evdev: remove a race condition opening the wrong device

Potential race condition:
- udev notifies us that a udev_device became available
- we go for a coffee and chat to the neighbours on the way
- the device is unplugged
- a new device is plugged in, gets the same devnode
- we finish our coffee and come back
- open(udev_device_get_devnode())
- new device is now opened as the old device

To avoid the above race, we compare the syspath of the device at the open fd
with the syspath of the device we originally wanted. If they differ, we fail.

evdev_compare_syspath was simply moved up.

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>

1 files changed, 26 insertions, 23 deletions

diff --git a/src/evdev.c b/src/evdev.c
index 94bf3604..908a8bad 100644
--- a/src/evdev.c
+++ b/src/evdev.c
@@ -1447,6 +1447,29 @@ evdev_notify_added_device(struct evdev_device *device)
 	notify_added_device(&device->base);
 }
 
+static int
+evdev_device_compare_syspath(struct udev_device *udev_device, int fd)
+{
+	struct udev *udev = udev_device_get_udev(udev_device);
+	struct udev_device *udev_device_new;
+	struct stat st;
+	int rc = 1;
+
+	if (fstat(fd, &st) < 0)
+		goto out;
+
+	udev_device_new = udev_device_new_from_devnum(udev, 'c', st.st_rdev);
+	if (!udev_device_new)
+		goto out;
+
+	rc = strcmp(udev_device_get_syspath(udev_device_new),
+		    udev_device_get_syspath(udev_device));
+out:
+	if (udev_device_new)
+		udev_device_unref(udev_device_new);
+	return rc;
+}
+
 struct evdev_device *
 evdev_device_create(struct libinput_seat *seat,
 		    struct udev_device *udev_device)
@@ -1469,6 +1492,9 @@ evdev_device_create(struct libinput_seat *seat,
 		return NULL;
 	}
 
+	if (evdev_device_compare_syspath(udev_device, fd) != 0)
+		goto err;
+
 	device = zalloc(sizeof *device);
 	if (device == NULL)
 		goto err;
@@ -1907,29 +1933,6 @@ evdev_device_suspend(struct evdev_device *device)
 	return 0;
 }
 
-static int
-evdev_device_compare_syspath(struct udev_device *udev_device, int fd)
-{
-	struct udev *udev = udev_device_get_udev(udev_device);
-	struct udev_device *udev_device_new;
-	struct stat st;
-	int rc = 1;
-
-	if (fstat(fd, &st) < 0)
-		goto out;
-
-	udev_device_new = udev_device_new_from_devnum(udev, 'c', st.st_rdev);
-	if (!udev_device_new)
-		goto out;
-
-	rc = strcmp(udev_device_get_syspath(udev_device_new),
-		    udev_device_get_syspath(udev_device));
-out:
-	if (udev_device_new)
-		udev_device_unref(udev_device_new);
-	return rc;
-}
-
 int
 evdev_device_resume(struct evdev_device *device)
 {