Re: [PATCH V2] usbip: Fix locking bug in RT-enabled kernels

[Shuah Khan <skhan@linuxfoundation.org>]

On 9/16/25 09:47, Alan Stern wrote:
> On Tue, Sep 16, 2025 at 09:41:43AM +0800, Lizhi Xu wrote:
>> Interrupts are disabled before entering usb_hcd_giveback_urb().
>> A spinlock_t becomes a sleeping lock on PREEMPT_RT, so it cannot be
>> acquired with disabled interrupts.
>>
>> Save the interrupt status and restore it after usb_hcd_giveback_urb().
>>
>> syz reported:
>> BUG: sleeping function called from invalid context at kernel/locking/spinlock_rt.c:48
>> Call Trace:
>>   dump_stack_lvl+0x189/0x250 lib/dump_stack.c:120
>>   rt_spin_lock+0xc7/0x2c0 kernel/locking/spinlock_rt.c:57
>>   spin_lock include/linux/spinlock_rt.h:44 [inline]
>>   mon_bus_complete drivers/usb/mon/mon_main.c:134 [inline]
>>   mon_complete+0x5c/0x200 drivers/usb/mon/mon_main.c:147
>>   usbmon_urb_complete include/linux/usb/hcd.h:738 [inline]
>>   __usb_hcd_giveback_urb+0x254/0x5e0 drivers/usb/core/hcd.c:1647
>>   vhci_urb_enqueue+0xb4f/0xe70 drivers/usb/usbip/vhci_hcd.c:818
>>
>> Reported-by: syzbot+205ef33a3b636b4181fb@syzkaller.appspotmail.com
>> Closes: https://syzkaller.appspot.com/bug?extid=205ef33a3b636b4181fb
>> Signed-off-by: Lizhi Xu <lizhi.xu@windriver.com>
>> ---
>> V1 -> V2: fix it in usbip
>>
>>   drivers/usb/usbip/vhci_hcd.c | 6 +++---
>>   1 file changed, 3 insertions(+), 3 deletions(-)
>>
>> diff --git a/drivers/usb/usbip/vhci_hcd.c b/drivers/usb/usbip/vhci_hcd.c
>> index e70fba9f55d6..eb6de7e8ea7b 100644
>> --- a/drivers/usb/usbip/vhci_hcd.c
>> +++ b/drivers/usb/usbip/vhci_hcd.c
>> @@ -809,15 +809,15 @@ static int vhci_urb_enqueue(struct usb_hcd *hcd, struct urb *urb, gfp_t mem_flag
>>   no_need_xmit:
>>   	usb_hcd_unlink_urb_from_ep(hcd, urb);
>>   no_need_unlink:
>> -	spin_unlock_irqrestore(&vhci->lock, flags);
>>   	if (!ret) {
>>   		/* usb_hcd_giveback_urb() should be called with
>>   		 * irqs disabled
>>   		 */
>> -		local_irq_disable();
>> +		spin_unlock(&vhci->lock);
>>   		usb_hcd_giveback_urb(hcd, urb, urb->status);
>> -		local_irq_enable();
>> +		spin_lock(&vhci->lock);
>>   	}
>> +	spin_unlock_irqrestore(&vhci->lock, flags);
>>   	return ret;
>>   }
> 
> This looks right to me; it's the same pattern that the other host
> controller drivers use.  However, the final decision is up to the usbip
> maintainers.
> 
> Also, there are several other places in the usbip drivers where
> usb_hcd_giveback_urb() gets called; shouldn't they all be changed to
> follow this pattern?
> 

Looks good to me.
+1 on changing all other instances - can we do that?

thanks,
-- Shuah