Inject #GP for if VMXON is attempting with a CR0/CR4 that fails the
generic "is CRx valid" check, but passes the CR4.VMXE check, and do the
generic checks _after_ handling the post-VMXON VM-Fail.

The CR4.VMXE check, and all other #UD cases, are special pre-conditions
that are enforced prior to pivoting on the current VMX mode, i.e. occur
before interception if VMXON is attempted in VMX non-root mode.

All other CR0/CR4 checks generate #GP and effectively have lower priority
than the post-VMXON check.

Per the SDM:

    IF (register operand) or (CR0.PE = 0) or (CR4.VMXE = 0) or ...
        THEN #UD;
    ELSIF not in VMX operation
        THEN
            IF (CPL > 0) or (in A20M mode) or
            (the values of CR0 and CR4 are not supported in VMX operation)
                THEN #GP(0);
    ELSIF in VMX non-root operation
        THEN VMexit;
    ELSIF CPL > 0
        THEN #GP(0);
    ELSE VMfail("VMXON executed in VMX root operation");
    FI;

which, if re-written without ELSIF, yields:

    IF (register operand) or (CR0.PE = 0) or (CR4.VMXE = 0) or ...
        THEN #UD

    IF in VMX non-root operation
        THEN VMexit;

    IF CPL > 0
        THEN #GP(0)

    IF in VMX operation
        THEN VMfail("VMXON executed in VMX root operation");

    IF (in A20M mode) or
       (the values of CR0 and CR4 are not supported in VMX operation)
                THEN #GP(0);

Note, KVM unconditionally forwards VMXON VM-Exits that occur in L2 to L1,
i.e. there is no need to check the vCPU is not in VMX non-root mode.  Add
a comment to explain why unconditionally forwarding such exits is
functionally correct.
Reported-by: Eric Li <ercli@ucdavis.edu>
Fixes: c7d855c2 ("KVM: nVMX: Inject #UD if VMXON is attempted with incompatible CR0/CR4")
Cc: stable@vger.kernel.org
Signed-off-by: Sean Christopherson <seanjc@google.com>
Link: https://lore.kernel.org/r/20221006001956.329314-1-seanjc@google.com