KVM: TDX: Check size of user's kvm_tdx_capabilities array before allocating

When userspace is getting TDX capabilities, retrieve and check the number
of user entries before allocating kernel scratch space to avoid having to
unwind the allocation if get_user() fails or if 'user_caps' is too small
to fit 'caps'.

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: split to separate patch]
Signed-off-by: Sean Christopherson <seanjc@google.com>

Author: toblux

arch/x86/kvm/vmx/tdx.c

@@ -2224,23 +2224,19 @@ static int tdx_get_capabilities(struct kvm_tdx_cmd *cmd)
 	if (cmd->flags)
 		return -EINVAL;
 
+	user_caps = u64_to_user_ptr(cmd->data);
+	if (get_user(nr_user_entries, &user_caps->cpuid.nent))
+		return -EFAULT;
+
+	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);
 	if (!caps)
 		return -ENOMEM;
 
-	user_caps = u64_to_user_ptr(cmd->data);
-	if (get_user(nr_user_entries, &user_caps->cpuid.nent)) {
-		ret = -EFAULT;
-		goto out;
-	}
-
-	if (nr_user_entries < td_conf->num_cpuid_config) {
-		ret = -E2BIG;
-		goto out;
-	}
-
 	ret = init_kvm_tdx_caps(td_conf, caps);
 	if (ret)
 		goto out;