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

[Christophe JAILLET]

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@xxxxxxxxxxxxxxx>
> > >
> > > 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