linux-kernel - Re: [PATCH v9 2/2] uio: Fix uio driver to refcount device

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Message-ID: <55115FAA.50800@brocade.com>
Date:	Tue, 24 Mar 2015 12:59:22 +0000
From:	Brian Russell <brian.russell@...cade.com>
To:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Brian Russell <brussell@...cade.com>
CC:	"Hans J. Koch" <hjk@...sjkoch.de>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v9 2/2] uio: Fix uio driver to refcount device



On 23/03/15 20:41, Greg Kroah-Hartman wrote:
> On Fri, Mar 20, 2015 at 02:54:44PM +0000, Brian Russell wrote:
>> Protect uio driver from its owner being unplugged while there are open fds.
>> Embed struct device in struct uio_device, use refcounting on device, free
>> uio_device on release.
>> info struct passed in uio_register_device can be freed on unregister, so null
>> out the field in uio_unregister_device and check accesses.
> 
> That's really not protecting anything except heavy-handed problems...
> 
> Look at the code:
> 
>> @@ -493,7 +499,7 @@ static unsigned int uio_poll(struct file *filep, poll_table *wait)
>>  	struct uio_listener *listener = filep->private_data;
>>  	struct uio_device *idev = listener->dev;
>>  
>> -	if (!idev->info->irq)
>> +	if (!idev->info || !idev->info->irq)
>>  		return -EIO;
>>  
> 
> Great, you checked the irq value, but what if it changes the very next
> line:
> 
>>  	poll_wait(filep, &idev->wait, wait);
> 
> Or any other line within this function?  Or any other function that you
> try to check the value for in the beginning...
> 
> This really isn't protecting anything "properly", sorry.  Either we
> don't care about it (hint, I don't think we really do), or we need to
> properly lock things and check, and protect, things that way.
> 

The checks for irq value are already there. I added the checks for the
idev->info ptr and deliberately nulled it in uio_unregister_device as
the caller module may free uio_info after unregistering (dpdk's igb_uio
does anyway) and then release will be called later when fds are closed.

So I think I definitely need the check in uio_release. I didn't think
it hurt to return early from poll/read/write if we know the device
has been unregistered?

Thanks,

Brian

> Please do the first one, as the reference count should be all that we
> need to care about here.
> 
> Sorry I missed this on the previous review, just realized it now this
> time around.
> 
> thanks,
> 
> greg k-h
> 
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