linux-kernel - FW: RE: Re: FW: RE: Re: Subject: [PATCH v3] USB:Core: BugFix: Proper handling of Race Condition when two USB class drivers try to call init_usb

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite for Android: free password hash cracker in your pocket

[<prev] [next>] [thread-next>] [day] [month] [year] [list]

Message-id: <20170302074947epcms5p16d50cace2b286f0cabaec6ac31f2390b@epcms5p1>
Date:   Thu, 02 Mar 2017 07:49:47 +0000
From:   Ajay Kaher <ajay.kaher@...sung.com>
To:     Alan Stern <stern@...land.harvard.edu>,
        "gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>
Cc:     "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: FW: RE: Re: FW: RE: Re: Subject: [PATCH v3] USB:Core: BugFix: Proper
 handling of Race Condition when two USB class drivers try to call
 init_usb_class simultaneously


> On Wed, 1 Mar 2017, Alan Stern wrote:
>> On Wed, 1 Mar 2017, Ajay Kaher wrote:
>>> On Mon, 22 Feb 2017, Ajay Kaher wrote:
>>> 
>>>> 
>>>>> Alan, as per my understanding I have shifted the lock from
>>>>> release_usb_class() to destroy_usb_class() in patch v3. 
>>>>> If it is not right, please explain in detail which race condition
>>>>> I have missed and also share your suggestions.
>>>>> 
>>>> 
>>>> Have you considered what would happen if destroy_usb_class() ran, but 
>>>> some other CPU was still holding a reference to usb_class?  And what if 
>>>> the last reference gets dropped later on, while init_usb_class() is 
>>>> running?
>>> 
>>> Access of usb_class->kref is only from either init_usb_class()
>>> or destroy_usb_class(), and both these functions are now protected
>>> with Mutex Locking in patch v3, so there is no chance of race condition
>>> as per above scenarios.
>>> 
>>>> Maybe that's not possible here, but it is possible in general for 
>>>> refcounted objects.  So yes, this code is probably okay, but it isn't 
>>>> good form.
>>> 
>>> As per my understanding, I found to be one of the best possible solution
>>> for this problem and this solutiuon don't have any side effect.
>> 
>> Alan, I had shared modified Patch v3 as per your inputs to prevent
>> the race condition during simultaneously calling of init_usb_class().
>> If you think there is scope to improve the patch, please share your inputs.
> 
> Under the circumstances, your patch is acceptable.
> 
> If you really want to make the point crystal clear, you could replace 
> usb_class->kref with an ordinary integer counter.  Then it would be 
> obvious that there are no references other than the ones taken by 
> init_usb_class() and released by destroy_usb_class().
 
usb_class->kref is not accessible outside the file.c
as usb_class is _static_ inside the file.c and
pointer of usb_class->kref is not passed anywhere.

Hence as you wanted, there are no references of usb_class->kref
other than taken by init_usb_class() and released by destroy_usb_class().

 
thanks,
ajay kaher
 
 
Signed-off-by: Ajay Kaher
 
---
 
 drivers/usb/core/file.c |    6 ++++++
 1 file changed, 6 insertions(+)
 
diff --git a/drivers/usb/core/file.c b/drivers/usb/core/file.c
index 822ced9..a12d184 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 +110,10 @@ static void release_usb_class(struct kref *kref)
 
 static void destroy_usb_class(void)
 {
+       mutex_lock(&init_usb_class_mutex);
        if (usb_class)
                kref_put(&usb_class->kref, release_usb_class);
+       mutex_unlock(&init_usb_class_mutex);
 }
 
 int usb_major_init(void)
@@ -171,7 +174,10 @@ 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;
 
 
 
Thanks and Regards,
Ajay Kaher