xfrm: cache the offload ifindex for netlink dumps

copy_to_user_state_extra() only holds a reference to the outer xfrm_state.
That does not pin x->xso.dev. NETDEV_DOWN and NETDEV_UNREGISTER can race
through xfrm_dev_state_flush(), xfrm_state_delete(), and
xfrm_dev_state_free(), which clears xso->dev and drops the netdev
reference before the GETSA dump reaches xso_to_xuo() and reads
xso->dev->ifindex.

The buggy scenario involves two paths, with each column showing the order
within that path:

XFRM_MSG_GETSA dump path:           NETDEV teardown path:
1. xfrm_get_sa() gets xfrm_state    1. xfrm_dev_state_flush() finds x
2. copy_to_user_state_extra() sees  2. xfrm_state_delete() removes x
   x->xso.dev                          from the SAD
3. copy_user_offload() calls        3. xfrm_dev_state_free() clears
   xso_to_xuo()                        xso->dev
4. xso->dev->ifindex dereferences   4. netdev_put() drops the device
   a detached net_device               reference

Avoid following the live net_device from the dump paths. Cache the
attached ifindex in xfrm_dev_offload when state or policy offload is bound
to a device, and serialize that snapshot instead. This preserves the
user-visible XFRMA_OFFLOAD_DEV value without depending on the embedded
net_device lifetime.

Validation reproduced this kernel report:
Oops: general protection fault

Call Trace:
 <TASK>
 copy_to_user_state_extra+0xb8d/0x1370 [xfrm_user]
 ? __pfx_copy_to_user_state_extra+0x10/0x10 [xfrm_user]
 ? __asan_memset+0x23/0x50
 ? srso_alias_return_thunk+0x5/0xfbef5
 ? __alloc_skb+0x342/0x960
 ? srso_alias_return_thunk+0x5/0xfbef5
 ? __asan_memset+0x23/0x50
 ? srso_alias_return_thunk+0x5/0xfbef5
 ? __nlmsg_put+0x147/0x1b0
 dump_one_state+0x1c7/0x3e0 [xfrm_user]
 xfrm_state_netlink+0xcb/0x130 [xfrm_user]
 ? __pfx_xfrm_state_netlink+0x10/0x10 [xfrm_user]
 ? srso_alias_return_thunk+0x5/0xfbef5
 ? xfrm_user_state_lookup.constprop.0+0x230/0x310 [xfrm_user]
 xfrm_get_sa+0x102/0x250 [xfrm_user]
 ? __pfx_xfrm_get_sa+0x10/0x10 [xfrm_user]
 xfrm_user_rcv_msg+0x504/0xaa0 [xfrm_user]
 ? __pfx_xfrm_user_rcv_msg+0x10/0x10 [xfrm_user]
 ? srso_alias_return_thunk+0x5/0xfbef5
 ? stack_trace_save+0x8e/0xc0
 ? __pfx_stack_trace_save+0x10/0x10
 netlink_rcv_skb+0x11f/0x350
 ? __pfx_xfrm_user_rcv_msg+0x10/0x10 [xfrm_user]
 ? __pfx_netlink_rcv_skb+0x10/0x10
 ? __pfx_mutex_lock+0x10/0x10
 ? srso_alias_return_thunk+0x5/0xfbef5
 xfrm_netlink_rcv+0x65/0x80 [xfrm_user]
 netlink_unicast+0x600/0x870
 ? __pfx_netlink_unicast+0x10/0x10
 ? srso_alias_return_thunk+0x5/0xfbef5
 ? __pfx_stack_trace_save+0x10/0x10
 netlink_sendmsg+0x75d/0xc10
 ? __pfx_netlink_sendmsg+0x10/0x10
 ? srso_alias_return_thunk+0x5/0xfbef5
 ____sys_sendmsg+0x77a/0x900
 ? srso_alias_return_thunk+0x5/0xfbef5
 ? __pfx_____sys_sendmsg+0x10/0x10
 ? __pfx_copy_msghdr_from_user+0x10/0x10
 ? release_sock+0x1a/0x1d0
 ? srso_alias_return_thunk+0x5/0xfbef5
 ? netlink_insert+0x143/0xec0
 ___sys_sendmsg+0xff/0x180
 ? __pfx____sys_sendmsg+0x10/0x10
 ? _raw_spin_lock_irqsave+0x85/0xe0
 ? do_getsockname+0xf9/0x170
 ? srso_alias_return_thunk+0x5/0xfbef5
 ? fdget+0x53/0x3b0
 __sys_sendmsg+0x111/0x1a0
 ? __pfx___sys_sendmsg+0x10/0x10
 ? srso_alias_return_thunk+0x5/0xfbef5
 ? __sys_getsockname+0x8c/0x100
 do_syscall_64+0x102/0x5a0
 entry_SYSCALL_64_after_hwframe+0x77/0x7f

Fixes: 07b87f9 ("xfrm: Fix unregister netdevice hang on hardware offload.")
Assisted-by: Codex:gpt-5.5
Signed-off-by: Cen Zhang <zzzccc427@gmail.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>

