audit: allow interfield comparison in audit rules

We wish to be able to audit when a uid=500 task accesses a file which is
uid=0.  Or vice versa.  This patch introduces a new audit filter type
AUDIT_FIELD_COMPARE which takes as an 'enum' which indicates which fields
should be compared.  At this point we only define the task->uid vs
inode->uid, but other comparisons can be added.
Signed-off-by: Eric Paris <eparis@redhat.com>

include/linux/audit.h

@@ -182,7 +182,10 @@
  * AUDIT_UNUSED_BITS is updated if need be. */
 #define AUDIT_UNUSED_BITS	0x07FFFC00
 
+/* AUDIT_FIELD_COMPARE rule list */
+#define AUDIT_COMPARE_UID_TO_OBJ_UID	1
 
+#define AUDIT_MAX_FIELD_COMPARE	AUDIT_COMPARE_UID_TO_OBJ_UID
 /* Rule fields */
 				/* These are useful when checking the
 				 * task structure at task creation time
@@ -225,6 +228,7 @@
 #define AUDIT_FILETYPE	108
 #define AUDIT_OBJ_UID	109
 #define AUDIT_OBJ_GID	110
+#define AUDIT_FIELD_COMPARE	111
 
 #define AUDIT_ARG0      200
 #define AUDIT_ARG1      (AUDIT_ARG0+1)

kernel/auditfilter.c

@@ -526,7 +526,6 @@ static struct audit_entry *audit_data_to_entry(struct audit_rule_data *data,
 				goto exit_free;
 			break;
 		case AUDIT_FILTERKEY:
-			err = -EINVAL;
 			if (entry->rule.filterkey || f->val > AUDIT_MAX_KEY_LEN)
 				goto exit_free;
 			str = audit_unpack_string(&bufp, &remain, f->val);
@@ -543,6 +542,10 @@ static struct audit_entry *audit_data_to_entry(struct audit_rule_data *data,
 			if (f->val & ~S_IFMT)
 				goto exit_free;
 			break;
+		case AUDIT_FIELD_COMPARE:
+			if (f->val > AUDIT_MAX_FIELD_COMPARE)
+				goto exit_free;
+			break;
 		default:
 			goto exit_free;
 		}

kernel/auditsc.c

@@ -463,6 +463,32 @@ static int match_tree_refs(struct audit_context *ctx, struct audit_tree *tree)
 	return 0;
 }
 
+static int audit_field_compare(struct task_struct *tsk,
+			       const struct cred *cred,
+			       struct audit_field *f,
+			       struct audit_context *ctx,
+			       struct audit_names *name)
+{
+	struct audit_names *n;
+
+	switch (f->val) {
+	case AUDIT_COMPARE_UID_TO_OBJ_UID:
+		if (name) {
+			return audit_comparator(cred->uid, f->op, name->uid);
+		} else if (ctx) {
+			list_for_each_entry(n, &ctx->names_list, list) {
+				if (audit_comparator(cred->uid, f->op, n->uid))
+					return 1;
+			}
+		}
+		break;
+	default:
+		WARN(1, "Missing AUDIT_COMPARE define.  Report as a bug\n");
+		return 0;
+	}
+	return 0;
+}
+
 /* Determine if any context name data matches a rule's watch data */
 /* Compare a task_struct with an audit_rule.  Return 1 on match, 0
  * otherwise.
@@ -693,8 +719,10 @@ static int audit_filter_rules(struct task_struct *tsk,
 		case AUDIT_FILETYPE:
 			result = audit_match_filetype(ctx, f->val);
 			break;
+		case AUDIT_FIELD_COMPARE:
+			result = audit_field_compare(tsk, cred, f, ctx, name);
+			break;
 		}
-
 		if (!result)
 			return 0;
 	}