Re: [PATCH net v4] usbnet: fix smp_processor_id() use in preemptible context

[David Laight <david.laight.linux@gmail.com>]

On Wed, 09 Sep 2026 21:47:34 +0000
netdev-bot+sashiko@kernel.org wrote:

> Thank you for your contribution! Sashiko AI review found 1 potential
> issue(s) to consider:
> 
> Critical: 0 · High: 1 · Medium: 0 · Low: 0
> 
> - [High] usbnet: dropping
>   u64_stats_update_begin_irqsave()/u64_stats_update_end_irqrestore() in…
> 
> --
> 
> usbnet: fix smp_processor_id() use in preemptible context
> 
> This changes usbnet_skb_return() and tx_complete() to fetch the per-CPU
> pcpu_sw_netstats pointer with get_cpu_ptr()/put_cpu_ptr() instead of a bare
> this_cpu_ptr(), addressing a smp_processor_id()-in-preemptible-code report
> from syzbot.  Along with the pointer change it also switches the seqcount
> write helpers from the _irqsave variants to the plain ones.
> 
> Closes: https://syzkaller.appspot.com/bug?extid=04cd90bb99c6ef81a65d
> 
> > diff --git a/drivers/net/usb/usbnet.c b/drivers/net/usb/usbnet.c
> > index a19ecf718f36a..3df72b0c6bcfc 100644
> > --- a/drivers/net/usb/usbnet.c
> > +++ b/drivers/net/usb/usbnet.c
> > @@ -325,8 +325,7 @@ static void __usbnet_status_stop_force(struct usbnet *dev)
> >   */
> >  void usbnet_skb_return(struct usbnet *dev, struct sk_buff *skb)
> >  {
> > -	struct pcpu_sw_netstats *stats64 = this_cpu_ptr(dev->net->tstats);
> > -	unsigned long flags;
> > +	struct pcpu_sw_netstats *stats64;
> >  	int	status;
> >  
> >  	if (test_bit(EVENT_RX_PAUSED, &dev->flags)) {
> > @@ -338,10 +337,12 @@ void usbnet_skb_return(struct usbnet *dev, struct sk_buff *skb)
> >  	if (skb->protocol == 0)
> >  		skb->protocol = eth_type_trans(skb, dev->net);
> >  
> > -	flags = u64_stats_update_begin_irqsave(&stats64->syncp);
> > +	stats64 = get_cpu_ptr(dev->net->tstats);
> > +	u64_stats_update_begin(&stats64->syncp);
> >  	u64_stats_inc(&stats64->rx_packets);
> >  	u64_stats_add(&stats64->rx_bytes, skb->len);
> > -	u64_stats_update_end_irqrestore(&stats64->syncp, flags);
> > +	u64_stats_update_end(&stats64->syncp);
> > +	put_cpu_ptr(dev->net->tstats);  
> 
> [Severity: High]
> 
> Does dropping the _irqsave seqcount helpers here re-open the problem that
> commit 2695578b896a ("net: usbnet: fix potential deadlock on 32bit hosts")
> closed?

Looks that way.
I think you need the following sequence:
	local_irq_save(flags);
	stats64 = this_cpu_ptr(...)
	u64_stats_update_begin(...)
	...
	u64_stats_update_end(...);
	local_irq_restore(flags);

