Smack: correct behavior in the mmap hook

The mmap policy enforcement was not properly handling the
  interaction between the global and local rule lists.
  Instead of going through one and then the other, which
  missed the important case where a rule specified that
  there should be no access, combine the access limitations
  where there is a rule in each list.
Signed-off-by: Casey Schaufler <casey@schaufler-ca.com>
Signed-off-by: James Morris <jmorris@namei.org>

security/smack/smack_lsm.c

@@ -1109,38 +1109,6 @@ static int smack_file_fcntl(struct file *file, unsigned int cmd,
 	return rc;
 }
 
-/**
- * smk_mmap_list_check - the mmap check
- * @sub: subject label
- * @obj: object label
- * @access: access mode
- * @local: the task specific rule list
- *
- * Returns 0 if acces is permitted, -EACCES otherwise
- */
-static int smk_mmap_list_check(char *sub, char *obj, int access,
-				struct list_head *local)
-{
-	int may;
-
-	/*
-	 * If there is not a global rule that
-	 * allows access say no.
-	 */
-	may = smk_access_entry(sub, obj, &smack_rule_list);
-	if (may == -ENOENT || (may & access) != access)
-		return -EACCES;
-	/*
-	 * If there is a task local rule that
-	 * denies access say no.
-	 */
-	may = smk_access_entry(sub, obj, local);
-	if (may != -ENOENT && (may & access) != access)
-		return -EACCES;
-
-	return 0;
-}
-
 /**
  * smack_file_mmap :
  * Check permissions for a mmap operation.  The @file may be NULL, e.g.
@@ -1160,8 +1128,12 @@ static int smack_file_mmap(struct file *file,
 	struct task_smack *tsp;
 	char *sp;
 	char *msmack;
+	char *osmack;
 	struct inode_smack *isp;
 	struct dentry *dp;
+	int may;
+	int mmay;
+	int tmay;
 	int rc;
 
 	/* do DAC check on address space usage */
@@ -1199,17 +1171,58 @@ static int smack_file_mmap(struct file *file,
 	list_for_each_entry_rcu(srp, &smack_rule_list, list) {
 		if (srp->smk_subject != sp)
 			continue;
+
+		osmack = srp->smk_object;
 		/*
 		 * Matching labels always allows access.
 		 */
-		if (msmack == srp->smk_object)
+		if (msmack == osmack)
+			continue;
+		/*
+		 * If there is a matching local rule take
+		 * that into account as well.
+		 */
+		may = smk_access_entry(srp->smk_subject, osmack,
+					&tsp->smk_rules);
+		if (may == -ENOENT)
+			may = srp->smk_access;
+		else
+			may &= srp->smk_access;
+		/*
+		 * If may is zero the SMACK64MMAP subject can't
+		 * possibly have less access.
+		 */
+		if (may == 0)
 			continue;
 
-		rc = smk_mmap_list_check(msmack, srp->smk_object,
-					 srp->smk_access, &tsp->smk_rules);
-		if (rc != 0)
+		/*
+		 * Fetch the global list entry.
+		 * If there isn't one a SMACK64MMAP subject
+		 * can't have as much access as current.
+		 */
+		mmay = smk_access_entry(msmack, osmack, &smack_rule_list);
+		if (mmay == -ENOENT) {
+			rc = -EACCES;
 			break;
 		}
+		/*
+		 * If there is a local entry it modifies the
+		 * potential access, too.
+		 */
+		tmay = smk_access_entry(msmack, osmack, &tsp->smk_rules);
+		if (tmay != -ENOENT)
+			mmay &= tmay;
+
+		/*
+		 * If there is any access available to current that is
+		 * not available to a SMACK64MMAP subject
+		 * deny access.
+		 */
+		if ((may | mmay) != may) {
+			rc = -EACCES;
+			break;
+		}
+	}
 
 	rcu_read_unlock();