Subject: Re: [PATCH] xhci-mtk: Fix NULL pointer dereference with xhci_irq() for shared_hcd

From: Mathias Nyman <[email-masked]>
Date: Fri, 31 Jan 2020 16:50:08 +0200

On 17.1.2020 9.41, Macpaul Lin wrote:
> According to NULL pointer fix: https://tinyurl.com/uqft5ra
> xhci: Fix NULL pointer dereference with xhci_irq() for shared_hcd
> The similar issue has also been found in QC activities in Mediatek.
> 
> Here quote the description from the referenced patch as follows.
> "Commit ("f068090426ea xhci: Fix leaking USB3 shared_hcd
> at xhci removal") sets xhci_shared_hcd to NULL without
> stopping xhci host. This results into a race condition
> where shared_hcd (super speed roothub) related interrupts
> are being handled with xhci_irq happens when the
> xhci_plat_remove is called and shared_hcd is set to NULL.
> Fix this by setting the shared_hcd to NULL only after the
> controller is halted and no interrupts are generated."
> 
> Signed-off-by: Sriharsha Allenki <[email-masked]>
> Signed-off-by: Macpaul Lin <[email-masked]>
> ---
>   drivers/usb/host/xhci-mtk.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/usb/host/xhci-mtk.c b/drivers/usb/host/xhci-mtk.c
> index b18a6baef204..c227c67f5dc5 100644
> --- a/drivers/usb/host/xhci-mtk.c
> +++ b/drivers/usb/host/xhci-mtk.c
> @@ -593,11 +593,11 @@ static int xhci_mtk_remove(struct platform_device *dev)
>   	struct usb_hcd  *shared_hcd = xhci->shared_hcd;
>   
>   	usb_remove_hcd(shared_hcd);
> -	xhci->shared_hcd = NULL;
>   	device_init_wakeup(&dev->dev, false);
>   
>   	usb_remove_hcd(hcd);
>   	usb_put_hcd(shared_hcd);
> +	xhci->shared_hcd = NULL;
>   	usb_put_hcd(hcd);
>   	xhci_mtk_sch_exit(mtk);
>   	xhci_mtk_clks_disable(mtk);
> 

Could you share details of the NULL pointer dereference, (backtrace).

The USB3 hcd is already removed when xhci->shared_hcd is set to NULL.
We might want to add some checks to make sure we are not using the removed
hcd anymore in that codepath anymore.

-Mathias

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