KVM: VMX: Fix a TSX_CTRL_CPUID_CLEAR field mask issue

When updating the host's mask for its MSR_IA32_TSX_CTRL user return entry,
clear the mask in the found uret MSR instead of vmx->guest_uret_msrs[i].
Modifying guest_uret_msrs directly is completely broken as 'i' does not
point at the MSR_IA32_TSX_CTRL entry.  In fact, it's guaranteed to be an
out-of-bounds accesses as is always set to kvm_nr_uret_msrs in a prior
loop. By sheer dumb luck, the fallout is limited to "only" failing to
preserve the host's TSX_CTRL_CPUID_CLEAR.  The out-of-bounds access is
benign as it's guaranteed to clear a bit in a guest MSR value, which are
always zero at vCPU creation on both x86-64 and i386.

Cc: stable@vger.kernel.org
Fixes: 8ea8b8d6 ("KVM: VMX: Use common x86's uret MSR list as the one true list")
Signed-off-by: Zhenzhong Duan <zhenzhong.duan@intel.com>
Reviewed-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20210926015545.281083-1-zhenzhong.duan@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/vmx/vmx.c

@@ -6848,7 +6848,7 @@ static int vmx_create_vcpu(struct kvm_vcpu *vcpu)
 		 */
 		tsx_ctrl = vmx_find_uret_msr(vmx, MSR_IA32_TSX_CTRL);
 		if (tsx_ctrl)
-			vmx->guest_uret_msrs[i].mask = ~(u64)TSX_CTRL_CPUID_CLEAR;
+			tsx_ctrl->mask = ~(u64)TSX_CTRL_CPUID_CLEAR;
 	}
 
 	err = alloc_loaded_vmcs(&vmx->vmcs01);