HID: hidraw, fix a NULL pointer dereference in hidraw_write

commit e42dee9a upstream.

BUG: unable to handle kernel NULL pointer dereference at 0000000000000028
IP: [<ffffffffa0f0a625>] hidraw_write+0x3b/0x116 [hid]
[...]

This is reproducible by disconnecting the device while userspace writes
to dev node in a loop and doesn't check return values in order to exit
the loop.
Signed-off-by: Antonio Ospite <ospite@studenti.unina.it>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

drivers/hid/hidraw.c

@@ -105,11 +105,15 @@ static ssize_t hidraw_read(struct file *file, char __user *buffer, size_t count,
 static ssize_t hidraw_write(struct file *file, const char __user *buffer, size_t count, loff_t *ppos)
 {
 	unsigned int minor = iminor(file->f_path.dentry->d_inode);
-	/* FIXME: What stops hidraw_table going NULL */
-	struct hid_device *dev = hidraw_table[minor]->hid;
+	struct hid_device *dev;
 	__u8 *buf;
 	int ret = 0;
 
+	if (!hidraw_table[minor])
+		return -ENODEV;
+
+	dev = hidraw_table[minor]->hid;
+
 	if (!dev->hid_output_raw_report)
 		return -ENODEV;