selinux: improve error checking in sel_write_load()

Move our existing input sanity checking to the top of sel_write_load()
and add a check to ensure the buffer size is non-zero.

Move a local variable initialization from the declaration to before it
is used.

Minor style adjustments.
Reported-by: Sam Sun <samsun1006219@gmail.com>
Signed-off-by: Paul Moore <paul@paul-moore.com>

security/selinux/selinuxfs.c

@@ -571,11 +571,18 @@ static ssize_t sel_write_load(struct file *file, const char __user *buf,
 			      size_t count, loff_t *ppos)
 
 {
-	struct selinux_fs_info *fsi = file_inode(file)->i_sb->s_fs_info;
+	struct selinux_fs_info *fsi;
 	struct selinux_load_state load_state;
 	ssize_t length;
 	void *data = NULL;
 
+	/* no partial writes */
+	if (*ppos)
+		return -EINVAL;
+	/* no empty policies */
+	if (!count)
+		return -EINVAL;
+
 	mutex_lock(&selinux_state.policy_mutex);
 
 	length = avc_has_perm(current_sid(), SECINITSID_SECURITY,
@@ -583,26 +590,22 @@ static ssize_t sel_write_load(struct file *file, const char __user *buf,
 	if (length)
 		goto out;
 
-	/* No partial writes. */
-	length = -EINVAL;
-	if (*ppos != 0)
-		goto out;
-
-	length = -ENOMEM;
 	data = vmalloc(count);
-	if (!data)
+	if (!data) {
+		length = -ENOMEM;
 		goto out;
-
+	}
+	if (copy_from_user(data, buf, count) != 0) {
 		length = -EFAULT;
-	if (copy_from_user(data, buf, count) != 0)
 		goto out;
+	}
 
 	length = security_load_policy(data, count, &load_state);
 	if (length) {
 		pr_warn_ratelimited("SELinux: failed to load policy\n");
 		goto out;
 	}
-
+	fsi = file_inode(file)->i_sb->s_fs_info;
 	length = sel_make_policy_nodes(fsi, load_state.policy);
 	if (length) {
 		pr_warn_ratelimited("SELinux: failed to initialize selinuxfs\n");
@@ -611,13 +614,12 @@ static ssize_t sel_write_load(struct file *file, const char __user *buf,
 	}
 
 	selinux_policy_commit(&load_state);
-
 	length = count;
-
 	audit_log(audit_context(), GFP_KERNEL, AUDIT_MAC_POLICY_LOAD,
 		"auid=%u ses=%u lsm=selinux res=1",
 		from_kuid(&init_user_ns, audit_get_loginuid(current)),
 		audit_get_sessionid(current));
+
 out:
 	mutex_unlock(&selinux_state.policy_mutex);
 	vfree(data);