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

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Date:   Thu, 13 May 2021 08:29:01 +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 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.

CJ