David


> 
> The reported splat is about the per-CPU pointer being taken outside of a
> preempt-disabled section, which get_cpu_ptr() addresses.  But get_cpu_ptr()
> only disables preemption:
> 
>   include/linux/percpu-defs.h
>   #define get_cpu_ptr(var)						\
>   ({									\
> 	preempt_disable();						\
> 	this_cpu_ptr(var);						\
>   })
> 
> so nothing masks interrupts across the seqcount write section anymore.
> 
> usbnet_skb_return() runs from usbnet_bh_work()->rx_process() (and from
> usbnet_resume_rx() in BH-disabled process context), i.e. with hardirqs
> enabled.  tx_complete() below is the urb->complete callback, and for host
> controllers that do not set HCD_BH it is invoked straight from the
> controller's hardirq handler:
> 
>   drivers/usb/core/hcd.c:usb_hcd_giveback_urb() {
> 	if (!hcd_giveback_urb_in_bh(hcd) && !is_root_hub(urb->dev)) {
> 		__usb_hcd_giveback_urb(urb);
> 		return;
> 	}
>   }
> 
>   drivers/usb/core/hcd.c:__usb_hcd_giveback_urb() {
> 	kcov_remote_start_usb_softirq((u64)urb->dev->bus->busnum);
> 	urb->complete(urb);
> 	kcov_remote_stop_softirq();
>   }
> 
> There is no local_irq_save() around urb->complete().  In-tree only
> ehci-hcd, xhci and dwc2 set HCD_BH, so on OHCI/UHCI/musb/max3421/sl811 and
> friends tx_complete() is a hardirq-context writer.
> 
> Both sites write the same object, since struct pcpu_sw_netstats has a
> single u64_stats_sync shared by the rx and tx counters:
> 
>   include/linux/netdevice.h
>   struct pcpu_sw_netstats {
> 	u64_stats_t		rx_packets;
> 	u64_stats_t		rx_bytes;
> 	u64_stats_t		tx_packets;
> 	u64_stats_t		tx_bytes;
> 	struct u64_stats_sync   syncp;
>   };
> 
> On 32-bit, where u64_stats_sync is a real seqcount, can the following
> nesting happen on one CPU?
> 
>   usbnet_bh_work() -> rx_process() -> usbnet_skb_return()
>      u64_stats_update_begin()          /* seq odd, rx_bytes half written */
>         <HCD hardirq>
>         usb_hcd_giveback_urb() -> __usb_hcd_giveback_urb() -> tx_complete()
>            u64_stats_update_begin()    /* seq even again */
>            u64_stats_update_end()      /* seq odd */
>      u64_stats_update_end()            /* seq even */
> 
> A reader coming in via dev_get_tstats64()->dev_fetch_sw_netstats() (for
> example "ip -s link show") would then see an even sequence in the middle of
> the outer update and accept a torn 64-bit counter without
> u64_stats_fetch_retry() firing.  With lockdep on 32-bit, is this also the
> "inconsistent {IN-HARDIRQ-W} -> {HARDIRQ-ON-W}" report that 2695578b896a
> removed?
> 
> This is the constraint documented in include/linux/u64_stats_sync.h:
> 
>    * 3) Write side must use the _irqsave() variant if other writers, or a reader,
>    *    can be invoked from an IRQ context. On 64bit systems this variant does not
>    *    disable interrupts.
> 
> Since the _irqsave variant compiles away on 64-bit, would it work to keep
> it and only change the pointer acquisition?
> 
>   stats64 = get_cpu_ptr(dev->net->tstats);
>   flags = u64_stats_update_begin_irqsave(&stats64->syncp);
>   u64_stats_inc(&stats64->rx_packets);
>   u64_stats_add(&stats64->rx_bytes, skb->len);
>   u64_stats_update_end_irqrestore(&stats64->syncp, flags);
>   put_cpu_ptr(dev->net->tstats);
> 
> The changelog only mentions the get_cpu_ptr()/put_cpu_ptr() conversion and
> says nothing about dropping the IRQ-safe helpers; could that be spelled out
> if the removal is intentional?
> 
> >  
> >  	netif_dbg(dev, rx_status, dev->net, "< rx, len %zu, type 0x%x\n",
> >  		  skb->len + sizeof(struct ethhdr), skb->protocol);
> > @@ -1298,13 +1299,14 @@ static void tx_complete(struct urb *urb)
> >  	struct usbnet		*dev = entry->dev;
> >  
> >  	if (urb->status == 0) {
> > -		struct pcpu_sw_netstats *stats64 = this_cpu_ptr(dev->net->tstats);
> > -		unsigned long flags;
> > +		struct pcpu_sw_netstats *stats64;
> >  
> > -		flags = u64_stats_update_begin_irqsave(&stats64->syncp);
> > +		stats64 = get_cpu_ptr(dev->net->tstats);
> > +		u64_stats_update_begin(&stats64->syncp);
> >  		u64_stats_add(&stats64->tx_packets, entry->packets);
> >  		u64_stats_add(&stats64->tx_bytes, entry->length);
> > -		u64_stats_update_end_irqrestore(&stats64->syncp, flags);
> > +		u64_stats_update_end(&stats64->syncp);
> > +		put_cpu_ptr(dev->net->tstats);
> >  	} else {  
> 
> [ ... ]
>