linux-kernel - Re: [PATCH v3] EDAC, ghes: Fix locking and memory barrier issues

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Date:   Fri, 8 Nov 2019 16:32:03 +0100
From:   Borislav Petkov <bp@...en8.de>
To:     Robert Richter <rrichter@...vell.com>
Cc:     James Morse <james.morse@....com>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        Tony Luck <tony.luck@...el.com>, Borislav Petkov <bp@...e.de>,
        "linux-edac@...r.kernel.org" <linux-edac@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3] EDAC, ghes: Fix locking and memory barrier issues

On Tue, Nov 05, 2019 at 08:11:22PM +0000, Robert Richter wrote:
> On 05.11.19 20:07:51, Robert Richter wrote:
> > The ghes registration and refcount is broken in several ways:
> > 
> >  * ghes_edac_register() returns with success for a 2nd instance even
> >    if a first instance is still running. This is not correct as the
> >    first instance may fail later. A subsequent registration may not
> >    finish before the first. Parallel registrations must be avoided.
> > 
> >  * The refcount was increased even if a registration failed. This
> >    leads to stale counters preventing the device from being released.
> > 
> >  * The ghes refcount may not be decremented properly on
> >    unregistration. Always decrement the refcount once
> >    ghes_edac_unregister() is called to keep the refcount sane.
> > 
> >  * The ghes_pvt pointer is handed to the irq handler before
> >    registration finished.
> > 
> >  * The mci structure could be freed while the irq handler is running.
> > 
> > Fix this by adding a mutex to ghes_edac_register(). This mutex
> > serializes instances to register and unregister. The refcount is only
> > increased if the registration succeeded. This makes sure the refcount
> > is in a consistent state after registering or unregistering a device.
> > Note: A spinlock cannot be used here as the code section may sleep.
> > 
> > The ghes_pvt is protected by ghes_lock now. This ensures the pointer
> > is not updated before registration was finished or while the irq
> > handler is running. It is unset before unregistering the device
> > including necessary (implicit) memory barriers making the changes
> > visible to other cpus. Thus, the device can not be used anymore by an
> > interrupt.
> > 
> > Also, rename ghes_init to ghes_refcount for better readability and
> > switch to refcount API.
> > 
> > A refcount is needed. There can be multiple GHES structures being
> > defined (see ACPI 6.3 specification, 18.3.2.7 Generic Hardware Error
> > Source, "Some platforms may describe multiple Generic Hardware Error
> > Source structures with different notification types, ...").
> > 
> > Another approach to use the mci's device refcount (get_device()) and
> > have a release function does not work here. A release function will be
> > called only for device_release() with the last put_device() call. The
> > device must be deleted *before* that with device_del(). This is only
> > possible by maintaining an own refcount.
> > 
> > Fixes: 0fe5f281f749 ("EDAC, ghes: Model a single, logical memory controller")
> > Fixes: 1e72e673b9d1 ("EDAC/ghes: Fix Use after free in ghes_edac remove path")
> > Co-developed-by: James Morse <james.morse@....com>
> > Signed-off-by: James Morse <james.morse@....com>
> > Co-developed-by: Borislav Petkov <bp@...e.de>
> > Signed-off-by: Borislav Petkov <bp@...e.de>
> > Signed-off-by: Robert Richter <rrichter@...vell.com>
> 
> I hope this SOB chain is correct now.

Yeah.

Applied, thanks.

-- 
Regards/Gruss,
    Boris.

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