genirq/msi: Store the IOMMU IOVA directly in msi_desc instead of iommu_cookie
In the Linux kernel, the following vulnerability has been resolved:

genirq/msi: Store the IOMMU IOVA directly in msi_desc instead of iommu_cookie

The IOMMU translation for MSI message addresses has been a 2-step process,
separated in time:

 1) iommu_dma_prepare_msi(): A cookie pointer containing the IOVA address
    is stored in the MSI descriptor when an MSI interrupt is allocated.

 2) iommu_dma_compose_msi_msg(): this cookie pointer is used to compute a
    translated message address.

This has an inherent lifetime problem for the pointer stored in the cookie
that must remain valid between the two steps. However, there is no locking
at the irq layer that helps protect the lifetime. Today, this works under
the assumption that the iommu domain is not changed while MSI interrupts
being programmed. This is true for normal DMA API users within the kernel,
as the iommu domain is attached before the driver is probed and cannot be
changed while a driver is attached.

Classic VFIO type1 also prevented changing the iommu domain while VFIO was
running as it does not support changing the "container" after starting up.

However, iommufd has improved this so that the iommu domain can be changed
during VFIO operation. This potentially allows userspace to directly race
VFIO_DEVICE_ATTACH_IOMMUFD_PT (which calls iommu_attach_group()) and
VFIO_DEVICE_SET_IRQS (which calls into iommu_dma_compose_msi_msg()).

This potentially causes both the cookie pointer and the unlocked call to
iommu_get_domain_for_dev() on the MSI translation path to become UAFs.

Fix the MSI cookie UAF by removing the cookie pointer. The translated IOVA
address is already known during iommu_dma_prepare_msi() and cannot change.
Thus, it can simply be stored as an integer in the MSI descriptor.

The other UAF related to iommu_get_domain_for_dev() will be addressed in
patch "iommu: Make iommu_dma_prepare_msi() into a generic operation" by
using the IOMMU group mutex.