KVM: nVMX: Add consistency checks for CET states

KVM: nVMX: Add consistency checks for CET states

Introduce consistency checks for CET states during nested VM-entry.

A VMCS contains both guest and host CET states, each comprising the
IA32_S_CET MSR, SSP, and IA32_INTERRUPT_SSP_TABLE_ADDR MSR. Various
checks are applied to CET states during VM-entry as documented in SDM
Vol3 Chapter "VM ENTRIES". Implement all these checks during nested
VM-entry to emulate the architectural behavior.

In summary, there are three kinds of checks on guest/host CET states
during VM-entry:

A. Checks applied to both guest states and host states:

 * The IA32_S_CET field must not set any reserved bits; bits 10 (SUPPRESS)
   and 11 (TRACKER) cannot both be set.
 * SSP should not have bits 1:0 set.
 * The IA32_INTERRUPT_SSP_TABLE_ADDR field must be canonical.

B. Checks applied to host states only

 * IA32_S_CET MSR and SSP must be canonical if the CPU enters 64-bit mode
   after VM-exit. Otherwise, IA32_S_CET and SSP must have their higher 32
   bits cleared.

C. Checks applied to guest states only:

 * IA32_S_CET MSR and SSP are not required to be canonical (i.e., 63:N-1
   are identical, where N is the CPU's maximum linear-address width). But,
   bits 63:N of SSP must be identical.

Tested-by: Mathias Krause <minipli@grsecurity.net>
Tested-by: John Allen <john.allen@amd.com>
Tested-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
Signed-off-by: Chao Gao <chao.gao@intel.com>
Reviewed-by: Binbin Wu <binbin.wu@linux.intel.com>
Link: https://lore.kernel.org/r/20250919223258.1604852-34-seanjc@google.com
[sean: have common helper return 0/-EINVAL, not true/false]
Signed-off-by: Sean Christopherson <seanjc@google.com>