5.10-stable review patch. If anyone has any objections, please let me know. ------------------ From: Junseo Lim [ Upstream commit 5fe7007aed9ad069b2bd77e5d0c875c64f5c0269 ] 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 Suggested-by: Daniel Borkmann Signed-off-by: Junseo Lim Signed-off-by: Daniel Borkmann Link: https://lore.kernel.org/bpf/20260811044149.118235-1-zirajs7@gmail.com Signed-off-by: Sasha Levin --- net/core/lwt_bpf.c | 15 +++++++++------ 1 file changed, 9 insertions(+), 6 deletions(-) diff --git a/net/core/lwt_bpf.c b/net/core/lwt_bpf.c index c883366a85b80..7953a3d0f94e9 100644 --- a/net/core/lwt_bpf.c +++ b/net/core/lwt_bpf.c @@ -157,10 +157,10 @@ static int bpf_output(struct net *net, struct sock *sk, struct sk_buff *skb) return dst->lwtstate->orig_output(net, sk, skb); } -static int xmit_check_hhlen(struct sk_buff *skb, int hh_len) +static int xmit_check_headroom(struct sk_buff *skb, int hroom) { - if (skb_headroom(skb) < hh_len) { - int nhead = HH_DATA_ALIGN(hh_len - skb_headroom(skb)); + if (skb_headroom(skb) < hroom) { + int nhead = hroom - skb_headroom(skb); if (pskb_expand_head(skb, nhead, 0, GFP_ATOMIC)) return -ENOMEM; @@ -272,7 +272,7 @@ static int bpf_xmit(struct sk_buff *skb) bpf = bpf_lwt_lwtunnel(dst->lwtstate); if (bpf->xmit.prog) { - int hh_len = dst->dev->hard_header_len; + int hroom = LL_RESERVED_SPACE(dst->dev); __be16 proto = skb->protocol; int ret; @@ -288,9 +288,12 @@ static int bpf_xmit(struct sk_buff *skb) return -EINVAL; } /* If the header was expanded, headroom might be too - * small for L2 header to come, expand as needed. + * small for the L2 header to come, expand as needed. + * neigh_hh_output() copies the cached header in + * HH_DATA_MOD aligned chunks, so match the reservation + * made before LWT xmit. */ - ret = xmit_check_hhlen(skb, hh_len); + ret = xmit_check_headroom(skb, hroom); if (unlikely(ret)) return ret; -- 2.53.0