linux-hardening - Re: [PATCH] platform/x86: thinkpad_acpi: Prefer struct

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Message-ID: <YU8C6B5zw5t4vsh7@kroah.com>
Date:   Sat, 25 Sep 2021 13:07:20 +0200
From:   Greg KH <greg@...ah.com>
To:     Len Baker <len.baker@....com>
Cc:     Hans de Goede <hdegoede@...hat.com>,
        Kees Cook <keescook@...omium.org>,
        Henrique de Moraes Holschuh <hmh@....eng.br>,
        Mark Gross <mgross@...ux.intel.com>,
        "Gustavo A. R. Silva" <gustavoars@...nel.org>,
        ibm-acpi-devel@...ts.sourceforge.net,
        platform-driver-x86@...r.kernel.org,
        linux-hardening@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] platform/x86: thinkpad_acpi: Prefer struct_size over
 open coded arithmetic

On Sat, Sep 25, 2021 at 12:40:44PM +0200, Len Baker wrote:
> Hi,
> 
> On Tue, Sep 21, 2021 at 05:15:35PM +0200, Greg KH wrote:
> >
> > First off, why is a single driver doing so many odd things with
> > attribute groups?  Why not just use them the way that the rest of the
> > kernel does?  Why does this driver need this special handling and no one
> > else does?
> 
> Is [1] the correct way to deal with devices attributes? I think so.
> 
> [1] https://www.kernel.org/doc/html/latest/driver-api/driver-model/driver.html#attributes

No, do not use driver_create_file(), see:
	http://kroah.com/log/blog/2013/06/26/how-to-create-a-sysfs-file-correctly/
as a more up to date thing.

Someone should fix that in-kernel documentation one day :)

> > I think the default way of handling if an attribute is enabled or not,
> > should suffice here, and make things much simpler overall as all of this
> > crazy attribute handling can just be removed.
> 
> Sorry but what is the default way? Would it be correct to check if the
> file exists?

Use the is_visable() callback for the attribute group to enable/disable
the creation of the sysfs file.

> > Bonus would also be that I think it would fix the race conditions that
> > happen when trying to create attributes after the device is bound to the
> > driver that I think the existing driver has today.
> >
> > > > (I see the caller uses +2? Why? It seems to be using each of hotkey_attributes,
> > > > plus 1 more attr, plus a final NULL?)
> > >
> > > The +2 is actually for 2 extra attributes (making the total number
> > > of extra attributes +3 because the sizeof(struct attribute_set_obj)
> > > already includes 1 extra).
> > >
> > > FWIW these 2 extra attributes are for devices with a
> > > a physical rfkill on/off switch and for the device being
> > > a convertible capable of reporting laptop- vs tablet-mode.
> >
> > Again, using the default way to show (or not show) attributes should
> > solve this issue.  Why not just use that instead?
> 
> What is the default way? Would it be correct to use device_create_file()
> and device_remove_file()?

Put all the attributes into an attribute group and attach it to the
driver.  The driver core will create/remove the files when needed.  The
link above should help explain that a bit better, and I can point you at
examples if needed.

Does that help?

thanks,

greg k-h