HID: hidraw, fix a NULL pointer dereference in hidraw_ioctl

commit d20d5ffa upstream.

BUG: unable to handle kernel NULL pointer dereference at 0000000000000028
IP: [<ffffffffa02c66b4>] hidraw_ioctl+0xfc/0x32c [hid]
[...]

This is reproducible by disconnecting the device while userspace does
ioctl 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

@@ -237,11 +237,16 @@ static long hidraw_ioctl(struct file *file, unsigned int cmd,
 	struct inode *inode = file->f_path.dentry->d_inode;
 	unsigned int minor = iminor(inode);
 	long ret = 0;
-	/* FIXME: What stops hidraw_table going NULL */
-	struct hidraw *dev = hidraw_table[minor];
+	struct hidraw *dev;
 	void __user *user_arg = (void __user*) arg;
 
 	lock_kernel();
+	dev = hidraw_table[minor];
+	if (!dev) {
+		ret = -ENODEV;
+		goto out;
+	}
+
 	switch (cmd) {
 		case HIDIOCGRDESCSIZE:
 			if (put_user(dev->hid->rsize, (int __user *)arg))
@@ -314,6 +319,7 @@ static long hidraw_ioctl(struct file *file, unsigned int cmd,
 
 		ret = -ENOTTY;
 	}
+out:
 	unlock_kernel();
 	return ret;
 }