Smack: Privilege check on key operations

Smack: Privilege check on key operations

Operations on key objects are subjected to Smack policy
even if the process is privileged. This is inconsistent
with the general behavior of Smack and may cause issues
with authentication by privileged daemons. This patch
allows processes with CAP_MAC_OVERRIDE to access keys
even if the Smack rules indicate otherwise.

Reported-by: Jose Bollo <[e-mail]>
Signed-off-by: Casey Schaufler <[e-mail]>

security/smack/smack.h

···
 void smk_insert_entry(struct smack_known *skp);
 struct smack_known *smk_find_entry(const char *);
 bool smack_privileged(int cap);
+bool smack_privileged_cred(int cap, const struct cred *cred);
 void smk_destroy_label_list(struct list_head *list);
 
 /*

security/smack/smack_access.c

···
 LIST_HEAD(smack_onlycap_list);
 DEFINE_MUTEX(smack_onlycap_lock);
 
-/*
+/**
+ * smack_privileged_cred - are all privilege requirements met by cred
+ * @cap: The requested capability
+ * @cred: the credential to use
+ *
  * Is the task privileged and allowed to be privileged
  * by the onlycap rule.
  *
  * Returns true if the task is allowed to be privileged, false if it's not.
  */
-bool smack_privileged(int cap)
+bool smack_privileged_cred(int cap, const struct cred *cred)
 {
-	struct smack_known *skp = smk_of_current();
+	struct task_smack *tsp = cred->security;
+	struct smack_known *skp = tsp->smk_task;
 	struct smack_known_list_elem *sklep;
 	int rc;
 
-	/*
-	 * All kernel tasks are privileged
-	 */
-	if (unlikely(current->flags & PF_KTHREAD))
-		return true;
-
-	rc = cap_capable(current_cred(), &init_user_ns, cap,
-				SECURITY_CAP_AUDIT);
+	rc = cap_capable(cred, &init_user_ns, cap, SECURITY_CAP_AUDIT);
 	if (rc)
 		return false;
 
···
 	rcu_read_unlock();
 
 	return false;
+}
+
+/**
+ * smack_privileged - are all privilege requirements met
+ * @cap: The requested capability
+ *
+ * Is the task privileged and allowed to be privileged
+ * by the onlycap rule.
+ *
+ * Returns true if the task is allowed to be privileged, false if it's not.
+ */
+bool smack_privileged(int cap)
+{
+	/*
+	 * All kernel tasks are privileged
+	 */
+	if (unlikely(current->flags & PF_KTHREAD))
+		return true;
+
+	return smack_privileged_cred(cap, current_cred());
 }

security/smack/smack_lsm.c

···
 	 */
 	if (tkp == NULL)
 		return -EACCES;
+
+	if (smack_privileged_cred(CAP_MAC_OVERRIDE, cred))
+		return 0;
+
 #ifdef CONFIG_AUDIT
 	smk_ad_init(&ad, __func__, LSM_AUDIT_DATA_KEY);
 	ad.a.u.key_struct.key = keyp->serial;