linux-kernel - Re: [PATCH] net: dsa: microchip: fix race condition

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Date:   Mon, 5 Oct 2020 12:59:39 -0700
From:   Florian Fainelli <f.fainelli@...il.com>
To:     Christian Eggers <ceggers@...i.de>,
        Woojung Huh <woojung.huh@...rochip.com>,
        Microchip Linux Driver Support <UNGLinuxDriver@...rochip.com>
Cc:     Andrew Lunn <andrew@...n.ch>,
        Vivien Didelot <vivien.didelot@...il.com>,
        "David S . Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org, stable@...r.kernel.org
Subject: Re: [PATCH] net: dsa: microchip: fix race condition



On 10/5/2020 9:08 AM, Christian Eggers wrote:
> Between queuing the delayed work and finishing the setup of the dsa
> ports, the process may sleep in request_module() and the queued work may
> be executed prior the initialization of the DSA ports is finished. In
> ksz_mib_read_work(), a NULL dereference will happen within
> netof_carrier_ok(dp->slave).
> 
> Not queuing the delayed work in ksz_init_mib_timer() make things even
> worse because the work will now be queued for immediate execution
> (instead of 2000 ms) in ksz_mac_link_down() via
> dsa_port_link_register_of().
> 
> Solution:
> 1. Do not queue (only initialize) delayed work in ksz_init_mib_timer().
> 2. Only queue delayed work in ksz_mac_link_down() if init is completed.
> 3. Queue work once in ksz_switch_register(), after dsa_register_switch()
> has completed.
> 
> Signed-off-by: Christian Eggers <ceggers@...i.de>
> Cc: stable@...r.kernel.org

This looks fine to me and the analysis appears to be correct, don't you 
need to pair the test for dev->mib_read_internal being non zero with 
setting it to zero in ksz_switch_unregister()?

You would also most likely want to add a Fixes: tag such that this can 
be back ported to stable trees?

> ---
> Call tree:
> ksz9477_i2c_probe()
> \--ksz9477_switch_register()
>     \--ksz_switch_register()
>        +--dsa_register_switch()
>        |  \--dsa_switch_probe()
>        |     \--dsa_tree_setup()
>        |        \--dsa_tree_setup_switches()
>        |           +--dsa_switch_setup()
>        |           |  +--ksz9477_setup()
>        |           |  |  \--ksz_init_mib_timer()
>        |           |  |     |--/* Start the timer 2 seconds later. */
>        |           |  |     \--schedule_delayed_work(&dev->mib_read, msecs_to_jiffies(2000));
>        |           |  \--__mdiobus_register()
>        |           |     \--mdiobus_scan()
>        |           |        \--get_phy_device()
>        |           |           +--get_phy_id()
>        |           |           \--phy_device_create()
>        |           |              |--/* sleeping, ksz_mib_read_work() can be called meanwhile */
>        |           |              \--request_module()
>        |           |
>        |           \--dsa_port_setup()
>        |              +--/* Called for non-CPU ports */
>        |              +--dsa_slave_create()
>        |              |  +--/* Too late, ksz_mib_read_work() may be called beforehand */
>        |              |  \--port->slave = ...
>        |             ...
>        |              +--Called for CPU port */
>        |              \--dsa_port_link_register_of()
>        |                 \--ksz_mac_link_down()
>        |                    +--/* mib_read must be initialized here */
>        |                    +--/* work is already scheduled, so it will be executed after 2000 ms */
>        |                    \--schedule_delayed_work(&dev->mib_read, 0);
>        \-- /* here port->slave is setup properly, scheduling the delayed work should be safe */
> 
> static void ksz_mib_read_work()
> \--netif_carrier_ok(dp->slave);  dp->slave has not been initialized yet
> 
> 
>   drivers/net/dsa/microchip/ksz_common.c | 16 +++++++++-------
>   1 file changed, 9 insertions(+), 7 deletions(-)
> 
> diff --git a/drivers/net/dsa/microchip/ksz_common.c b/drivers/net/dsa/microchip/ksz_common.c
> index 8e755b50c9c1..a94d2278b95c 100644
> --- a/drivers/net/dsa/microchip/ksz_common.c
> +++ b/drivers/net/dsa/microchip/ksz_common.c
> @@ -103,14 +103,8 @@ void ksz_init_mib_timer(struct ksz_device *dev)
>   
>   	INIT_DELAYED_WORK(&dev->mib_read, ksz_mib_read_work);
>   
> -	/* Read MIB counters every 30 seconds to avoid overflow. */
> -	dev->mib_read_interval = msecs_to_jiffies(30000);
> -
>   	for (i = 0; i < dev->mib_port_cnt; i++)
>   		dev->dev_ops->port_init_cnt(dev, i);
> -
> -	/* Start the timer 2 seconds later. */
> -	schedule_delayed_work(&dev->mib_read, msecs_to_jiffies(2000));
>   }
>   EXPORT_SYMBOL_GPL(ksz_init_mib_timer);
>   
> @@ -143,7 +137,9 @@ void ksz_mac_link_down(struct dsa_switch *ds, int port, unsigned int mode,
>   
>   	/* Read all MIB counters when the link is going down. */
>   	p->read = true;
> -	schedule_delayed_work(&dev->mib_read, 0);
> +	/* timer started */
> +	if (dev->mib_read_interval)
> +		schedule_delayed_work(&dev->mib_read, 0);
>   }
>   EXPORT_SYMBOL_GPL(ksz_mac_link_down);
>   
> @@ -446,6 +442,12 @@ int ksz_switch_register(struct ksz_device *dev,
>   		return ret;
>   	}
>   
> +	/* Read MIB counters every 30 seconds to avoid overflow. */
> +	dev->mib_read_interval = msecs_to_jiffies(30000);
> +
> +	/* Start the MIB timer. */
> +	schedule_delayed_work(&dev->mib_read, 0);
> +
>   	return 0;
>   }
>   EXPORT_SYMBOL(ksz_switch_register);
> 

-- 
Florian