[PATCH] HID: hiddev: keep state alive through disconnect unlock

[Yousef Alhouseen <alhouseenyousef@gmail.com>]

hiddev_disconnect() drops existancelock before freeing the hiddev state,
but a waiting final file release can run as soon as the mutex becomes
available. On PREEMPT_RT, that waiter may free hiddev while the disconnect
thread is still executing the mutex slow-unlock path, causing a
use-after-free in the mutex implementation.

Give the connection and each open file an explicit reference. Drop each
reference only after its existancelock critical section has completed, so
the state cannot be freed from the other unlock path.

Fixes: 079034073faf ("HID: hiddev cleanup -- handle all error conditions properly")
Reported-by: syzbot+563191a4939ddbfe73d4@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=563191a4939ddbfe73d4
Cc: stable@vger.kernel.org
Signed-off-by: Yousef Alhouseen <alhouseenyousef@gmail.com>
---
 drivers/hid/usbhid/hiddev.c | 37 +++++++++++++++++++------------------
 include/linux/hiddev.h      |  2 ++
 2 files changed, 21 insertions(+), 18 deletions(-)

diff --git a/drivers/hid/usbhid/hiddev.c b/drivers/hid/usbhid/hiddev.c
index 6378801b22c6..21396481995b 100644
--- a/drivers/hid/usbhid/hiddev.c
+++ b/drivers/hid/usbhid/hiddev.c
@@ -46,6 +46,12 @@ struct hiddev_list {
 	struct mutex thread_lock;
 };
 
+static void hiddev_put(struct hiddev *hiddev)
+{
+	if (refcount_dec_and_test(&hiddev->refcount))
+		kfree(hiddev);
+}
+
 /*
  * Find a report, given the report's type and ID.  The ID can be specified
  * indirectly by REPORT_ID_FIRST (which returns the first report of the given
@@ -216,26 +222,21 @@ static int hiddev_fasync(int fd, struct file *file, int on)
 static int hiddev_release(struct inode * inode, struct file * file)
 {
 	struct hiddev_list *list = file->private_data;
+	struct hiddev *hiddev = list->hiddev;
 	unsigned long flags;
 
-	spin_lock_irqsave(&list->hiddev->list_lock, flags);
+	spin_lock_irqsave(&hiddev->list_lock, flags);
 	list_del(&list->node);
-	spin_unlock_irqrestore(&list->hiddev->list_lock, flags);
+	spin_unlock_irqrestore(&hiddev->list_lock, flags);
 
-	mutex_lock(&list->hiddev->existancelock);
-	if (!--list->hiddev->open) {
-		if (list->hiddev->exist) {
-			hid_hw_close(list->hiddev->hid);
-			hid_hw_power(list->hiddev->hid, PM_HINT_NORMAL);
-		} else {
-			mutex_unlock(&list->hiddev->existancelock);
-			kfree(list->hiddev);
-			vfree(list);
-			return 0;
-		}
+	mutex_lock(&hiddev->existancelock);
+	if (!--hiddev->open && hiddev->exist) {
+		hid_hw_close(hiddev->hid);
+		hid_hw_power(hiddev->hid, PM_HINT_NORMAL);
 	}
 
-	mutex_unlock(&list->hiddev->existancelock);
+	mutex_unlock(&hiddev->existancelock);
+	hiddev_put(hiddev);
 	vfree(list);
 
 	return 0;
@@ -270,6 +271,7 @@ static int __hiddev_open(struct hiddev *hiddev, struct file *file)
 	spin_unlock_irq(&hiddev->list_lock);
 
 	file->private_data = list;
+	refcount_inc(&hiddev->refcount);
 
 	return 0;
 
@@ -897,6 +899,7 @@ int hiddev_connect(struct hid_device *hid, unsigned int force)
 	INIT_LIST_HEAD(&hiddev->list);
 	spin_lock_init(&hiddev->list_lock);
 	mutex_init(&hiddev->existancelock);
+	refcount_set(&hiddev->refcount, 1);
 	hid->hiddev = hiddev;
 	hiddev->hid = hid;
 	hiddev->exist = 1;
@@ -937,9 +940,7 @@ void hiddev_disconnect(struct hid_device *hid)
 	if (hiddev->open) {
 		hid_hw_close(hiddev->hid);
 		wake_up_interruptible(&hiddev->wait);
-		mutex_unlock(&hiddev->existancelock);
-	} else {
-		mutex_unlock(&hiddev->existancelock);
-		kfree(hiddev);
 	}
+	mutex_unlock(&hiddev->existancelock);
+	hiddev_put(hiddev);
 }
diff --git a/include/linux/hiddev.h b/include/linux/hiddev.h
index 2164c03d2c72..8e9f8a33e359 100644
--- a/include/linux/hiddev.h
+++ b/include/linux/hiddev.h
@@ -13,6 +13,7 @@
 #ifndef _HIDDEV_H
 #define _HIDDEV_H
 
+#include <linux/refcount.h>
 #include <uapi/linux/hiddev.h>
 
 
@@ -24,6 +25,7 @@ struct hiddev {
 	int minor;
 	int exist;
 	int open;
+	refcount_t refcount;
 	struct mutex existancelock;
 	wait_queue_head_t wait;
 	struct hid_device *hid;
-- 
2.54.0