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

[Greg KH <gregkh@linuxfoundation.org>]

On Thu, Mar 31, 2022 at 12:32:24PM +0300, Heikki Krogerus wrote:
> 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@linux.intel.com>
> > > Suggested-by: Mika Westerberg <mika.westerberg@linux.intel.com>
> > > Signed-off-by: Won Chung <wonchung@google.com>
> > > Cc: stable@vger.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@vger.kernel.org"
> > > 
> > > Changes from v1:
> > > - Add "Fixes" tag
> > > - Send to stable@vger.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.

And going off of the driver name is sane?  That feels ripe for bugs and
problems in the future, but hey, I don't understand the need for this
driver to care about another driver at all.

And why is a USB device being passed to something that it thinks is a
PCI device?  That too feels really wrong and ripe for problems.

thanks,

greg k-h