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

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

On Tue, Oct 28, 2025 at 01:30:43PM +0530, Uttkarsh Aggarwal wrote:
> A kernel panic was observed due to a race condition between un-registering
> sideband 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.
> 
> By locking the mutex before modifying sb->xhci, any thread calling
> xhci_sideband_create_interrupter() will either see a valid sb->xhci or wait
> until xhci_sideband_unregister() completes.
> 
> Signed-off-by: Uttkarsh Aggarwal <uttkarsh.aggarwal@oss.qualcomm.com>

What commit id does this fix?  Should it be backported to older kernels?

> ---
>  drivers/usb/host/xhci-sideband.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/drivers/usb/host/xhci-sideband.c b/drivers/usb/host/xhci-sideband.c
> index e771a476fef2..74a58f759cee 100644
> --- a/drivers/usb/host/xhci-sideband.c
> +++ b/drivers/usb/host/xhci-sideband.c
> @@ -481,10 +481,12 @@ xhci_sideband_unregister(struct xhci_sideband *sb)
>  
>  	xhci_sideband_remove_interrupter(sb);
>  
> +	mutex_lock(&sb->mutex);
>  	spin_lock_irq(&xhci->lock);

A mutex and a spinlock irq?  That just feels wrong for the obvious
reasons, only one should be needed.

thanks,

greg k-h