KVM: nVMX: Check MSR load/store list counts during VM-Enter consistency checks

KVM: nVMX: Check MSR load/store list counts during VM-Enter consistency checks

Explicitly verify the MSR load/store list counts are below the advertised
limit as part of the initial consistency checks on the lists, so that code
that consumes the count doesn't need to worry about extreme edge cases.
Enforcing the limit during the initial checks fixes a flaw on 32-bit KVM
where a sufficiently high @count could lead to overflow:

	arch/x86/kvm/vmx/nested.c:834 nested_vmx_check_msr_switch()
	warn: potential user controlled sizeof overflow 'addr + count * 16' '0-u64max + 16-68719476720'

arch/x86/kvm/vmx/nested.c
    827 static int nested_vmx_check_msr_switch(struct kvm_vcpu *vcpu,
    828                                        u32 count, u64 addr)
    829 {
    830         if (count == 0)
    831                 return 0;
    832
    833         if (!kvm_vcpu_is_legal_aligned_gpa(vcpu, addr, 16) ||
--> 834             !kvm_vcpu_is_legal_gpa(vcpu, (addr + count * sizeof(struct vmx_msr_entry) - 1)))
                                                         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

While the SDM doesn't explicitly state an illegal count results in VM-Fail,
the SDM states that exceeding the limit may result in undefined behavior.
I.e. the SDM gives hardware, and thus KVM, carte blanche to do literally
anything in response to a count that exceeds the "recommended" limit.

  If the limit is exceeded, undefined processor behavior may result
  (including a machine check during the VMX transition).

KVM already enforces the limit when processing the MSRs, i.e. already
signals a late VM-Exit Consistency Check for VM-Enter, and generates a
VMX Abort for VM-Exit.  I.e. explicitly checking the limits simply means
KVM will signal VM-Fail instead of VM-Exit or VMX Abort.

Reported-by: Dan Carpenter <dan.carpenter@linaro.org>
Closes: https://lore.kernel.org/all/44961459-2759-4164-b604-f6bd43da8ce9@stanley.mountain
Link: https://lore.kernel.org/r/20250315024402.2363098-1-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>