audit: Make testing for a valid loginuid explicit.

audit rule additions containing "-F auid!=4294967295" were failing
with EINVAL because of a regression caused by e1760bd.

Apparently some userland audit rule sets want to know if loginuid uid
has been set and are using a test for auid != 4294967295 to determine
that.

In practice that is a horrible way to ask if a value has been set,
because it relies on subtle implementation details and will break
every time the uid implementation in the kernel changes.

So add a clean way to test if the audit loginuid has been set, and
silently convert the old idiom to the cleaner and more comprehensible
new idiom.

Cc: <stable@vger.kernel.org> # 3.7
Reported-By: Richard Guy Briggs <rgb@redhat.com>
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
Tested-by: Richard Guy Briggs <rgb@redhat.com>
Signed-off-by: Eric Paris <eparis@redhat.com>

include/linux/audit.h

@@ -391,6 +391,11 @@ static inline void audit_ptrace(struct task_struct *t)
 #define audit_signals 0
 #endif /* CONFIG_AUDITSYSCALL */
 
+static inline bool audit_loginuid_set(struct task_struct *tsk)
+{
+	return uid_valid(audit_get_loginuid(tsk));
+}
+
 #ifdef CONFIG_AUDIT
 /* These are defined in audit.c */
 				/* Public API */

include/uapi/linux/audit.h

@@ -246,6 +246,7 @@
 #define AUDIT_OBJ_TYPE	21
 #define AUDIT_OBJ_LEV_LOW	22
 #define AUDIT_OBJ_LEV_HIGH	23
+#define AUDIT_LOGINUID_SET	24
 
 				/* These are ONLY useful when checking
 				 * at syscall exit time (AUDIT_AT_EXIT). */

kernel/auditfilter.c

@@ -365,7 +365,10 @@ static int audit_field_valid(struct audit_entry *entry, struct audit_field *f)
 	case AUDIT_DIR:
 	case AUDIT_FILTERKEY:
 		break;
-	/* arch is only allowed to be = or != */
+	case AUDIT_LOGINUID_SET:
+		if ((f->val != 0) && (f->val != 1))
+			return -EINVAL;
+	/* FALL THROUGH */
 	case AUDIT_ARCH:
 		if (f->op != Audit_not_equal && f->op != Audit_equal)
 			return -EINVAL;
@@ -419,17 +422,23 @@ static struct audit_entry *audit_data_to_entry(struct audit_rule_data *data,
 		f->lsm_str = NULL;
 		f->lsm_rule = NULL;
 
+		/* Support legacy tests for a valid loginuid */
+		if ((f->type == AUDIT_LOGINUID) && (f->val == 4294967295)) {
+			f->type = AUDIT_LOGINUID_SET;
+			f->val = 0;
+		}
+
 		err = audit_field_valid(entry, f);
 		if (err)
 			goto exit_free;
 
 		err = -EINVAL;
 		switch (f->type) {
+		case AUDIT_LOGINUID:
 		case AUDIT_UID:
 		case AUDIT_EUID:
 		case AUDIT_SUID:
 		case AUDIT_FSUID:
-		case AUDIT_LOGINUID:
 		case AUDIT_OBJ_UID:
 			f->uid = make_kuid(current_user_ns(), f->val);
 			if (!uid_valid(f->uid))
@@ -1222,6 +1231,10 @@ static int audit_filter_user_rules(struct audit_krule *rule, int type,
 			result = audit_uid_comparator(audit_get_loginuid(current),
 						  f->op, f->uid);
 			break;
+		case AUDIT_LOGINUID_SET:
+			result = audit_comparator(audit_loginuid_set(current),
+						  f->op, f->val);
+			break;
 		case AUDIT_MSGTYPE:
 			result = audit_comparator(type, f->op, f->val);
 			break;

kernel/auditsc.c

@@ -613,6 +613,9 @@ static int audit_filter_rules(struct task_struct *tsk,
 			if (ctx)
 				result = audit_uid_comparator(tsk->loginuid, f->op, f->uid);
 			break;
+		case AUDIT_LOGINUID_SET:
+			result = audit_comparator(audit_loginuid_set(tsk), f->op, f->val);
+			break;
 		case AUDIT_SUBJ_USER:
 		case AUDIT_SUBJ_ROLE:
 		case AUDIT_SUBJ_TYPE:
@@ -1970,7 +1973,7 @@ int audit_set_loginuid(kuid_t loginuid)
 	unsigned int sessionid;
 
 #ifdef CONFIG_AUDIT_LOGINUID_IMMUTABLE
-	if (uid_valid(task->loginuid))
+	if (audit_loginuid_set(task))
 		return -EPERM;
 #else /* CONFIG_AUDIT_LOGINUID_IMMUTABLE */
 	if (!capable(CAP_AUDIT_CONTROL))