Re: [PATCH 0/6] usbip fixes to crashes found by syzbot

From: Shuah Khan <skhan@linuxfoundation.org>
To: Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>, shuah@kernel.org
Cc: valentina.manea.m@gmail.com, linux-usb@vger.kernel.org,
	linux-kernel@vger.kernel.org,
	Greg KH <gregkh@linuxfoundation.org>,
	Shuah Khan <skhan@linuxfoundation.org>
Subject: Re: [PATCH 0/6] usbip fixes to crashes found by syzbot
Date: Thu, 18 Mar 2021 07:13:37 -0600	[thread overview]
Message-ID: <41d21338-19f4-ac4f-2aef-e26180f4c573@linuxfoundation.org> (raw)
In-Reply-To: <fbf64c33-87c3-137c-4faf-66de651243fc@linuxfoundation.org>

On 3/17/21 9:06 AM, Shuah Khan wrote:
> On 3/17/21 12:21 AM, Tetsuo Handa wrote:
>> Shuah, this driver is getting more and more cryptic and buggy.
>> Please explain the strategy for serialization before you write patches.
>>
>>> - Fix attach_store() to check usbip_event_happened() before
>>>    waking up threads.
>>
>> No, this helps nothing.
>>
>>> diff --git a/drivers/usb/usbip/vhci_sysfs.c 
>>> b/drivers/usb/usbip/vhci_sysfs.c
>>> index c4b4256e5dad3..f0a770adebd97 100644
>>> --- a/drivers/usb/usbip/vhci_sysfs.c
>>> +++ b/drivers/usb/usbip/vhci_sysfs.c
>>> @@ -418,6 +418,15 @@ static ssize_t attach_store(struct device *dev, 
>>> struct device_attribute *attr,
>>>       spin_unlock_irqrestore(&vhci->lock, flags);
>>>       /* end the lock */
>>> +    if (usbip_event_happened(&vdev->ud)) {
>>> +        /*
>>> +         * something went wrong - event handler shutting
>>> +         * the connection and doing reset - bail out
>>> +         */
>>> +        dev_err(dev, "Event happended - handler is active\n");
>>> +        return -EAGAIN;
>>> +    }
>>> +
>>
>> detach_store() can queue shutdown event as soon as reaching "/* end 
>> the lock */" line
>> but attach_store() might be preempted immediately after verifying that
>> usbip_event_happened() was false (i.e. at this location) in order to 
>> wait for
>> shutdown event posted by detach_store() to be processed.
>>
> 

Please don't review code that isn't sent upstream. This repo you are
looking at is a private branch created just to verify fixes on syzbot.

I understand the race window you are talking about. I have my way of
working to resolve it.

thanks,
-- Shuah