linux-kernel - Re: Subject: [PATCH v4] USB:Core: BugFix: Proper handling of Race Condition when two USB class drivers try to call init_usb

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Message-id: <20170309113425epcms5p783be90da82c183653eafed97413469c4@epcms5p7>
Date:   Thu, 09 Mar 2017 11:34:25 +0000
From:   Ajay Kaher <ajay.kaher@...sung.com>
To:     "gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>
Cc:     Alan Stern <stern@...land.harvard.edu>,
        "linux-usb@...r.kernel.org" <linux-usb@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        AMAN DEEP <aman.deep@...sung.com>,
        HEMANSHU SRIVASTAVA <hemanshu.s@...sung.com>
Subject: Re: Subject: [PATCH v4] USB:Core: BugFix: Proper handling of Race
 Condition when two USB class drivers try to call init_usb_class simultaneously

From febeb10887d5026a489658fd9e911656e76038ac Mon Sep 17 00:00:00 2001
From: Ajay Kaher <ajay.kaher@...sung.com>
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@...sung.com>
Acked-by: Alan Stern <stern@...land.harvard.edu>
---
 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