> +				      MT7530_STATS_POLL_INTERVAL);

If dsa_register_switch() encounters an error after this setup callback
completes successfully, the DSA core will attempt to clean up.

Since mt7530_switch_ops does not implement a teardown callback to cancel
this delayed work, will the work remain scheduled and eventually
dereference freed priv memory once the driver core frees it via devres?

> +	}
> +
>  	return ret;
>  }
-- 
pw-bot: cr