smack: fix bug: unprivileged task can create labels

If an unprivileged task is allowed to relabel itself
(/smack/relabel-self is not empty),
it can freely create new labels by writing their
names into own /proc/PID/attr/smack/current

This occurs because do_setattr() imports
the provided label in advance,
before checking "relabel-self" list.

This change ensures that the "relabel-self" list
is checked before importing the label.

Fixes: 38416e5 ("Smack: limited capability for changing process label")
Signed-off-by: Konstantin Andreev <andreev@swemel.ru>
Signed-off-by: Casey Schaufler <casey@schaufler-ca.com>

Author: goongas

security/smack/smack_lsm.c

@@ -3778,8 +3778,8 @@ static int do_setattr(u64 attr, void *value, size_t size)
 	struct task_smack *tsp = smack_cred(current_cred());
 	struct cred *new;
 	struct smack_known *skp;
-	struct smack_known_list_elem *sklep;
-	int rc;
+	char *labelstr;
+	int rc = 0;
 
 	if (!smack_privileged(CAP_MAC_ADMIN) && list_empty(&tsp->smk_relabel))
 		return -EPERM;
@@ -3790,28 +3790,41 @@ static int do_setattr(u64 attr, void *value, size_t size)
 	if (attr != LSM_ATTR_CURRENT)
 		return -EOPNOTSUPP;
 
-	skp = smk_import_entry(value, size);
-	if (IS_ERR(skp))
-		return PTR_ERR(skp);
+	labelstr = smk_parse_smack(value, size);
+	if (IS_ERR(labelstr))
+		return PTR_ERR(labelstr);
 
 	/*
 	 * No process is ever allowed the web ("@") label
 	 * and the star ("*") label.
 	 */
-	if (skp == &smack_known_web || skp == &smack_known_star)
-		return -EINVAL;
+	if (labelstr[1] == '\0' /* '@', '*' */) {
+		const char c = labelstr[0];
+
+		if (c == *smack_known_web.smk_known ||
+		    c == *smack_known_star.smk_known) {
+			rc = -EPERM;
+			goto free_labelstr;
+		}
+	}
 
 	if (!smack_privileged(CAP_MAC_ADMIN)) {
-		rc = -EPERM;
+		const struct smack_known_list_elem *sklep;
 		list_for_each_entry(sklep, &tsp->smk_relabel, list)
-			if (sklep->smk_label == skp) {
-				rc = 0;
-				break;
-			}
-		if (rc)
-			return rc;
+			if (strcmp(sklep->smk_label->smk_known, labelstr) == 0)
+				goto free_labelstr;
+		rc = -EPERM;
 	}
 
+free_labelstr:
+	kfree(labelstr);
+	if (rc)
+		return -EPERM;
+
+	skp = smk_import_entry(value, size);
+	if (IS_ERR(skp))
+		return PTR_ERR(skp);
+
 	new = prepare_creds();
 	if (new == NULL)
 		return -ENOMEM;