fanotify: define struct members to hold response decision context

This patch adds a flag, FAN_INFO and an extensible buffer to provide
additional information about response decisions. The buffer contains
one or more headers defining the information type and the length of the
following information. The patch defines one additional information
type, FAN_RESPONSE_INFO_AUDIT_RULE, to audit a rule number. This will
allow for the creation of other information types in the future if other
users of the API identify different needs.

The kernel can be tested if it supports a given info type by supplying
the complete info extension but setting fd to FAN_NOFD. It will return
the expected size but not issue an audit record.

Suggested-by: Steve Grubb <[e-mail]>
Link: https://lore.kernel.org/r/2745105.e9J7NaK4W3@x2
Suggested-by: Jan Kara <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Tested-by: Steve Grubb <[e-mail]>
Acked-by: Steve Grubb <[e-mail]>
Signed-off-by: Richard Guy Briggs <[e-mail]>
Signed-off-by: Jan Kara <[e-mail]>
Message-Id: <[e-mail]>

fs/notify/fanotify/fanotify.c

···
 	}
 
 	/* userspace responded, convert to something usable */
-	switch (event->response & ~FAN_AUDIT) {
+	switch (event->response & FANOTIFY_RESPONSE_ACCESS) {
 	case FAN_ALLOW:
 		ret = 0;
 		break;
···
 
 	pevent->fae.type = FANOTIFY_EVENT_TYPE_PATH_PERM;
 	pevent->response = 0;
+	pevent->hdr.type = FAN_RESPONSE_INFO_NONE;
+	pevent->hdr.pad = 0;
+	pevent->hdr.len = 0;
 	pevent->state = FAN_EVENT_INIT;
 	pevent->path = *path;
 	path_get(path);

fs/notify/fanotify/fanotify.h

···
 	u32 response;			/* userspace answer to the event */
 	unsigned short state;		/* state of the event */
 	int fd;		/* fd we passed to userspace for this event */
+	union {
+		struct fanotify_response_info_header hdr;
+		struct fanotify_response_info_audit_rule audit_rule;
+	};
 };
 
 static inline struct fanotify_perm_event *

fs/notify/fanotify/fanotify_user.c

···
 	return client_fd;
 }
 
+static int process_access_response_info(const char __user *info,
+					size_t info_len,
+				struct fanotify_response_info_audit_rule *friar)
+{
+	if (info_len != sizeof(*friar))
+		return -EINVAL;
+
+	if (copy_from_user(friar, info, sizeof(*friar)))
+		return -EFAULT;
+
+	if (friar->hdr.type != FAN_RESPONSE_INFO_AUDIT_RULE)
+		return -EINVAL;
+	if (friar->hdr.pad != 0)
+		return -EINVAL;
+	if (friar->hdr.len != sizeof(*friar))
+		return -EINVAL;
+
+	return info_len;
+}
+
 /*
  * Finish processing of permission event by setting it to ANSWERED state and
  * drop group->notification_lock.
  */
 static void finish_permission_event(struct fsnotify_group *group,
-				    struct fanotify_perm_event *event,
-				    u32 response)
+				    struct fanotify_perm_event *event, u32 response,
+				    struct fanotify_response_info_audit_rule *friar)
 				    __releases(&group->notification_lock)
 {
 	bool destroy = false;
 
 	assert_spin_locked(&group->notification_lock);
-	event->response = response;
+	event->response = response & ~FAN_INFO;
+	if (response & FAN_INFO)
+		memcpy(&event->audit_rule, friar, sizeof(*friar));
+
 	if (event->state == FAN_EVENT_CANCELED)
 		destroy = true;
 	else
···
 }
 
 static int process_access_response(struct fsnotify_group *group,
-				   struct fanotify_response *response_struct)
+				   struct fanotify_response *response_struct,
+				   const char __user *info,
+				   size_t info_len)
 {
 	struct fanotify_perm_event *event;
 	int fd = response_struct->fd;
 	u32 response = response_struct->response;
+	int ret = info_len;
+	struct fanotify_response_info_audit_rule friar;
 
-	pr_debug("%s: group=%p fd=%d response=%u\n", __func__, group,
-		 fd, response);
+	pr_debug("%s: group=%p fd=%d response=%u buf=%p size=%zu\n", __func__,
+		 group, fd, response, info, info_len);
 	/*
 	 * make sure the response is valid, if invalid we do nothing and either
 	 * userspace can send a valid response or we will clean it up after the
 	 * timeout
 	 */
-	switch (response & ~FAN_AUDIT) {
+	if (response & ~FANOTIFY_RESPONSE_VALID_MASK)
+		return -EINVAL;
+
+	switch (response & FANOTIFY_RESPONSE_ACCESS) {
 	case FAN_ALLOW:
 	case FAN_DENY:
 		break;
···
 		return -EINVAL;
 	}
 
-	if (fd < 0)
+	if ((response & FAN_AUDIT) && !FAN_GROUP_FLAG(group, FAN_ENABLE_AUDIT))
 		return -EINVAL;
 
