linux-kernel - Re: [PATCH v3] misc/mei: Add NULL check to component match callback functions

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Date:   Thu, 31 Mar 2022 12:32:24 +0300
From:   Heikki Krogerus <heikki.krogerus@...ux.intel.com>
To:     Greg KH <gregkh@...uxfoundation.org>
Cc:     Won Chung <wonchung@...gle.com>,
        Tomas Winkler <tomas.winkler@...el.com>,
        Mika Westerberg <mika.westerberg@...ux.intel.com>,
        Benson Leung <bleung@...omium.org>,
        Prashant Malani <pmalani@...omium.org>,
        linux-kernel@...r.kernel.org, stable@...r.kernel.org
Subject: Re: [PATCH v3] misc/mei: Add NULL check to component match callback
 functions

On Thu, Mar 31, 2022 at 11:00:46AM +0200, Greg KH wrote:
> On Thu, Mar 31, 2022 at 08:49:18AM +0000, Won Chung wrote:
> > Component match callback functions need to check if expected data is
> > passed to them. Without this check, it can cause a NULL pointer
> > dereference when another driver registers a component before i915
> > drivers have their component master fully bind.
> 
> How can that happen in a real system?  Or does this just happen for when
> you are doing development and testing?
> 
> > 
> > Fixes: 1e8d19d9b0dfc ("mei: hdcp: bind only with i915 on the same PCH")
> > Fixes: c2004ce99ed73 ("mei: pxp: export pavp client to me client bus")
> > Suggested-by: Heikki Krogerus <heikki.krogerus@...ux.intel.com>
> > Suggested-by: Mika Westerberg <mika.westerberg@...ux.intel.com>
> > Signed-off-by: Won Chung <wonchung@...gle.com>
> > Cc: stable@...r.kernel.org
> 
> Why does this need to go to stable?  How can this be triggered in older
> kernels?
> 
> > ---
> > Changes from v2:
> > - Correctly add "Suggested-by" tag
> > - Add "Cc: stable@...r.kernel.org"
> > 
> > Changes from v1:
> > - Add "Fixes" tag
> > - Send to stable@...r.kernel.org
> > 
> >  drivers/misc/mei/hdcp/mei_hdcp.c | 2 +-
> >  drivers/misc/mei/pxp/mei_pxp.c   | 2 +-
> >  2 files changed, 2 insertions(+), 2 deletions(-)
> > 
> > diff --git a/drivers/misc/mei/hdcp/mei_hdcp.c b/drivers/misc/mei/hdcp/mei_hdcp.c
> > index ec2a4fce8581..843dbc2b21b1 100644
> > --- a/drivers/misc/mei/hdcp/mei_hdcp.c
> > +++ b/drivers/misc/mei/hdcp/mei_hdcp.c
> > @@ -784,7 +784,7 @@ static int mei_hdcp_component_match(struct device *dev, int subcomponent,
> >  {
> >  	struct device *base = data;
> >  
> > -	if (strcmp(dev->driver->name, "i915") ||
> > +	if (!base || !dev->driver || strcmp(dev->driver->name, "i915") ||
> 
> How can base be NULL?
> 
> 
> >  	    subcomponent != I915_COMPONENT_HDCP)
> >  		return 0;
> >  
> > diff --git a/drivers/misc/mei/pxp/mei_pxp.c b/drivers/misc/mei/pxp/mei_pxp.c
> > index f7380d387bab..e32a81da8af6 100644
> > --- a/drivers/misc/mei/pxp/mei_pxp.c
> > +++ b/drivers/misc/mei/pxp/mei_pxp.c
> > @@ -131,7 +131,7 @@ static int mei_pxp_component_match(struct device *dev, int subcomponent,
> >  {
> >  	struct device *base = data;
> >  
> > -	if (strcmp(dev->driver->name, "i915") ||
> > +	if (!base || !dev->driver || strcmp(dev->driver->name, "i915") ||
> 
> Same here, shouldn't this be caught by the driver core or bus and match
> should not be called?
> 
> Why not fix this in the component/driver core instead?

A component is just a device that is declared to be a "component", and
the code that declares it as component does not have to be the driver
of that device. You simply can't assume that it's bind to a driver
like this function does.

In our case the "components" are USB ports, so devices that are never
bind to drivers.

thanks,

-- 
heikki