[PATCH] usb: gadget: u_ether: Fix race between gether_disconnect and eth_stop

[Kuen-Han Tsai <khtsai@google.com>]

A race condition between gether_disconnect() and eth_stop() leads to a
NULL pointer dereference. Specifically, if eth_stop() is triggered
concurrently while gether_disconnect() is tearing down the endpoints,
eth_stop() attempts to access the cleared endpoint descriptor, causing
the following NPE:

  Unable to handle kernel NULL pointer dereference
  Call trace:
   __dwc3_gadget_ep_enable+0x60/0x788
   dwc3_gadget_ep_enable+0x70/0xe4
   usb_ep_enable+0x60/0x15c
   eth_stop+0xb8/0x108

Because eth_stop() crashes while holding the dev->lock, the thread
running gether_disconnect() fails to acquire the same lock and spins
forever, resulting in a hardlockup:

  Core - Debugging Information for Hardlockup core(7)
  Call trace:
   queued_spin_lock_slowpath+0x94/0x488
   _raw_spin_lock+0x64/0x6c
   gether_disconnect+0x19c/0x1e8
   ncm_set_alt+0x68/0x1a0
   composite_setup+0x6a0/0xc50

The root cause is that the clearing of dev->port_usb in
gether_disconnect() is delayed until the end of the function.

Move the clearing of dev->port_usb to the very beginning of
gether_disconnect() while holding dev->lock. This cuts off the link
immediately, ensuring eth_stop() will see dev->port_usb as NULL and
safely bail out.

Fixes: 2b3d942c4878 ("usb ethernet gadget: split out network core")
Cc: stable@vger.kernel.org
Signed-off-by: Kuen-Han Tsai <khtsai@google.com>
---
 drivers/usb/gadget/function/u_ether.c | 10 +++++-----
 1 file changed, 5 insertions(+), 5 deletions(-)

diff --git a/drivers/usb/gadget/function/u_ether.c b/drivers/usb/gadget/function/u_ether.c
index 338f6e2a85a9..2c970a0eafd9 100644
--- a/drivers/usb/gadget/function/u_ether.c
+++ b/drivers/usb/gadget/function/u_ether.c
@@ -1246,6 +1246,11 @@ void gether_disconnect(struct gether *link)
 
 	DBG(dev, "%s\n", __func__);
 
+	spin_lock(&dev->lock);
+	dev->port_usb = NULL;
+	link->is_suspend = false;
+	spin_unlock(&dev->lock);
+
 	netif_stop_queue(dev->net);
 	netif_carrier_off(dev->net);
 
@@ -1283,11 +1288,6 @@ void gether_disconnect(struct gether *link)
 	dev->header_len = 0;
 	dev->unwrap = NULL;
 	dev->wrap = NULL;
-
-	spin_lock(&dev->lock);
-	dev->port_usb = NULL;
-	link->is_suspend = false;
-	spin_unlock(&dev->lock);
 }
 EXPORT_SYMBOL_GPL(gether_disconnect);
 

---
base-commit: 1be3b77de4eb89af8ae2fd6610546be778e25589
change-id: 20260311-gether-disconnect-npe-5861d9831dff

Best regards,
-- 
Kuen-Han Tsai <khtsai@google.com>