vfio/cdx: Fix NULL pointer dereference in interrupt trigger path

In the Linux kernel, the following vulnerability has been resolved:

vfio/cdx: Fix NULL pointer dereference in interrupt trigger path

Add validation to ensure MSI is configured before accessing cdx_irqs

array in vfio_cdx_set_msi_trigger(). Without this check, userspace

can trigger a NULL pointer dereference by calling VFIO_DEVICE_SET_IRQS

with VFIO_IRQ_SET_DATA_BOOL or VFIO_IRQ_SET_DATA_NONE flags before

ever setting up interrupts via VFIO_IRQ_SET_DATA_EVENTFD.

The vfio_cdx_msi_enable() function allocates the cdx_irqs array and

sets config_msi to 1 only when called through the EVENTFD path. The

trigger loop (for DATA_BOOL/DATA_NONE) assumed this had already been

done, but there was no enforcement of this call ordering.

This matches the protection used in the PCI VFIO driver where

vfio_pci_set_msi_trigger() checks irq_is() before the trigger loop.