USB: misc: yurex: fix race between read and write

The write code path touches the bbu member in a non atomic manner
without taking the spinlock. Fix it.

The bug is as old as the driver.

Signed-off-by: Oliver Neukum <oneukum@suse.com>
CC: stable@vger.kernel.org
Link: https://lore.kernel.org/r/20240912132126.1034743-1-oneukum@suse.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Author: oneukum

drivers/usb/misc/yurex.c

@@ -404,7 +404,6 @@ static ssize_t yurex_read(struct file *file, char __user *buffer, size_t count,
 	struct usb_yurex *dev;
 	int len = 0;
 	char in_buffer[MAX_S64_STRLEN];
-	unsigned long flags;
 
 	dev = file->private_data;
 
@@ -419,9 +418,9 @@ static ssize_t yurex_read(struct file *file, char __user *buffer, size_t count,
 		return -EIO;
 	}
 
-	spin_lock_irqsave(&dev->lock, flags);
+	spin_lock_irq(&dev->lock);
 	scnprintf(in_buffer, MAX_S64_STRLEN, "%lld\n", dev->bbu);
-	spin_unlock_irqrestore(&dev->lock, flags);
+	spin_unlock_irq(&dev->lock);
 	mutex_unlock(&dev->io_mutex);
 
 	return simple_read_from_buffer(buffer, count, ppos, in_buffer, len);
@@ -511,8 +510,11 @@ static ssize_t yurex_write(struct file *file, const char __user *user_buffer,
 			__func__, retval);
 		goto error;
 	}
-	if (set && timeout)
+	if (set && timeout) {
+		spin_lock_irq(&dev->lock);
 		dev->bbu = c2;
+		spin_unlock_irq(&dev->lock);
+	}
 	return timeout ? count : -EIO;
 
 error: