On 01/25, Vladis Dronov wrote:

Ring buffer implementation in hid_debug_event() and hid_debug_events_read()
is strange allowing lost or corrupted data. After commit 717adfdaf147
("HID: debug: check length before copy_to_user()") it is possible to enter
an infinite loop in hid_debug_events_read() by providing 0 as count, this
locks up a system. Fix this by rewriting the ring buffer implementation
with kfifo and simplify the code.

This fixes CVE-2019-3819.

To me this looks like a good cleanup even if we forget about bugfix. Cosmetic
nits, feel free to ignore...

+	if (kfifo_is_empty(&list->hid_debug_fifo)) {
+		add_wait_queue(&list->hdev->debug_wait, &wait);
+		set_current_state(TASK_INTERRUPTIBLE);
+
+		while (kfifo_is_empty(&list->hid_debug_fifo)) {
+			if (file->f_flags & O_NONBLOCK) {
+				ret = -EAGAIN;
+				break;
+			}
+
+			if (signal_pending(current)) {
+				ret = -ERESTARTSYS;
+				break;
+			}
+
+			if (!list->hdev || !list->hdev->debug) {
+				ret = -EIO;
+				set_current_state(TASK_RUNNING);
+				goto out;

Can't resist... Yes, this is what the current code does. But you know that it looks
suspicious ;) if you add a comment the patch will be even better.

+			}
+
+			/* allow O_NONBLOCK from other threads */
+			mutex_unlock(&list->read_mutex);
+			schedule();
+			mutex_lock(&list->read_mutex);
+			set_current_state(TASK_INTERRUPTIBLE);
+		}
+
+		set_current_state(TASK_RUNNING);

you can use __set_current_state() here, mb() is not needed.

+		remove_wait_queue(&list->hdev->debug_wait, &wait);
+	}
+
+	if (ret)
+		goto out;
+

perhaps it make sense to move this check into the "if (kfifo_is_empty())" block.

+	if (kfifo_is_empty(&list->hid_debug_fifo))
+		goto out;

is kfifo_is_empty() == T really possible here?

Oleg.