Re: [RFT PATCH] xhci: sideband: Fix race condition in sideband unregister

[Greg KH <gregkh@linuxfoundation.org>]

On Tue, Oct 28, 2025 at 03:44:51PM +0200, Mathias Nyman wrote:
> Uttkarsh Aggarwal observed a kernel panic during sideband un-register
> and found it was caused by a race condition between sideband unregister,
> and creating sideband interrupters.
> The issue occurrs when thread T1 runs uaudio_disconnect() and released
> sb->xhci via sideband_unregister, while thread T2 simultaneously accessed
> the now-NULL sb->xhci in xhci_sideband_create_interrupter() resulting in
> a crash.
> 
> Ensure new endpoints or interrupter can't be added to a sidenband after
> xhci_sideband_unregister() cleared the existing ones, and unlocked the
> sideband mutex.
> Reorganise code so that mutex is only taken and released once in
> xhci_sideband_unregister(), and clear sb->vdev while mutex is taken.
> 
> Refuse to add endpoints or interrupter if sb->vdev is not set.
> sb->vdev is set when sideband is created and registered.
> 
> Reported-by: Uttkarsh Aggarwal <uttkarsh.aggarwal@oss.qualcomm.com>
> Closes: https://lore.kernel.org/linux-usb/20251028080043.27760-1-uttkarsh.aggarwal@oss.qualcomm.com
> Fixes: de66754e9f80 ("xhci: sideband: add initial api to register a secondary interrupter entity")
> Signed-off-by: Mathias Nyman <mathias.nyman@linux.intel.com>
> ---
>  drivers/usb/host/xhci-sideband.c | 55 ++++++++++++++++++++++++--------
>  1 file changed, 41 insertions(+), 14 deletions(-)
> 
> diff --git a/drivers/usb/host/xhci-sideband.c b/drivers/usb/host/xhci-sideband.c
> index e771a476fef2..c308be9a8e9f 100644
> --- a/drivers/usb/host/xhci-sideband.c
> +++ b/drivers/usb/host/xhci-sideband.c
> @@ -86,6 +86,22 @@ __xhci_sideband_remove_endpoint(struct xhci_sideband *sb, struct xhci_virt_ep *e
>  	sb->eps[ep->ep_index] = NULL;
>  }
>  
> +static void
> +__xhci_sideband_remove_interrupter(struct xhci_sideband *sb)
> +{
> +	struct usb_device *udev;
> +
> +	if (!sb->ir)
> +		return;
> +
> +	xhci_remove_secondary_interrupter(xhci_to_hcd(sb->xhci), sb->ir);
> +	sb->ir = NULL;
> +	udev = sb->vdev->udev;
> +
> +	if (udev->state != USB_STATE_NOTATTACHED)
> +		usb_offload_put(udev);
> +}
> +
>  /* sideband api functions */
>  
>  /**
> @@ -132,6 +148,12 @@ xhci_sideband_add_endpoint(struct xhci_sideband *sb,
>  	unsigned int ep_index;
>  
>  	mutex_lock(&sb->mutex);
> +
> +	if (!sb->vdev) {
> +		mutex_unlock(&sb->mutex);
> +		return -ENODEV;
> +	}
> +
>  	ep_index = xhci_get_endpoint_index(&host_ep->desc);
>  	ep = &sb->vdev->eps[ep_index];
>  
> @@ -317,6 +339,12 @@ xhci_sideband_create_interrupter(struct xhci_sideband *sb, int num_seg,
>  		return -ENODEV;
>  
>  	mutex_lock(&sb->mutex);
> +
> +	if (!sb->vdev) {
> +		ret = -ENODEV;
> +		goto out;
> +	}
> +
>  	if (sb->ir) {
>  		ret = -EBUSY;
>  		goto out;
> @@ -352,20 +380,11 @@ EXPORT_SYMBOL_GPL(xhci_sideband_create_interrupter);
>  void
>  xhci_sideband_remove_interrupter(struct xhci_sideband *sb)
>  {
> -	struct usb_device *udev;
> -
> -	if (!sb || !sb->ir)
> +	if (!sb)
>  		return;
>  
>  	mutex_lock(&sb->mutex);
> -	xhci_remove_secondary_interrupter(xhci_to_hcd(sb->xhci), sb->ir);
> -
> -	sb->ir = NULL;
> -	udev = sb->vdev->udev;
> -
> -	if (udev->state != USB_STATE_NOTATTACHED)
> -		usb_offload_put(udev);
> -
> +	__xhci_sideband_remove_interrupter(sb);
>  	mutex_unlock(&sb->mutex);
>  }
>  EXPORT_SYMBOL_GPL(xhci_sideband_remove_interrupter);
> @@ -465,6 +484,7 @@ EXPORT_SYMBOL_GPL(xhci_sideband_register);
>  void
>  xhci_sideband_unregister(struct xhci_sideband *sb)
>  {
> +	struct xhci_virt_device *vdev;
>  	struct xhci_hcd *xhci;
>  	int i;
>  
> @@ -474,16 +494,23 @@ xhci_sideband_unregister(struct xhci_sideband *sb)
>  	xhci = sb->xhci;
>  
>  	mutex_lock(&sb->mutex);
> +
> +	vdev = sb->vdev;
> +	if (!vdev)
> +		return;

Lock is still held :(

I think you need to use guard() to make this more sane.

thanks,

greg k-h