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.

Fixes: 3a0af8fd61f9 ("bpf: BPF for lightweight tunnel infrastructure")
Reported-by: Sechang Lim <rhkrqnwk98@gmail.com>
Suggested-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: Junseo Lim <zirajs7@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20260811044149.118235-1-zirajs7@gmail.com

0 files changed, 0 insertions, 0 deletions