In the Linux kernel, the following vulnerability has been resolved:<\/p>\n
net: lwtunnel: Drop skb metadata before LWT encapsulation<\/p>\n
skb metadata is meant for passing information between XDP and TC. It lives
\nin the skb headroom, immediately before skb->data. LWT programs cannot
\naccess the __sk_buff->data_meta pseudo-pointer to metadata.<\/p>\n
However, LWT encapsulation prepends outer headers, moving skb->data back
\nover the headroom where the metadata sits. On an RX-originated (forwarded)
\npacket that still carries XDP metadata this goes wrong in two different
\nways, depending on the encap type:<\/p>\n
1. Non-BPF LWT encaps (mpls, seg6, ioam6 …) call skb_push()\/skb_pull()
\n and silently overwrite the metadata that sits in the headroom.<\/p>\n
2) BPF LWT xmit calls bpf_skb_change_head(), which uses skb_data_move().
\n That helper expects metadata immediately before skb->data. But since
\n the IP output path runs LWT xmit before neighbour output has built
\n the outgoing L2 header, for forwarded packets skb->data points at the
\n L3 header while skb_mac_header() still points at the old L2 header.
\n skb_data_move() sees metadata ending at skb_mac_header(), not before
\n skb->data, warns and clears metadata:<\/p>\n
WARNING: CPU: 21 PID: 454557 at include\/linux\/skbuff.h:4609 skb_data_move+0x47\/0x90
\n CPU: 21 UID: 0 PID: 454557 Comm: napi\/iconduit-g Tainted: G O 6.18.21 #1
\n RIP: 0010:skb_data_move+0x47\/0x90
\n Call Trace:<\/p>\n
bpf_skb_change_head+0xe6\/0x1a0
\n bpf_prog_…+0x213\/0x2e3
\n run_lwt_bpf.isra.0+0x1d3\/0x360
\n bpf_xmit+0x46\/0xe0
\n lwtunnel_xmit+0xa1\/0xf0
\n ip_finish_output2+0x1e7\/0x5e0
\n ip_output+0x63\/0x100
\n __netif_receive_skb_one_core+0x85\/0xa0
\n process_backlog+0x9c\/0x150
\n __napi_poll+0x2b\/0x190
\n net_rx_action+0x40b\/0x7f0
\n handle_softirqs+0xd2\/0x270
\n do_softirq+0x3f\/0x60<\/p>\n
That is what happens, as for how to fix it – a received packet that
\ncarries metadata can reach an encap through any of the three LWT
\nredirect modes:<\/p>\n
LWTUNNEL_STATE_INPUT_REDIRECT
\n ip6_rcv_finish
\n dst_input
\n lwtunnel_input<\/p>\n
LWTUNNEL_STATE_OUTPUT_REDIRECT
\n ip6_rcv_finish
\n dst_input
\n ip6_forward
\n ip6_forward_finish
\n dst_output
\n lwtunnel_output<\/p>\n
LWTUNNEL_STATE_XMIT_REDIRECT
\n ip6_rcv_finish
\n dst_input
\n ip6_forward
\n ip6_forward_finish
\n dst_output
\n ip6_output
\n ip6_finish_output
\n ip6_finish_output2
\n lwtunnel_xmit<\/p>\n
Every encap funnels through the three LWT dispatch helpers, so drop the
\nmetadata there, right before handing the skb to the encap op. This
\nsingle chokepoint covers all encap types and all three redirect modes:<\/p>\n
– lwtunnel_input(): seg6, rpl, ila, seg6_local
\n – lwtunnel_output(): ioam6
\n – lwtunnel_xmit(): mpls, LWT BPF xmit<\/p>\n
Alternatively, we could clear the metadata right after TC ingress hook.
\nThat would require a compromise, however. Metadata would become
\ninaccessible from TC egress (in setups where it actually reaches the
\nhook it tact, that is without any L2 tunnels on path).<\/p>\n","protected":false},"excerpt":{"rendered":"
In the Linux kernel, the following vulnerability has been resolved:<\/p>\n
net: lwtunnel: Drop skb metadata before LWT encapsulation<\/p>\n
skb metadata is meant for passing information between XDP and TC. It liv<\/p>\n","protected":false},"template":"","meta":[],"categories":[],"tags":[],"class_list":["post-12303","vuln","type-vuln","status-publish","hentry"],"_links":{"self":[{"href":"https:\/\/www.54iter.cn\/index.php?rest_route=\/wp\/v2\/vuln\/12303","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.54iter.cn\/index.php?rest_route=\/wp\/v2\/vuln"}],"about":[{"href":"https:\/\/www.54iter.cn\/index.php?rest_route=\/wp\/v2\/types\/vuln"}],"wp:attachment":[{"href":"https:\/\/www.54iter.cn\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=12303"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.54iter.cn\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=12303"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.54iter.cn\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=12303"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}