KVM: SEV: Allocate full pages for {DE,EN}CRYPT ops on SNP-enabled hosts

In the Linux kernel, the following vulnerability has been resolved: KVM: SEV: Allocate full pages for {DE,EN}CRYPT ops on SNP-enabled hosts When {de,en}crypting memory of an SEV or SEV-ES guest on an SNP-enabled host via a temporary buffer, allocate a full 4KiB page for the buffer to ensure the page containing the buffer is wholly owned by KVM, i.e. won't be concurrently allocated and accessed by other kernel code while KVM is using the buffer to {de,en}crypt memory. On SNP-enabled platforms, when sending SEV/SEV-ES commands that trigger firmware writes to memory, the to-be-written page(s) must be (temporarily) assigned to Firmware (as required by the SNP architecture, to guard against using such commands as gadgets to attack SNP guests). See snp_map_cmd_buf_desc() and friends. Unfortunately, transferring ownership of a page to Firmware makes the page inaccessible to software, and thus writes generate RMP #PF violations. If KVM uses a sub-page allocation for its temporary buffer, some other actor in the kernel can allocate and use the other portions of the page, and thus trigger unexpected (and seemingly spurious) RMP #PF violations due to software attempting to access a Firmware-owned page. BUG: unable to handle page fault for address: ffff906ae30f0300 #PF: supervisor write access in kernel mode #PF: error_code(0x80000003) - RMP violation PGD 6b1b80d067 P4D 6b1b80d067 PUD 100231e2063 PMD 10055a88063 PTE 80000100630f0163 SEV-SNP: PFN 0x100630f0 unassigned, dumping non-zero en