linux-kernel - Re: [PATCH] usb: dwc3: gadget: Fix null pointer dereference

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Message-ID: <YnKPI4O7JloBqi0F@kroah.com>
Date:   Wed, 4 May 2022 16:35:15 +0200
From:   Greg KH <gregkh@...uxfoundation.org>
To:     Albert Wang <albertccwang@...gle.com>
Cc:     balbi@...nel.org, quic_jackp@...cinc.com, badhri@...gle.com,
        linux-usb@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] usb: dwc3: gadget: Fix null pointer dereference

On Wed, May 04, 2022 at 03:28:02PM +0800, Albert Wang wrote:
> There are still race conditions to hit the null pointer deference
> with my previous commit. So I re-write the code to dereference the
> pointer right after checking it is not null.

What race conditions?

And just moving it is not going to solve a race condition, you need a
lock.

> 
> Fixes: 26288448120b ("usb: dwc3: gadget: Fix null pointer exception")
> 
> Signed-off-by: Albert Wang <albertccwang@...gle.com>
> ---
>  drivers/usb/dwc3/gadget.c | 7 +++----
>  1 file changed, 3 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/usb/dwc3/gadget.c b/drivers/usb/dwc3/gadget.c
> index 19477f4bbf54..f2792968afd9 100644
> --- a/drivers/usb/dwc3/gadget.c
> +++ b/drivers/usb/dwc3/gadget.c
> @@ -3366,15 +3366,14 @@ static bool dwc3_gadget_endpoint_trbs_complete(struct dwc3_ep *dep,
>  	struct dwc3		*dwc = dep->dwc;
>  	bool			no_started_trb = true;
>  
> -	if (!dep->endpoint.desc)
> -		return no_started_trb;
> -
>  	dwc3_gadget_ep_cleanup_completed_requests(dep, event, status);
>  
>  	if (dep->flags & DWC3_EP_END_TRANSFER_PENDING)
>  		goto out;
>  
> -	if (usb_endpoint_xfer_isoc(dep->endpoint.desc) &&
> +	if (!dep->endpoint.desc)
> +		return no_started_trb;
> +	else if (usb_endpoint_xfer_isoc(dep->endpoint.desc) &&

There is no locking here, so why would this change do anything but
reduce the window?

thanks,

greg k-h