linux-kernel - Re: [PATCH] usb:host: fix divide-by-zero in function fhci_queue

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Date:   Tue, 16 Apr 2019 11:07:56 -0400 (EDT)
From:   Alan Stern <stern@...land.harvard.edu>
To:     "zhuyan (M)" <zhuyan34@...wei.com>
cc:     Greg KH <gregkh@...uxfoundation.org>,
        "anton@...msg.org" <anton@...msg.org>,
        "linux-usb@...r.kernel.org" <linux-usb@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] usb:host: fix divide-by-zero in function fhci_queue_urb

On Tue, 16 Apr 2019, zhuyan (M) wrote:

> On Tue, 16 Apr 2019 at 11:45:45 +0200, Greg KH wrote:
> > On Tue, Apr 09, 2019 at 10:37:12PM +0800, zhuyan wrote:
> > > In function fhci_queue_urb, the divisor of expression 
> > > (urb->transfer_buffer_length % usb_maxpacket(urb->dev, urb->pipe,
> > > usb_pipeout(urb->pipe))) may be zero.
> > 
> > How can you hit that?
> > 
> > > When it is zero, unexpected results may occur, so it is necessary to 
> > > ensure that the divisor is not zero.
> > > 
> > > Signed-off-by: zhuyan <zhuyan34@...wei.com>
> > 
> > I need a "Full" name here, not just a single name.  Whatever you use to sign documents is good.
> > 
> > thanks,
> > 
> > greg k-h
> 
> In function usb_maxpacket, when ep is NULL, its return value is 0.  

fhci_queue_urb() shouldn't use urb->pipe to compute the maxpacket size 
anyway.  It should use usb_endpoint_maxp(&urb->ep->desc).

Alan Stern

> Then, in function fhci_queue_urb, the divisor of expression  
> (urb->transfer_buffer_length % usb_maxpacket(urb->dev, urb->pipe, usb_pipeout(urb->pipe))) 
> is zero. It will lead to unpredictable results.