[PATCH 6.1.y] usb: gadget: u_ether: Fix race between gether_disconnect and eth_stop

[Sasha Levin <sashal@kernel.org>]

From: Kuen-Han Tsai <khtsai@google.com>

[ Upstream commit e1eabb072c75681f78312c484ccfffb7430f206e ]

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 <stable@kernel.org>
Signed-off-by: Kuen-Han Tsai <khtsai@google.com>
Link: https://patch.msgid.link/20260311-gether-disconnect-npe-v1-1-454966adf7c7@google.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 drivers/usb/gadget/function/u_ether.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/drivers/usb/gadget/function/u_ether.c b/drivers/usb/gadget/function/u_ether.c
index 1f420ff8f4232..e84178bffe780 100644
--- a/drivers/usb/gadget/function/u_ether.c
+++ b/drivers/usb/gadget/function/u_ether.c
@@ -1175,6 +1175,10 @@ void gether_disconnect(struct gether *link)
 
 	DBG(dev, "%s\n", __func__);
 
+	spin_lock(&dev->lock);
+	dev->port_usb = NULL;
+	spin_unlock(&dev->lock);
+
 	netif_stop_queue(dev->net);
 	netif_carrier_off(dev->net);
 
@@ -1212,10 +1216,6 @@ void gether_disconnect(struct gether *link)
 	dev->header_len = 0;
 	dev->unwrap = NULL;
 	dev->wrap = NULL;
-
-	spin_lock(&dev->lock);
-	dev->port_usb = NULL;
-	spin_unlock(&dev->lock);
 }
 EXPORT_SYMBOL_GPL(gether_disconnect);
 
-- 
2.53.0