CVE-2022-49886: x86/tdx: Panic on bad configs that #VE on "private" memory access

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

x86/tdx: Panic on bad configs that #VE on "private" memory access

All normal kernel memory is "TDX private memory". This includes everything from kernel stacks to kernel text. Handling exceptions on arbitrary accesses to kernel memory is essentially impossible because they can happen in horribly nasty places like kernel entry/exit. But, TDX hardware can theoretically deliver a virtualization exception (#VE) on any access to private memory.

But, it's not as bad as it sounds. TDX can be configured to never deliver these exceptions on private memory with a "TD attribute" called ATTRSEPTVEDISABLE. The guest has no way to set this attribute, but it can check it.

Ensure ATTRSEPTVEDISABLE is set in early boot. panic() if it is unset. There is no sane way for Linux to run with this attribute clear so a panic() is appropriate.

There's small window during boot before the check where kernel has an early #VE handler. But the handler is only for port I/O and will also panic() as soon as it sees any other #VE, such as a one generated by a private memory access.

[ dhansen: Rewrite changelog and rebase on new tdxparsetdinfo(). Add Kirill's tested-by because I made changes since he wrote this. ]