Re: [PATCH] usb: gadget: u_ether: Fix race between gether_disconnect and eth_stop

[Dayu Jiang <jiangdayu@xiaomi.com>]

On Wed, Mar 11, 2026 at 05:12:15PM +0800, Kuen-Han Tsai wrote:
> A race condition between gether_disconnect() and eth_stop() leads to a
> NULL pointer dereference. Specifically, if eth_stop() is triggered
> concurrently while gether_disconnect() is tearing down the endpoints,
> eth_stop() attempts to access the cleared endpoint descriptor, causing
> the following NPE:
> 
>   Unable to handle kernel NULL pointer dereference
>   Call trace:
>    __dwc3_gadget_ep_enable+0x60/0x788
>    dwc3_gadget_ep_enable+0x70/0xe4
>    usb_ep_enable+0x60/0x15c
>    eth_stop+0xb8/0x108
> 
> Because eth_stop() crashes while holding the dev->lock, the thread
> running gether_disconnect() fails to acquire the same lock and spins
> forever, resulting in a hardlockup:
> 
>   Core - Debugging Information for Hardlockup core(7)
>   Call trace:
>    queued_spin_lock_slowpath+0x94/0x488
>    _raw_spin_lock+0x64/0x6c
>    gether_disconnect+0x19c/0x1e8
>    ncm_set_alt+0x68/0x1a0
>    composite_setup+0x6a0/0xc50
> 
> The root cause is that the clearing of dev->port_usb in
> gether_disconnect() is delayed until the end of the function.
> 
> Move the clearing of dev->port_usb to the very beginning of
> gether_disconnect() while holding dev->lock. This cuts off the link
> immediately, ensuring eth_stop() will see dev->port_usb as NULL and
> safely bail out.
Hi Greg,
Hit the same issue during NCM switch stress test.
Can you take a look at this patch and check if it’s ready for merge?

Thanks,
Dayu Jiang
> 
> Fixes: 2b3d942c4878 ("usb ethernet gadget: split out network core")
> Cc: stable@vger.kernel.org
> Signed-off-by: Kuen-Han Tsai <khtsai@google.com>
> ---
>  drivers/usb/gadget/function/u_ether.c | 10 +++++-----
>  1 file changed, 5 insertions(+), 5 deletions(-)
> 
> diff --git a/drivers/usb/gadget/function/u_ether.c b/drivers/usb/gadget/function/u_ether.c
> index 338f6e2a85a9..2c970a0eafd9 100644
> --- a/drivers/usb/gadget/function/u_ether.c
> +++ b/drivers/usb/gadget/function/u_ether.c
> @@ -1246,6 +1246,11 @@ void gether_disconnect(struct gether *link)
>  
>  	DBG(dev, "%s\n", __func__);
>  
> +	spin_lock(&dev->lock);
> +	dev->port_usb = NULL;
> +	link->is_suspend = false;
> +	spin_unlock(&dev->lock);
> +
>  	netif_stop_queue(dev->net);
>  	netif_carrier_off(dev->net);
>  
> @@ -1283,11 +1288,6 @@ void gether_disconnect(struct gether *link)
>  	dev->header_len = 0;
>  	dev->unwrap = NULL;
>  	dev->wrap = NULL;
> -
> -	spin_lock(&dev->lock);
> -	dev->port_usb = NULL;
> -	link->is_suspend = false;
> -	spin_unlock(&dev->lock);
>  }
>  EXPORT_SYMBOL_GPL(gether_disconnect);
>  
> 
> ---
> base-commit: 1be3b77de4eb89af8ae2fd6610546be778e25589
> change-id: 20260311-gether-disconnect-npe-5861d9831dff
> 
> Best regards,
> -- 
> Kuen-Han Tsai <khtsai@google.com>
>