isa: Prevent NULL dereference in isa_bus driver callbacks

The isa_driver structure for an isa_bus device is stored in the device
platform_data member of the respective device structure. This
platform_data member may be reset to NULL if isa_driver match callback
for the device fails, indicating a device unsupported by the ISA driver.

This patch fixes a possible NULL pointer dereference if one of the
isa_driver callbacks to attempted for an unsupported device. This error
should not occur in practice since ISA devices are typically manually
configured and loaded by the users, but we may as well prevent this
error from popping up for the 0day testers.

Fixes: a5117ba7 ("[PATCH] Driver model: add ISA bus")
Signed-off-by: William Breathitt Gray <vilhelm.gray@gmail.com>
Cc: stable <stable@vger.kernel.org>
Acked-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/base/isa.c

@@ -39,7 +39,7 @@ static int isa_bus_probe(struct device *dev)
 {
 	struct isa_driver *isa_driver = dev->platform_data;
 
-	if (isa_driver->probe)
+	if (isa_driver && isa_driver->probe)
 		return isa_driver->probe(dev, to_isa_dev(dev)->id);
 
 	return 0;
@@ -49,7 +49,7 @@ static int isa_bus_remove(struct device *dev)
 {
 	struct isa_driver *isa_driver = dev->platform_data;
 
-	if (isa_driver->remove)
+	if (isa_driver && isa_driver->remove)
 		return isa_driver->remove(dev, to_isa_dev(dev)->id);
 
 	return 0;
@@ -59,7 +59,7 @@ static void isa_bus_shutdown(struct device *dev)
 {
 	struct isa_driver *isa_driver = dev->platform_data;
 
-	if (isa_driver->shutdown)
+	if (isa_driver && isa_driver->shutdown)
 		isa_driver->shutdown(dev, to_isa_dev(dev)->id);
 }
 
@@ -67,7 +67,7 @@ static int isa_bus_suspend(struct device *dev, pm_message_t state)
 {
 	struct isa_driver *isa_driver = dev->platform_data;
 
-	if (isa_driver->suspend)
+	if (isa_driver && isa_driver->suspend)
 		return isa_driver->suspend(dev, to_isa_dev(dev)->id, state);
 
 	return 0;
@@ -77,7 +77,7 @@ static int isa_bus_resume(struct device *dev)
 {
 	struct isa_driver *isa_driver = dev->platform_data;
 
-	if (isa_driver->resume)
+	if (isa_driver && isa_driver->resume)
 		return isa_driver->resume(dev, to_isa_dev(dev)->id);
 
 	return 0;