Smack: Signal delivery as an append operation

Under a strict subject/object security policy delivering a
signal or delivering network IPC could be considered either
a write or an append operation. The original choice to make
both write operations leads to an issue where IPC delivery
is desired under policy, but delivery of signals is not.
This patch provides the option of making signal delivery
an append operation, allowing Smack rules that deny signal
delivery while allowing IPC. This was requested for Tizen.
Signed-off-by: Casey Schaufler <casey@schaufler-ca.com>

security/smack/Kconfig

@@ -40,3 +40,15 @@ config SECURITY_SMACK_NETFILTER
 	  This enables security marking of network packets using
 	  Smack labels.
 	  If you are unsure how to answer this question, answer N.
+
+config SECURITY_SMACK_APPEND_SIGNALS
+	bool "Treat delivering signals as an append operation"
+	depends on SECURITY_SMACK
+	default n
+	help
+	  Sending a signal has been treated as a write operation to the
+	  receiving process. If this option is selected, the delivery
+	  will be an append operation instead. This makes it possible
+	  to differentiate between delivering a network packet and
+	  delivering a signal in the Smack rules.
+	  If you are unsure how to answer this question, answer N.

security/smack/smack.h

@@ -256,6 +256,16 @@ enum {
 #define MAY_LOCK	0x00002000	/* Locks should be writes, but ... */
 #define MAY_BRINGUP	0x00004000	/* Report use of this rule */
 
+/*
+ * The policy for delivering signals is configurable.
+ * It is usually "write", but can be "append".
+ */
+#ifdef CONFIG_SECURITY_SMACK_APPEND_SIGNALS
+#define MAY_DELIVER	MAY_APPEND	/* Signal delivery requires append */
+#else
+#define MAY_DELIVER	MAY_WRITE	/* Signal delivery requires write */
+#endif
+
 #define SMACK_BRINGUP_ALLOW		1	/* Allow bringup mode */
 #define SMACK_UNCONFINED_SUBJECT	2	/* Allow unconfined label */
 #define SMACK_UNCONFINED_OBJECT		3	/* Allow unconfined label */

security/smack/smack_lsm.c

@@ -1857,14 +1857,14 @@ static int smack_file_send_sigiotask(struct task_struct *tsk,
 
 	/* we don't log here as rc can be overriden */
 	skp = file->f_security;
-	rc = smk_access(skp, tkp, MAY_WRITE, NULL);
-	rc = smk_bu_note("sigiotask", skp, tkp, MAY_WRITE, rc);
+	rc = smk_access(skp, tkp, MAY_DELIVER, NULL);
+	rc = smk_bu_note("sigiotask", skp, tkp, MAY_DELIVER, rc);
 	if (rc != 0 && has_capability(tsk, CAP_MAC_OVERRIDE))
 		rc = 0;
 
 	smk_ad_init(&ad, __func__, LSM_AUDIT_DATA_TASK);
 	smk_ad_setfield_u_tsk(&ad, tsk);
-	smack_log(skp->smk_known, tkp->smk_known, MAY_WRITE, rc, &ad);
+	smack_log(skp->smk_known, tkp->smk_known, MAY_DELIVER, rc, &ad);
 	return rc;
 }
 
@@ -2265,8 +2265,8 @@ static int smack_task_kill(struct task_struct *p, struct siginfo *info,
 	 * can write the receiver.
 	 */
 	if (secid == 0) {
-		rc = smk_curacc(tkp, MAY_WRITE, &ad);
-		rc = smk_bu_task(p, MAY_WRITE, rc);
+		rc = smk_curacc(tkp, MAY_DELIVER, &ad);
+		rc = smk_bu_task(p, MAY_DELIVER, rc);
 		return rc;
 	}
 	/*
@@ -2275,8 +2275,8 @@ static int smack_task_kill(struct task_struct *p, struct siginfo *info,
 	 * we can't take privilege into account.
 	 */
 	skp = smack_from_secid(secid);
-	rc = smk_access(skp, tkp, MAY_WRITE, &ad);
-	rc = smk_bu_note("USB signal", skp, tkp, MAY_WRITE, rc);
+	rc = smk_access(skp, tkp, MAY_DELIVER, &ad);
+	rc = smk_bu_note("USB signal", skp, tkp, MAY_DELIVER, rc);
 	return rc;
 }