netdev - Re: [PATCH] net: mdio: Fix a double free issue in the .remove function

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Date:   Thu, 13 May 2021 10:22:11 +0200
From:   Christophe JAILLET <christophe.jaillet@...adoo.fr>
To:     Russell King - ARM Linux admin <linux@...linux.org.uk>
Cc:     andrew@...n.ch, hkallweit1@...il.com, davem@...emloft.net,
        kuba@...nel.org, david.daney@...ium.com, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org, kernel-janitors@...r.kernel.org
Subject: Re: [PATCH] net: mdio: Fix a double free issue in the .remove
 function

Le 13/05/2021 à 10:09, Russell King - ARM Linux admin a écrit :
> On Thu, May 13, 2021 at 08:29:01AM +0200, Christophe JAILLET wrote:
>> Le 12/05/2021 à 23:44, Russell King - ARM Linux admin a écrit :
>>> On Wed, May 12, 2021 at 11:35:38PM +0200, Christophe JAILLET wrote:
>>>> 'bus->mii_bus' have been allocated with 'devm_mdiobus_alloc_size()' in the
>>>> probe function. So it must not be freed explicitly or there will be a
>>>> double free.
>>>>
>>>> Remove the incorrect 'mdiobus_free' in the remove function.
>>>
>>> Yes, this looks correct, thanks.
>>>
>>> Reviewed-by: Russell King <rmk+kernel@...linux.org.uk>
>>>
>>> However, there's another issue in this driver that ought to be fixed.
>>>
>>> If devm_mdiobus_alloc_size() succeeds, but of_mdiobus_register() fails,
>>> we continue on to the next bus (which I think is reasonable.) We don't
>>> free the bus.
>>>
>>> When we come to the remove method however, we will call
>>> mdiobus_unregister() on this existent but not-registered bus. Surely we
>>> don't want to do that?
>>>
>>
>> Hmmm, I don't agree here.
>>
>> 'nexus' is 'kzalloc()'ed.
>> So the pointers in 'buses[]' are all NULL by default.
>> We set 'nexus->buses[i] = bus' only when all functions that can fail in the
>> loop have been called. (the very last 'break' is when the array is full)
>>
>> And in the remove function, we have:
>> 	struct cavium_mdiobus *bus = nexus->buses[i];
>> 	if (!bus)
>> 		continue;
>>
>> So, this looks safe to me.
> 
> It isn't safe. Please look closer.
> 
>          device_for_each_child_node(&pdev->dev, fwn) {
>                  mii_bus = devm_mdiobus_alloc_size(&pdev->dev, sizeof(*bus));
>                  if (!mii_bus)
>                          break;
>                  bus = mii_bus->priv;
>                  bus->mii_bus = mii_bus;
> 
>                  nexus->buses[i] = bus;
> 
> This succeeds and sets nexus->buses[i] to a non-NULL value.
> 
>                  err = of_mdiobus_register(bus->mii_bus, node);
>                  if (err)
>                          dev_err(&pdev->dev, "of_mdiobus_register failed\n");
> 
>                  dev_info(&pdev->dev, "Added bus at %llx\n", r.start);
>                  if (i >= ARRAY_SIZE(nexus->buses))
>                          break;
>          }
> 
> If of_mdiobus_register() fails, the bus is not registered, and we just
> move on to the next bus, leaving nexus->buses[i] set to a non-NULL
> value.

Got it.

Thx for the explanation.

CJ