CVE-2026-90160

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

lwt_bpf: Restore reserved headroom after xmit program

ip_finish_output2() expands an skb to LL_RESERVED_SPACE(dev) before LWT

xmit. An LWT_XMIT BPF program can then modify the skb head and still

return BPF_OK, so bpf_xmit() rechecks the remaining headroom before the

skb continues to neighbour output.

That recheck uses dst->dev->hard_header_len. This is not enough for the

neighbour cached-header path: neigh_hh_output() copies the cached hardware

header using the aligned hh_cache size, HH_DATA_MOD for short headers or

HH_DATA_ALIGN(hh_len) otherwise.

On Ethernet, hard_header_len is 14 but the cached copy needs 16 bytes. If

an LWT_XMIT BPF program calls bpf_skb_change_head(skb, 1, 0), the skb can

still have 15 bytes of headroom after the program. The existing check

accepts that, after which neigh_hh_output() hits its headroom warning and

drops the skb.

Use LL_RESERVED_SPACE(dst->dev) in the post-BPF headroom check to match

the reservation made before LWT xmit.