genirq: Ensure IRQ descriptor is valid when setting-up the IRQ

In the function, setup_irq(), we don't check that the descriptor
returned from irq_to_desc() is valid before we start using it. For
example chip_bus_lock() called from setup_irq(), assumes that the
descriptor pointer is valid and doesn't check before dereferencing it.

In many other functions including setup/free_percpu_irq() we do check
that the descriptor returned is not NULL and therefore add the same test
to setup_irq() to ensure the descriptor returned is valid.
Signed-off-by: Jon Hunter <jonathanh@nvidia.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>

kernel/irq/manage.c

@@ -1407,7 +1407,7 @@ int setup_irq(unsigned int irq, struct irqaction *act)
 	int retval;
 	struct irq_desc *desc = irq_to_desc(irq);
 
-	if (WARN_ON(irq_settings_is_per_cpu_devid(desc)))
+	if (!desc || WARN_ON(irq_settings_is_per_cpu_devid(desc)))
 		return -EINVAL;
 	chip_bus_lock(desc);
 	retval = __setup_irq(irq, desc, act);