From: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To: stable@vger.kernel.org
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	patches@lists.linux.dev,
	Sechang Lim <rhkrqnwk98@gmail.com>,
	Daniel Borkmann <daniel@iogearbox.net>,
	Junseo Lim <zirajs7@gmail.com>,
	Sasha Levin <sashal@kernel.org>
Subject: [PATCH 6.12 1170/1376] lwt_bpf: Restore reserved headroom after xmit program
Date: Sat, 12 Sep 2026 08:59:56 +0200
Message-ID: <20260912065633.627943719@linuxfoundation.org>
In-Reply-To: <20260912065607.535295758@linuxfoundation.org>
References: <20260912065607.535295758@linuxfoundation.org>
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6.12-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Junseo Lim <zirajs7@gmail.com>

[ 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 <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
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 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 f1641ec8284be..d80899233441d 100644
--- a/net/core/lwt_bpf.c
+++ b/net/core/lwt_bpf.c
@@ -160,10 +160,10 @@ 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;
@@ -275,7 +275,7 @@ 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;
 
@@ -291,9 +291,12 @@ 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