-	if ((response & FAN_AUDIT) && !FAN_GROUP_FLAG(group, FAN_ENABLE_AUDIT))
+	if (response & FAN_INFO) {
+		ret = process_access_response_info(info, info_len, &friar);
+		if (ret < 0)
+			return ret;
+		if (fd == FAN_NOFD)
+			return ret;
+	} else {
+		ret = 0;
+	}
+
+	if (fd < 0)
 		return -EINVAL;
 
 	spin_lock(&group->notification_lock);
···
 			continue;
 
 		list_del_init(&event->fae.fse.list);
-		finish_permission_event(group, event, response);
+		finish_permission_event(group, event, response, &friar);
 		wake_up(&group->fanotify_data.access_waitq);
-		return 0;
+		return ret;
 	}
 	spin_unlock(&group->notification_lock);
 
···
 			if (ret <= 0) {
 				spin_lock(&group->notification_lock);
 				finish_permission_event(group,
-					FANOTIFY_PERM(event), FAN_DENY);
+					FANOTIFY_PERM(event), FAN_DENY, NULL);
 				wake_up(&group->fanotify_data.access_waitq);
 			} else {
 				spin_lock(&group->notification_lock);
···
 
 static ssize_t fanotify_write(struct file *file, const char __user *buf, size_t count, loff_t *pos)
 {
-	struct fanotify_response response = { .fd = -1, .response = -1 };
+	struct fanotify_response response;
 	struct fsnotify_group *group;
 	int ret;
+	const char __user *info_buf = buf + sizeof(struct fanotify_response);
+	size_t info_len;
 
 	if (!IS_ENABLED(CONFIG_FANOTIFY_ACCESS_PERMISSIONS))
 		return -EINVAL;
 
 	group = file->private_data;
 
+	pr_debug("%s: group=%p count=%zu\n", __func__, group, count);
+
 	if (count < sizeof(response))
 		return -EINVAL;
 
-	count = sizeof(response);
-
-	pr_debug("%s: group=%p count=%zu\n", __func__, group, count);
-
-	if (copy_from_user(&response, buf, count))
+	if (copy_from_user(&response, buf, sizeof(response)))
 		return -EFAULT;
 
-	ret = process_access_response(group, &response);
+	info_len = count - sizeof(response);
+
+	ret = process_access_response(group, &response, info_buf, info_len);
 	if (ret < 0)
 		count = ret;
+	else
+		count = sizeof(response) + ret;
 
 	return count;
 }
···
 		event = list_first_entry(&group->fanotify_data.access_list,
 				struct fanotify_perm_event, fae.fse.list);
 		list_del_init(&event->fae.fse.list);
-		finish_permission_event(group, event, FAN_ALLOW);
+		finish_permission_event(group, event, FAN_ALLOW, NULL);
 		spin_lock(&group->notification_lock);
 	}
 
···
 			fsnotify_destroy_event(group, fsn_event);
 		} else {
 			finish_permission_event(group, FANOTIFY_PERM(event),
-						FAN_ALLOW);
+						FAN_ALLOW, NULL);
 		}
 		spin_lock(&group->notification_lock);
 	}

include/linux/fanotify.h

···
 #define ALL_FANOTIFY_EVENT_BITS		(FANOTIFY_OUTGOING_EVENTS | \
 					 FANOTIFY_EVENT_FLAGS)
 
+/* These masks check for invalid bits in permission responses. */
+#define FANOTIFY_RESPONSE_ACCESS (FAN_ALLOW | FAN_DENY)
+#define FANOTIFY_RESPONSE_FLAGS (FAN_AUDIT | FAN_INFO)
+#define FANOTIFY_RESPONSE_VALID_MASK (FANOTIFY_RESPONSE_ACCESS | FANOTIFY_RESPONSE_FLAGS)
+
 /* Do not use these old uapi constants internally */
 #undef FAN_ALL_CLASS_BITS
 #undef FAN_ALL_INIT_FLAGS

include/uapi/linux/fanotify.h

···
 	__u32 error_count;
 };
 
+/*
+ * User space may need to record additional information about its decision.
+ * The extra information type records what kind of information is included.
+ * The default is none. We also define an extra information buffer whose
+ * size is determined by the extra information type.
+ *
+ * If the information type is Audit Rule, then the information following
+ * is the rule number that triggered the user space decision that
+ * requires auditing.
+ */
+
+#define FAN_RESPONSE_INFO_NONE		0
+#define FAN_RESPONSE_INFO_AUDIT_RULE	1
+
 struct fanotify_response {
 	__s32 fd;
 	__u32 response;
 };
 
+struct fanotify_response_info_header {
+	__u8 type;
+	__u8 pad;
+	__u16 len;
+};
+
+struct fanotify_response_info_audit_rule {
+	struct fanotify_response_info_header hdr;
+	__u32 rule_number;
+	__u32 subj_trust;
+	__u32 obj_trust;
+};
+
 /* Legit userspace responses to a _PERM event */
 #define FAN_ALLOW	0x01
 #define FAN_DENY	0x02
-#define FAN_AUDIT	0x10	/* Bit mask to create audit record for result */
+#define FAN_AUDIT	0x10	/* Bitmask to create audit record for result */
+#define FAN_INFO	0x20	/* Bitmask to indicate additional information */
 
 /* No fd set in event */
 #define FAN_NOFD	-1