Author: Cen Zhang

include/net/xfrm.h

@@ -162,6 +162,8 @@ struct xfrm_dev_offload {
 	 */
 	struct net_device	*real_dev;
 	unsigned long		offload_handle;
+	/* Snapshot the attached device index for dump paths. */
+	int			ifindex;
 	u8			dir : 2;
 	u8			type : 2;
 	u8			flags : 2;

net/xfrm/xfrm_device.c

@@ -321,6 +321,7 @@ int xfrm_dev_state_add(struct net *net, struct xfrm_state *x,
 	}
 
 	xso->dev = dev;
+	xso->ifindex = dev->ifindex;
 	netdev_tracker_alloc(dev, &xso->dev_tracker, GFP_ATOMIC);
 
 	if (xuo->flags & XFRM_OFFLOAD_INBOUND)

net/xfrm/xfrm_state.c

@@ -1547,6 +1547,7 @@ xfrm_state_find(const xfrm_address_t *daddr, const xfrm_address_t *saddr,
 			xso->type = XFRM_DEV_OFFLOAD_PACKET;
 			xso->dir = xdo->dir;
 			xso->dev = dev;
+			xso->ifindex = dev->ifindex;
 			xso->flags = XFRM_DEV_OFFLOAD_FLAG_ACQ;
 			netdev_hold(dev, &xso->dev_tracker, GFP_ATOMIC);
 			error = dev->xfrmdev_ops->xdo_dev_state_add(dev, x,

net/xfrm/xfrm_user.c

@@ -1201,17 +1201,26 @@ static int copy_sec_ctx(struct xfrm_sec_ctx *s, struct sk_buff *skb)
 	return 0;
 }
 
-static void xso_to_xuo(const struct xfrm_dev_offload *xso,
-		       struct xfrm_user_offload *xuo)
+static void xso_to_xuo_ifindex(const struct xfrm_dev_offload *xso, int ifindex,
+			       struct xfrm_user_offload *xuo)
 {
-	xuo->ifindex = xso->dev->ifindex;
+	xuo->ifindex = ifindex;
 	if (xso->dir == XFRM_DEV_OFFLOAD_IN)
 		xuo->flags = XFRM_OFFLOAD_INBOUND;
 	if (xso->type == XFRM_DEV_OFFLOAD_PACKET)
 		xuo->flags |= XFRM_OFFLOAD_PACKET;
 }
 
-static int copy_user_offload(struct xfrm_dev_offload *xso, struct sk_buff *skb)
+#ifdef CONFIG_XFRM_MIGRATE
+static void xso_to_xuo(const struct xfrm_dev_offload *xso,
+		       struct xfrm_user_offload *xuo)
+{
+	xso_to_xuo_ifindex(xso, xso->dev->ifindex, xuo);
+}
+#endif
+
+static int copy_user_offload_ifindex(const struct xfrm_dev_offload *xso,
+				     int ifindex, struct sk_buff *skb)
 {
 	struct xfrm_user_offload *xuo;
 	struct nlattr *attr;
@@ -1222,11 +1231,22 @@ static int copy_user_offload(struct xfrm_dev_offload *xso, struct sk_buff *skb)
 
 	xuo = nla_data(attr);
 	memset(xuo, 0, sizeof(*xuo));
-	xso_to_xuo(xso, xuo);
+	xso_to_xuo_ifindex(xso, ifindex, xuo);
 
 	return 0;
 }
 
+static int copy_user_offload(struct xfrm_dev_offload *xso, struct sk_buff *skb)
+{
+	return copy_user_offload_ifindex(xso, xso->dev->ifindex, skb);
+}
+
+static int copy_user_state_offload(const struct xfrm_dev_offload *xso,
+				   struct sk_buff *skb)
+{
+	return copy_user_offload_ifindex(xso, READ_ONCE(xso->ifindex), skb);
+}
+
 static bool xfrm_redact(void)
 {
 	return IS_ENABLED(CONFIG_SECURITY) &&
@@ -1433,8 +1453,8 @@ static int copy_to_user_state_extra(struct xfrm_state *x,
 			      &x->replay);
 	if (ret)
 		goto out;
-	if(x->xso.dev)
-		ret = copy_user_offload(&x->xso, skb);
+	if (READ_ONCE(x->xso.dev))
+		ret = copy_user_state_offload(&x->xso, skb);
 	if (ret)
 		goto out;
 	if (x->if_id) {
@@ -4046,8 +4066,8 @@ static inline unsigned int xfrm_sa_len(struct xfrm_state *x)
 		l += nla_total_size(sizeof(*x->coaddr));
 	if (x->props.extra_flags)
 		l += nla_total_size(sizeof(x->props.extra_flags));
-	if (x->xso.dev)
-		 l += nla_total_size(sizeof(struct xfrm_user_offload));
+	if (READ_ONCE(x->xso.dev))
+		l += nla_total_size(sizeof(struct xfrm_user_offload));
 	if (x->props.smark.v | x->props.smark.m) {
 		l += nla_total_size(sizeof(x->props.smark.v));
 		l += nla_total_size(sizeof(x->props.smark.m));