netdev - xfrm: Possible refcount bug in xfrm_dev_state

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Message-ID: <c232db28-622d-4dd9-a61f-f12cd0ff39bb@I-love.SAKURA.ne.jp>
Date: Sat, 17 Jan 2026 20:00:16 +0900
From: Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
To: Leon Romanovsky "<leon@...nel.org>" Leon Romanovsky "<leonro@...dia.com>"
 Shannon Nelson "<shannon.nelson@...cle.com>" Steffen Klassert
 "<steffen.klassert@...unet.com>" Yossef Efraim <yossefe@...lanox.com>
Cc: Network Development <netdev@...r.kernel.org>
Subject: xfrm: Possible refcount bug in xfrm_dev_state_add() ?

Just browsing call trace for

  unregister_netdevice: waiting for netdevsim0 to become free. Usage count = 2
  ref_tracker: netdev@...f888052f24618 has 1/1 users at
       __netdev_tracker_alloc include/linux/netdevice.h:4400 [inline]
       netdev_tracker_alloc include/linux/netdevice.h:4412 [inline]
       xfrm_dev_state_add+0x3a5/0x1080 net/xfrm/xfrm_device.c:316
       xfrm_state_construct net/xfrm/xfrm_user.c:986 [inline]
       xfrm_add_sa+0x34ff/0x5fa0 net/xfrm/xfrm_user.c:1022
       xfrm_user_rcv_msg+0x58e/0xc00 net/xfrm/xfrm_user.c:3507
       netlink_rcv_skb+0x158/0x420 net/netlink/af_netlink.c:2550
       xfrm_netlink_rcv+0x71/0x90 net/xfrm/xfrm_user.c:3529
       netlink_unicast_kernel net/netlink/af_netlink.c:1318 [inline]
       netlink_unicast+0x5aa/0x870 net/netlink/af_netlink.c:1344
       netlink_sendmsg+0x8c8/0xdd0 net/netlink/af_netlink.c:1894
       sock_sendmsg_nosec net/socket.c:727 [inline]
       __sock_sendmsg net/socket.c:742 [inline]
       ____sys_sendmsg+0xa5d/0xc30 net/socket.c:2592
       ___sys_sendmsg+0x134/0x1d0 net/socket.c:2646
       __sys_sendmsg+0x16d/0x220 net/socket.c:2678
       do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
       do_syscall_64+0xcd/0xf80 arch/x86/entry/syscall_64.c:94
       entry_SYSCALL_64_after_hwframe+0x77/0x7f

problem, I noticed a different-but-possible refcount bug.

Commit 67a63387b141 ("xfrm: Fix negative device refcount on offload failure.")
resets xso->dev to NULL. Commit 50bd870a9e5c ("xfrm: Add ESN support for IPSec
HW offload") also resets xso->dev to NULL. Then, why not commit 585b64f5a620
("xfrm: delay initialization of offload path till its actually requested") also
resets xso->dev to NULL (like shown below) ? (Note that I don't know the
background of these commits...)

diff --git a/net/xfrm/xfrm_device.c b/net/xfrm/xfrm_device.c
index 52ae0e034d29..daa640f1ff9c 100644
--- a/net/xfrm/xfrm_device.c
+++ b/net/xfrm/xfrm_device.c
@@ -308,6 +308,7 @@ int xfrm_dev_state_add(struct net *net, struct xfrm_state *x,

 	if (!x->type_offload) {
 		NL_SET_ERR_MSG(extack, "Type doesn't support offload");
+		xso->dev = NULL;
 		dev_put(dev);
 		return -EINVAL;
 	}