KVM: TDX: Use struct_size to simplify tdx_get_capabilities()

Use struct_size() instead of manually calculating the number of bytes to
allocate for 'caps', including the nested flexible array, and copy all of
'caps' to user space with a single copy_to_user() call (thanks to the full
size being provided by struct_size()).

Signed-off-by: Thorsten Blum <thorsten.blum@linux.dev>
Tested-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
Link: https://patch.msgid.link/20251017213914.167301-1-thorsten.blum@linux.dev
[sean: separate from swap of get_user() vs. kzalloc() ordering]
Signed-off-by: Sean Christopherson <seanjc@google.com>

Author: sean-jc

arch/x86/kvm/vmx/tdx.c

@@ -2231,26 +2231,21 @@ static int tdx_get_capabilities(struct kvm_tdx_cmd *cmd)
 	if (nr_user_entries < td_conf->num_cpuid_config)
 		return -E2BIG;
 
-	caps = kzalloc(sizeof(*caps) +
-		       sizeof(struct kvm_cpuid_entry2) * td_conf->num_cpuid_config,
-		       GFP_KERNEL);
+	caps = kzalloc(struct_size(caps, cpuid.entries,
+				   td_conf->num_cpuid_config), GFP_KERNEL);
 	if (!caps)
 		return -ENOMEM;
 
 	ret = init_kvm_tdx_caps(td_conf, caps);
 	if (ret)
 		goto out;
 
-	if (copy_to_user(user_caps, caps, sizeof(*caps))) {
+	if (copy_to_user(user_caps, caps, struct_size(caps, cpuid.entries,
+						      caps->cpuid.nent))) {
 		ret = -EFAULT;
 		goto out;
 	}
 
-	if (copy_to_user(user_caps->cpuid.entries, caps->cpuid.entries,
-			 caps->cpuid.nent *
-			 sizeof(caps->cpuid.entries[0])))
-		ret = -EFAULT;
-
 out:
 	/* kfree() accepts NULL. */
 	kfree(caps);