CVE-2022-49610: Undefined Security Weakness

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

KVM: VMX: Prevent RSB underflow before vmenter

On VMX, there are some balanced returns between the time the guest's SPEC_CTRL value is written, and the vmenter.

Balanced returns (matched by a preceding call) are usually ok, but it's at least theoretically possible an NMI with a deep call stack could empty the RSB before one of the returns.

For maximum paranoia, don't allow *any* returns (balanced or otherwise) between the SPEC_CTRL write and the vmenter.

[ bp: Fix 32-bit build. ]