HID: debug: fix the ring buffer implementation

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.

v2: fix an execution logic and add a comment
v3: use __set_current_state() instead of set_current_state()

Link: https://bugzilla.redhat.com/show_bug.cgi?id=1669187
Cc: [e-mail] # v4.18+
Fixes: cd667ce24796 ("HID: use debugfs for events/reports dumping")
Fixes: 717adfdaf147 ("HID: debug: check length before copy_to_user()")
Signed-off-by: Vladis Dronov <[e-mail]>
Reviewed-by: Oleg Nesterov <[e-mail]>
Signed-off-by: Benjamin Tissoires <[e-mail]>

drivers/hid/hid-debug.c

···
 
 #include <linux/debugfs.h>
 #include <linux/seq_file.h>
+#include <linux/kfifo.h>
 #include <linux/sched/signal.h>
 #include <linux/export.h>
 #include <linux/slab.h>
···
 /* enqueue string to 'events' ring buffer */
 void hid_debug_event(struct hid_device *hdev, char *buf)
 {
-	unsigned i;
 	struct hid_debug_list *list;
 	unsigned long flags;
 
 	spin_lock_irqsave(&hdev->debug_list_lock, flags);
-	list_for_each_entry(list, &hdev->debug_list, node) {
-		for (i = 0; buf[i]; i++)
-			list->hid_debug_buf[(list->tail + i) % HID_DEBUG_BUFSIZE] =
-				buf[i];
-		list->tail = (list->tail + i) % HID_DEBUG_BUFSIZE;
-        }
+	list_for_each_entry(list, &hdev->debug_list, node)
+		kfifo_in(&list->hid_debug_fifo, buf, strlen(buf));
 	spin_unlock_irqrestore(&hdev->debug_list_lock, flags);
 
 	wake_up_interruptible(&hdev->debug_wait);
···
 	hid_debug_event(hdev, buf);
 
 	kfree(buf);
-        wake_up_interruptible(&hdev->debug_wait);
-
+	wake_up_interruptible(&hdev->debug_wait);
 }
 EXPORT_SYMBOL_GPL(hid_dump_input);
 
···
 		goto out;
 	}
 
-	if (!(list->hid_debug_buf = kzalloc(HID_DEBUG_BUFSIZE, GFP_KERNEL))) {
-		err = -ENOMEM;
+	err = kfifo_alloc(&list->hid_debug_fifo, HID_DEBUG_FIFOSIZE, GFP_KERNEL);
+	if (err) {
 		kfree(list);
 		goto out;
 	}
···
 		size_t count, loff_t *ppos)
 {
 	struct hid_debug_list *list = file->private_data;
-	int ret = 0, len;
+	int ret = 0, copied;
 	DECLARE_WAITQUEUE(wait, current);
 
 	mutex_lock(&list->read_mutex);
-	while (ret == 0) {
-		if (list->head == list->tail) {
-			add_wait_queue(&list->hdev->debug_wait, &wait);
-			set_current_state(TASK_INTERRUPTIBLE);
+	if (kfifo_is_empty(&list->hid_debug_fifo)) {
+		add_wait_queue(&list->hdev->debug_wait, &wait);
+		set_current_state(TASK_INTERRUPTIBLE);
 
-			while (list->head == list->tail) {
-				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;
-				}
-
-				/* allow O_NONBLOCK from other threads */
-				mutex_unlock(&list->read_mutex);
-				schedule();
-				mutex_lock(&list->read_mutex);
-				set_current_state(TASK_INTERRUPTIBLE);
+		while (kfifo_is_empty(&list->hid_debug_fifo)) {
+			if (file->f_flags & O_NONBLOCK) {
+				ret = -EAGAIN;
+				break;
 			}
 
-			set_current_state(TASK_RUNNING);
-			remove_wait_queue(&list->hdev->debug_wait, &wait);
+			if (signal_pending(current)) {
+				ret = -ERESTARTSYS;
+				break;
+			}
+
+			/* if list->hdev is NULL we cannot remove_wait_queue().
+			 * if list->hdev->debug is 0 then hid_debug_unregister()
+			 * was already called and list->hdev is being destroyed.
+			 * if we add remove_wait_queue() here we can hit a race.
+			 */
+			if (!list->hdev || !list->hdev->debug) {
+				ret = -EIO;
+				set_current_state(TASK_RUNNING);
+				goto out;
+			}
+
+			/* 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);
+		remove_wait_queue(&list->hdev->debug_wait, &wait);
 
 		if (ret)
 			goto out;
-
-		/* pass the ringbuffer contents to userspace */
-copy_rest:
-		if (list->tail == list->head)
-			goto out;
-		if (list->tail > list->head) {
-			len = list->tail - list->head;
-			if (len > count)
-				len = count;
-
-			if (copy_to_user(buffer + ret, &list->hid_debug_buf[list->head], len)) {
-				ret = -EFAULT;
-				goto out;
-			}
-			ret += len;
-			list->head += len;
-		} else {
-			len = HID_DEBUG_BUFSIZE - list->head;
-			if (len > count)
-				len = count;
-
-			if (copy_to_user(buffer, &list->hid_debug_buf[list->head], len)) {
-				ret = -EFAULT;
-				goto out;
-			}
-			list->head = 0;
-			ret += len;
-			count -= len;
-			if (count > 0)
-				goto copy_rest;
-		}
-
 	}
+
+	/* pass the fifo content to userspace, locking is not needed with only
+	 * one concurrent reader and one concurrent writer
+	 */
+	ret = kfifo_to_user(&list->hid_debug_fifo, buffer, count, &copied);
+	if (ret)
+		goto out;
+	ret = copied;
 out:
 	mutex_unlock(&list->read_mutex);
 	return ret;
···
 	struct hid_debug_list *list = file->private_data;
 
 	poll_wait(file, &list->hdev->debug_wait, wait);
-	if (list->head != list->tail)
+	if (!kfifo_is_empty(&list->hid_debug_fifo))
 		return EPOLLIN | EPOLLRDNORM;
 	if (!list->hdev->debug)
 		return EPOLLERR | EPOLLHUP;
···
 	spin_lock_irqsave(&list->hdev->debug_list_lock, flags);
 	list_del(&list->node);
 	spin_unlock_irqrestore(&list->hdev->debug_list_lock, flags);
-	kfree(list->hid_debug_buf);
+	kfifo_free(&list->hid_debug_fifo);
 	kfree(list);
 
 	return 0;
···
 {
 	debugfs_remove_recursive(hid_debug_root);
 }
-

include/linux/hid-debug.h

···
 
 #ifdef CONFIG_DEBUG_FS
 
+#include <linux/kfifo.h>
+
 #define HID_DEBUG_BUFSIZE 512
+#define HID_DEBUG_FIFOSIZE 512
 
 void hid_dump_input(struct hid_device *, struct hid_usage *, __s32);
 void hid_dump_report(struct hid_device *, int , u8 *, int);
···
 void hid_debug_exit(void);
 void hid_debug_event(struct hid_device *, char *);
 
-
 struct hid_debug_list {
-	char *hid_debug_buf;
-	int head;
-	int tail;
+	DECLARE_KFIFO_PTR(hid_debug_fifo, char);
 	struct fasync_struct *fasync;
 	struct hid_device *hdev;
 	struct list_head node;
···
 #endif
 
 #endif
-