net: mpls: clear inner_protocol when the last label is popped

skb_mpls_push() records the pre-encapsulation network header once, gated
on !skb->inner_protocol. skb_mpls_pop() never clears that record, so it
outlives the encapsulation it describes.

Open vSwitch can then re-push MPLS onto a packet whose
inner_network_header still points at the older, deeper offset: push a
label, pop every label, recirculate (ovs_flow_key_update() re-derives
key->eth.type and resets network_header, but leaves inner_*), then push
again. ovs_fragment() trusts the record:

	skb->network_header = skb->inner_network_header;

so skb_network_offset() goes negative. The bound check is signed:

	if (skb_network_offset(skb) > MAX_L2_LEN)

a negative offset passes it, and prepare_frag() widens the value:

	unsigned int hlen = skb_network_offset(skb);
	memcpy(&data->l2_data, skb->data, hlen);

which is a ~4GiB memcpy out of a 30-byte per-CPU buffer.

Reproduced on v7.3-rc1. RDX is the truncated length, (unsigned int)(-8):

  BUG: unable to handle page fault for address: ffffe8ffffc16000
  #PF: supervisor write access in kernel mode
  Oops: 0002 [#1] SMP KASAN NOPTI
  RIP: 0010:memcpy+0x8/0x20
  RDX: 00000000fffffff8 RSI: ffff888105d732db RDI: ffffe8ffffc16000
   prepare_frag+0x3df/0x4e0
   ovs_fragment+0x589/0x7e0
   do_output+0x4ce/0x5e0
   do_execute_actions+0x55d2/0x7b30
   ovs_execute_actions+0xea/0x450

Same root-cause shape as commit 975b5b067f ("ipv6: sr: restore network
header before routing and forwarding"): a stale network header offset
reaching a consumer that widens it. Here it originates in the MPLS
push/pop path.

Clear inner_protocol once the packet is no longer MPLS, so a later push
re-records the current header. net/sched/act_mpls.c is the only other
skb_mpls_pop() caller and gets the same fix; sch_frag.c saves and
restores inner_protocol around fragmentation in the same way OVS does.

Fixes: 48d2ab609b ("net: mpls: Fixups for GSO")
Cc: stable@vger.kernel.org
Signed-off-by: Fourie Zhang <fouriezhang@tencent.com>
Acked-by: Jiri Benc <jbenc@redhat.com>
Link: https://patch.msgid.link/20260902092719.2874481-1-fouriezhang@tencent.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

net/core/skbuff.c

@@ -6690,6 +6690,13 @@ int skb_mpls_pop(struct sk_buff *skb, __be16 next_proto, int mac_len,
 	}
 	skb->protocol = next_proto;
 
+	/* The last label is gone, so the inner header recorded by
+	 * skb_mpls_push() no longer describes this packet. Drop it, or a
+	 * later push keeps the stale offset.
+	 */
+	if (!eth_p_mpls(next_proto))
+		skb->inner_protocol = 0;
+
 	return 0;
 }
 EXPORT_SYMBOL_GPL(skb_mpls_pop);