Re: Subject: [PATCH v4] USB:Core: BugFix: Proper handling of Race Condition when two USB class drivers try to call init_usb_class simultaneously

[Ajay Kaher]

From febeb10887d5026a489658fd9e911656e76038ac Mon Sep 17 00:00:00 2001
From: Ajay Kaher <ajay.kaher@xxxxxxxxxxx>
Date: Thu, 9 Mar 2017 16:07:54 +0530
Subject: [PATCH v4] USB:Core: BugFix: Proper handling of Race Condition when two
 USB class drivers try to call init_usb_class simultaneously

There is race condition when two USB class drivers try to call
init_usb_class at the same time and leads to crash.
code path: probe->usb_register_dev->init_usb_class
 
To solve this, mutex locking has been added in init_usb_class() and 
destroy_usb_class().

As pointed by Alan, removed "if (usb_class)" test from destroy_usb_class()
because usb_class can never be NULL there.

Signed-off-by: Ajay Kaher <ajay.kaher@xxxxxxxxxxx>
Acked-by: Alan Stern <stern@xxxxxxxxxxxxxxxxxxx>
---
 drivers/usb/core/file.c |    9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/drivers/usb/core/file.c b/drivers/usb/core/file.c
index 822ced9..422ce7b 100644
--- a/drivers/usb/core/file.c
+++ b/drivers/usb/core/file.c
@@ -27,6 +27,7 @@
 #define MAX_USB_MINORS 256
 static const struct file_operations *usb_minors[MAX_USB_MINORS];
 static DECLARE_RWSEM(minor_rwsem);
+static DEFINE_MUTEX(init_usb_class_mutex);

 static int usb_open(struct inode *inode, struct file *file)
 {
@@ -109,8 +110,9 @@ static void release_usb_class(struct kref *kref)

 static void destroy_usb_class(void)
 {
- if (usb_class)
-  kref_put(&usb_class->kref, release_usb_class);
+ mutex_lock(&init_usb_class_mutex);
+ kref_put(&usb_class->kref, release_usb_class);
+ mutex_unlock(&init_usb_class_mutex);
 }

 int usb_major_init(void)
@@ -171,7 +173,10 @@ int usb_register_dev(struct usb_interface *intf,
  if (intf->minor >= 0)
   return -EADDRINUSE;

+ mutex_lock(&init_usb_class_mutex);
  retval = init_usb_class();
+ mutex_unlock(&init_usb_class_mutex);
+
  if (retval)
   return retval;

--
1.7.9.5