CVE-2026-90160lwt_bpf: Restore reserved headroom after xmit program

Official description

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.

Affected packages & fixed versions

Affected products (as reported)

References

• https://git.kernel.org/stable/c/753e5cdcca5474d230d62bb3489e5168ab27c272
• https://git.kernel.org/stable/c/c488071c3441fa34f5a87cd6c12ce2cc6304f20e
• https://git.kernel.org/stable/c/a38c0eb447e2dd0120a2ebcdba470f9505ac8907
• https://git.kernel.org/stable/c/de2b2004e16f2930eb689175e2c1998b0a68d499
• https://git.kernel.org/stable/c/7d043e24520a273c362be5dd7d9c82796879a49b
• https://git.kernel.org/stable/c/7cf561843ed0ad57501892a65abb77957e6c800f
• https://git.kernel.org/stable/c/179a5b2171573d94a25c9aa8e1c9f9ac352ad316
• https://git.kernel.org/stable/c/5fe7007aed9ad069b2bd77e5d0c875c64f5c0269