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, an audit rule number.  This will
allow for the creation of other information types in the future if other
users of the API identify different needs.

Suggested-by: Steve Grubb <sgrubb@redhat.com>
Link: https://lore.kernel.org/r/2745105.e9J7NaK4W3@x2
Suggested-by: Jan Kara <jack@suse.cz>
Link: https://lore.kernel.org/r/20201001101219.GE17860@quack2.suse.cz
Signed-off-by: Richard Guy Briggs <rgb@redhat.com>

fs/notify/fanotify/fanotify.c

@@ -262,13 +262,16 @@ static int fanotify_get_response(struct fsnotify_group *group,
 	}
 
 	/* userspace responded, convert to something usable */
-	switch (event->response & ~FAN_AUDIT) {
+	switch (event->response & FANOTIFY_RESPONSE_ACCESS) {
 	case FAN_ALLOW:
 		ret = 0;
 		break;
 	case FAN_DENY:
-	default:
 		ret = -EPERM;
+		break;
+	default:
+		ret = -EINVAL;
+		break;
 	}
 
 	/* Check if the response should be audited */
@@ -563,6 +566,8 @@ static struct fanotify_event *fanotify_alloc_perm_event(const struct path *path,
 
 	pevent->fae.type = FANOTIFY_EVENT_TYPE_PATH_PERM;
 	pevent->response = 0;
+	pevent->info_len = 0;
+	pevent->info_buf = NULL;
 	pevent->state = FAN_EVENT_INIT;
 	pevent->path = *path;
 	path_get(path);
@@ -999,6 +1004,7 @@ static void fanotify_free_path_event(struct fanotify_event *event)
 static void fanotify_free_perm_event(struct fanotify_event *event)
 {
 	path_put(fanotify_event_path(event));
+	kfree(FANOTIFY_PERM(event)->info_buf);
 	kmem_cache_free(fanotify_perm_event_cachep, FANOTIFY_PERM(event));
 }
 

fs/notify/fanotify/fanotify.h

@@ -428,6 +428,8 @@ struct fanotify_perm_event {
 	u32 response;			/* userspace answer to the event */
 	unsigned short state;		/* state of the event */
 	int fd;		/* fd we passed to userspace for this event */
+	size_t info_len;
+	char *info_buf;
 };
 
 static inline struct fanotify_perm_event *

fs/notify/fanotify/fanotify_user.c

@@ -289,13 +289,18 @@ static int create_fd(struct fsnotify_group *group, struct path *path,
  */
 static void finish_permission_event(struct fsnotify_group *group,
 				    struct fanotify_perm_event *event,
-				    u32 response)
+				    struct fanotify_response *response,
+				    size_t info_len, char *info_buf)
 				    __releases(&group->notification_lock)
 {
 	bool destroy = false;
 
 	assert_spin_locked(&group->notification_lock);
-	event->response = response;
+	event->response = response->response & ~FAN_INFO;
+	if (response->response & FAN_INFO) {
+		event->info_len = info_len;
+		event->info_buf = info_buf;
+	}
 	if (event->state == FAN_EVENT_CANCELED)
 		destroy = true;
 	else
@@ -306,41 +311,81 @@ static void finish_permission_event(struct fsnotify_group *group,
 }
 
 static int process_access_response(struct fsnotify_group *group,
-				   struct fanotify_response *response_struct)
+				   struct fanotify_response *response_struct,
+				   const char __user *buf,
+				   size_t count)
 {
 	struct fanotify_perm_event *event;
 	int fd = response_struct->fd;
 	u32 response = response_struct->response;
+	struct fanotify_response_info_header info_hdr;
+	char *info_buf = NULL;
 
-	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=%lu\n", __func__,
+		 group, fd, response, info_buf, count);
 	/*
 	 * 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;
 	default:
 		return -EINVAL;
 	}
-
-	if (fd < 0)
-		return -EINVAL;
-
 	if ((response & FAN_AUDIT) && !FAN_GROUP_FLAG(group, FAN_ENABLE_AUDIT))
 		return -EINVAL;
+	if (fd < 0)
+		return -EINVAL;
+	if (response & FAN_INFO) {
+		size_t c = count;
+		const char __user *ib = buf;
 
