Re: [PATCH v2] usb: dwc3: gadget: Free gadget structure only after freeing endpoints

[Jack Pham <jackp@codeaurora.org>]

Hi Felipe,

I know it may still be early in the merge window but do you have any
feedback for this? We have some internal testers getting hit by this
use-after-free when enabling SLUB_DEBUG or KASAN enabled builds. This
fix resolves the issue in those cases so was hoping for a review soon
so we can eventually queue it up for stable too.

(I sent V2 quickly after V1 but before the kernel bot caught my obvious
blunder on V1.  But I sent V2 as a threaded-reply, so not sure if that
made this get buried in people's inboxes...)

Thanks
Jack

On Sat, May 01, 2021 at 02:35:58AM -0700, Jack Pham wrote:
> As part of commit e81a7018d93a ("usb: dwc3: allocate gadget structure
> dynamically") the dwc3_gadget_release() was added which will free
> the dwc->gadget structure upon the device's removal when
> usb_del_gadget_udc() is called in dwc3_gadget_exit().
> 
> However, simply freeing the gadget results a dangling pointer
> situation: the endpoints created in dwc3_gadget_init_endpoints()
> have their dep->endpoint.ep_list members chained off the list_head
> anchored at dwc->gadget->ep_list.  Thus when dwc->gadget is freed,
> the first dwc3_ep in the list now has a dangling prev pointer and
> likewise for the next pointer of the dwc3_ep at the tail of the list.
> The dwc3_gadget_free_endpoints() that follows will result in a
> use-after-free when it calls list_del().
> 
> This was caught by enabling KASAN and performing a driver unbind.
> The recent commit 568262bf5492 ("usb: dwc3: core: Add shutdown
> callback for dwc3") also exposes this as a panic during shutdown.
> 
> There are a few possibilities to fix this.  One could be to perform
> a list_del() of the gadget->ep_list itself which removes it from
> the rest of the dwc3_ep chain.
> 
> Another approach is what this patch does, by splitting up the
> usb_del_gadget_udc() call into its separate "del" and "put"
> components.  This allows dwc3_gadget_free_endpoints() to be
> called before the gadget is finally freed with usb_put_gadget().
> 
> Fixes: e81a7018d93a ("usb: dwc3: allocate gadget structure dynamically")
> Signed-off-by: Jack Pham <jackp@codeaurora.org>
> ---
> v2: Fix silly typo: usb_del_gadget_put -> usb_put_gadget (brain fart
> when manually recomposing the patch)
> 
>  drivers/usb/dwc3/gadget.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/usb/dwc3/gadget.c b/drivers/usb/dwc3/gadget.c
> index 6eab78f8a1a7..c7e5f5a07f3b 100644
> --- a/drivers/usb/dwc3/gadget.c
> +++ b/drivers/usb/dwc3/gadget.c
> @@ -4062,8 +4062,9 @@ int dwc3_gadget_init(struct dwc3 *dwc)
>  
>  void dwc3_gadget_exit(struct dwc3 *dwc)
>  {
> -	usb_del_gadget_udc(dwc->gadget);
> +	usb_del_gadget(dwc->gadget);
>  	dwc3_gadget_free_endpoints(dwc);
> +	usb_put_gadget(dwc->gadget);
>  	dma_free_coherent(dwc->sysdev, DWC3_BOUNCE_SIZE, dwc->bounce,
>  			  dwc->bounce_addr);
>  	kfree(dwc->setup_buf);
> -- 
> 2.24.0
> 

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