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

In the Linux kernel, the following vulnerability has been resolved:

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 975b5b067f52 ("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.