[PATCH net v6] usbnet: fix smp_processor_id() use in preemptible context

[Ömer Mete Kaya]

tx_complete() calls this_cpu_ptr() before disabling preemption, which
triggers a BUG when running with CONFIG_DEBUG_PREEMPT:

  BUG: using smp_processor_id() in preemptible code in tx_complete

Fix by using get_cpu_ptr()/put_cpu_ptr() which disable preemption and
return the per-CPU pointer atomically. The usbnet_skb_return() hunk
is a hardening change: that path runs in softirq context so no warning
fires there, but the same fix is applied for consistency.

Fixes: ed194d136769 ("usb: core: remove local_irq_save() around ->complete() handler")
Suggested-by: David Laight <david.laight.linux@xxxxxxxxx>
Reported-by: syzbot+04cd90bb99c6ef81a65d@xxxxxxxxxxxxxxxxxxxxxxxxx
Closes: https://syzkaller.appspot.com/bug?extid=04cd90bb99c6ef81a65d
Signed-off-by: Ömer Mete Kaya <omermetekaya0@xxxxxxxxx>

---
v6: Fix commit message and Fixes tag as pointed out by Sashiko:
    - BUG is in tx_complete(), not usbnet_skb_return()
    - usbnet_skb_return() hunk is hardening, not a fix
    - CONFIG_DEBUG_PREEMPT, not PREEMPT_FULL
    - Fixes tag updated to ed194d136769
---
 drivers/net/usb/usbnet.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/drivers/net/usb/usbnet.c b/drivers/net/usb/usbnet.c
index a19ecf718f36..84f97f448b2d 100644
--- a/drivers/net/usb/usbnet.c
+++ b/drivers/net/usb/usbnet.c
@@ -325,7 +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);
+	struct pcpu_sw_netstats *stats64;
 	unsigned long flags;
 	int	status;

@@ -338,10 +338,12 @@ void usbnet_skb_return(struct usbnet *dev, struct sk_buff *skb)
 	if (skb->protocol == 0)
 		skb->protocol = eth_type_trans(skb, dev->net);

+	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);

 	netif_dbg(dev, rx_status, dev->net, "< rx, len %zu, type 0x%x\n",
 		  skb->len + sizeof(struct ethhdr), skb->protocol);
@@ -1298,13 +1300,15 @@ 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);
+		struct pcpu_sw_netstats *stats64;
 		unsigned long flags;

+		stats64 = get_cpu_ptr(dev->net->tstats);
 		flags = u64_stats_update_begin_irqsave(&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);
+		put_cpu_ptr(dev->net->tstats);
 	} else {
 		dev->net->stats.tx_errors++;

--
2.55.0