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>

1 files changed, 6 insertions, 17 deletions

diff --git a/drivers/usb/misc/yurex.c b/drivers/usb/misc/yurex.c
index 8abb6cbbd98a..3be40eaa1ac9 100644
--- a/drivers/usb/misc/yurex.c
+++ b/drivers/usb/misc/yurex.c
@@ -396,8 +396,7 @@ 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;
 
@@ -405,26 +404,16 @@ static ssize_t yurex_read(struct file *file, char __user *buffer, size_t count,
 
         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,