+		if (c <= 0)
+			return -EINVAL;
+		while (c >= sizeof(info_hdr)) {
+			if (copy_from_user(&info_hdr, ib, sizeof(info_hdr)))
+				return -EFAULT;
+			if (info_hdr.pad != 0)
+				return -EINVAL;
+			if (c < info_hdr.len)
+				return -EINVAL;
+			switch (info_hdr.type) {
+			case FAN_RESPONSE_INFO_AUDIT_RULE:
+				break;
+			case FAN_RESPONSE_INFO_NONE:
+			default:
+				return -EINVAL;
+			}
+			c -= info_hdr.len;
+			ib += info_hdr.len;
+		}
+		if (c != 0)
+			return -EINVAL;
+		/* Simplistic check for now */
+		if (count != sizeof(struct fanotify_response_info_audit_rule))
+			return -EINVAL;
+		info_buf = kmalloc(sizeof(struct fanotify_response_info_audit_rule),
+				   GFP_KERNEL);
+		if (!info_buf)
+			return -ENOMEM;
+		if (copy_from_user(info_buf, buf, count))
+			return -EFAULT;
+	}
 	spin_lock(&group->notification_lock);
 	list_for_each_entry(event, &group->fanotify_data.access_list,
 			    fae.fse.list) {
 		if (event->fd != fd)
 			continue;
 
 		list_del_init(&event->fae.fse.list);
-		finish_permission_event(group, event, response);
+		/* finish_permission_event() eats info_buf */
+		finish_permission_event(group, event, response_struct,
+					count, info_buf);
 		wake_up(&group->fanotify_data.access_waitq);
 		return 0;
 	}
@@ -802,9 +847,14 @@ static ssize_t fanotify_read(struct file *file, char __user *buf,
 			fsnotify_destroy_event(group, &event->fse);
 		} else {
 			if (ret <= 0) {
+				struct fanotify_response response = {
+					.fd = FAN_NOFD,
+					.response = FAN_DENY };
+
 				spin_lock(&group->notification_lock);
 				finish_permission_event(group,
-					FANOTIFY_PERM(event), FAN_DENY);
+					FANOTIFY_PERM(event), &response,
+					0, NULL);
 				wake_up(&group->fanotify_data.access_waitq);
 			} else {
 				spin_lock(&group->notification_lock);
@@ -827,26 +877,33 @@ static ssize_t fanotify_read(struct file *file, char __user *buf,
 
 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 c;
 
 	if (!IS_ENABLED(CONFIG_FANOTIFY_ACCESS_PERMISSIONS))
 		return -EINVAL;
 
 	group = file->private_data;
 
-	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 (count < sizeof(response))
+		return -EINVAL;
+	if (copy_from_user(&response, buf, sizeof(response)))
 		return -EFAULT;
 
-	ret = process_access_response(group, &response);
+	c = count - sizeof(response);
+	if (response.response & FAN_INFO) {
+		if (c < sizeof(struct fanotify_response_info_header))
+			return -EINVAL;
+	} else {
+		if (c != 0)
+			return -EINVAL;
+	}
+	ret = process_access_response(group, &response, info_buf, c);
 	if (ret < 0)
 		count = ret;
 
@@ -857,6 +914,9 @@ static int fanotify_release(struct inode *ignored, struct file *file)
 {
 	struct fsnotify_group *group = file->private_data;
 	struct fsnotify_event *fsn_event;
+	struct fanotify_response response = {
+		.fd = FAN_NOFD,
+		.response = FAN_ALLOW };
 
 	/*
 	 * Stop new events from arriving in the notification queue. since
@@ -876,7 +936,7 @@ static int fanotify_release(struct inode *ignored, struct file *file)
 		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, &response, 0, NULL);
 		spin_lock(&group->notification_lock);
 	}
 
@@ -893,7 +953,7 @@ static int fanotify_release(struct inode *ignored, struct file *file)
 			fsnotify_destroy_event(group, fsn_event);
 		} else {
 			finish_permission_event(group, FANOTIFY_PERM(event),
-						FAN_ALLOW);
+						&response, 0, NULL);
 		}
 		spin_lock(&group->notification_lock);
 	}

include/linux/fanotify.h

@@ -122,6 +122,11 @@
 #define ALL_FANOTIFY_EVENT_BITS		(FANOTIFY_OUTGOING_EVENTS | \
 					 FANOTIFY_EVENT_FLAGS)
 
+/* This mask is to check for invalid bits of a user space permission response */
+#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

@@ -188,15 +188,40 @@ struct fanotify_event_info_error {
 	__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 context type is Rule, then the context following is the rule number
+ * that triggered the user space decision.
+ */
+
+#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 audit_rule;
+};
+
 /* 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