Vulnerability: An issue was discovered in yurex_read in drivers/usb/misc/yurex.c in the Linux kernel before 4.17.7. Local attackers could use user access read/writes with incorrect bounds checking in the yurex USB driver to crash the kernel or potentially escalate privileges.
Vulnerability ID: CVE-2018-16276
CWE: CWE-20
Severity score: 10
Project: https://github.com/torvalds/linux
Published: 2018-08-31
File: drivers/usb/misc/yurex.c
Commit message: USB: yurex: fix out-of-bounds uaccess in read handler

In general, accessing userspace memory beyond the length of the supplied
buffer in VFS read/write handlers can lead to both kernel memory corruption
(via kernel_read()/kernel_write(), which can e.g. be triggered via
sys_splice()) and privilege escalation inside userspace.

Fix it by using simple_read_from_buffer() instead of custom logic.

Fixes: 6bc235a2e24a ("USB: add driver for Meywa-Denki & Kayac YUREX")
Signed-off-by: Jann Horn <jannh@google.com>
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Commit author: Jann Horn

Vulnerable version (before fix):
 static ssize_t yurex_read(struct file *file, char __user *buffer, size_t count, 			  loff_t *ppos) { 	struct usb_yurex *dev; 	int retval = 0; 	int bytes_read = 0; 	char in_buffer[20]; 	unsigned long flags;  	dev = file->private_data;  	mutex_lock(&dev->io_mutex); 	if (!dev->interface) {		/* already disconnected */ 		retval = -ENODEV; 		goto exit; 	}  	spin_lock_irqsave(&dev->lock, flags); 	bytes_read = snprintf(in_buffer, 20, "%lld\n", dev->bbu); 	spin_unlock_irqrestore(&dev->lock, flags);  	if (*ppos < bytes_read) { 		if (copy_to_user(buffer, in_buffer + *ppos, bytes_read - *ppos)) 			retval = -EFAULT; 		else { 			retval = bytes_read - *ppos; 			*ppos += bytes_read; 		} 	}  exit: 	mutex_unlock(&dev->io_mutex); 	return retval; }  static ssize_t yurex_write(struct file *file, const char __user *user_buffer,

Fixed version (after fix):
 static ssize_t yurex_read(struct file *file, char __user *buffer, size_t count, 			  loff_t *ppos) { 	struct usb_yurex *dev; 	int len = 0; 	char in_buffer[20]; 	unsigned long flags;  	dev = file->private_data;  	mutex_lock(&dev->io_mutex); 	if (!dev->interface) {		/* already disconnected */ 		mutex_unlock(&dev->io_mutex); 		return -ENODEV; 	}  	spin_lock_irqsave(&dev->lock, flags); 	len = snprintf(in_buffer, 20, "%lld\n", dev->bbu); 	spin_unlock_irqrestore(&dev->lock, flags); 	mutex_unlock(&dev->io_mutex);  	return simple_read_from_buffer(buffer, count, ppos, in_buffer, len); }  static ssize_t yurex_write(struct file *file, const char __user *user_buffer,

Patch:
patch: @@ -396,35 +396,24 @@ static ssize_t yurex_read(struct file *file, char __user *buffer, size_t count,
 			  loff_t *ppos)
 {
 	struct usb_yurex *dev;
-	int retval = 0;
-	int bytes_read = 0;
+	int len = 0;
 	char in_buffer[20];
 	unsigned long flags;
 
 	dev = file->private_data;
 
 	mutex_lock(&dev->io_mutex);
 	if (!dev->interface) {		/* already disconnected */
-		retval = -ENODEV;
-		goto exit;
+		mutex_unlock(&dev->io_mutex);
+		return -ENODEV;
 	}
 
 	spin_lock_irqsave(&dev->lock, flags);
-	bytes_read = snprintf(in_buffer, 20, "%lld\n", dev->bbu);
+	len = snprintf(in_buffer, 20, "%lld\n", dev->bbu);
 	spin_unlock_irqrestore(&dev->lock, flags);
-
-	if (*ppos < bytes_read) {
-		if (copy_to_user(buffer, in_buffer + *ppos, bytes_read - *ppos))
-			retval = -EFAULT;
-		else {
-			retval = bytes_read - *ppos;
-			*ppos += bytes_read;
-		}
-	}
-
-exit:
 	mutex_unlock(&dev->io_mutex);
-	return retval;
+
+	return simple_read_from_buffer(buffer, count, ppos, in_buffer, len);
 }
 
 static ssize_t yurex_write(struct file *file, const char __user *user_buffer,
path: drivers/usb/misc/yurex.c