x86/IOMMU: move tracking in iommu_identity_mapping()

x86/IOMMU: move tracking in iommu_identity_mapping()

If for some reason xmalloc() fails after having mapped the reserved
regions, an error is reported, but the regions remain mapped in the P2M.

Similarly if an error occurs during set_identity_p2m_entry() (except on
the first call), the partial mappings of the region would be retained
without being tracked anywhere, and hence without there being a way to
remove them again from the domain's P2M.

Move the setting up of the list entry ahead of trying to map the region.
In cases other than the first mapping failing, keep record of the full
region, such that a subsequent unmapping request can be properly torn
down.

To compensate for the potentially excess unmapping requests, don't log a
warning from p2m_remove_identity_entry() when there really was nothing
mapped at a given GFN.

This is XSA-460 / CVE-2024-31145.

Fixes: 2201b67b9128 ("VT-d: improve RMRR region handling")
Fixes: c0e19d7c6c42 ("IOMMU: generalize VT-d's tracking of mapped RMRR regions")
Signed-off-by: Teddy Astie <teddy.astie@vates.tech>
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>