From 5e96760b25fdedbbf9312827f1f237d6b18b03e8 Mon Sep 17 00:00:00 2001 From: Yuqiang Sun Date: Mon, 2 Nov 2026 02:23:43 +0800 Subject: [PATCH] bpf: lwt: Free the skb when bpf_xmit() fails to expand the headroom When an LWT xmit program returns BPF_OK, bpf_xmit() makes sure that the skb still has room for the link layer header. If pskb_expand_head() fails there, bpf_xmit() returns -ENOMEM without freeing the skb. Its callers, lwtunnel_xmit() and then ip_finish_output2() or ip6_finish_output2(), take any return value other than LWTUNNEL_XMIT_CONTINUE to mean that the skb has been consumed, so nothing frees it. Each such failure leaks the skb, together with what it holds: its dst and its charge to the sending socket, so the socket is never freed either. The other error paths of bpf_xmit() (BPF_DROP, an illegal return code, a changed protocol, a failed reroute) all free the skb. Free it here too. The leak was found with kbound, a static analysis prototype for ownership mismatches at kernel interfaces, on v7.2.12; it is present in mainline. Build-tested with x86_64 and arm64 allmodconfig (W=1, sparse): no new warnings. Run-tested on QEMU (x86_64, KVM) with KASAN: a route through a dummy device with an LWT xmit program that pushes one byte with bpf_skb_change_head(), so the headroom check has to expand the head, and failslab limited to allocations under bpf_xmit(). Each of 10 pings then fails with ENOMEM. Without this patch each leaves a raw socket behind (RAW slab objects 6 -> 16); with it none does (6 -> 6), and nothing else is reported. Without injection both kernels go from 6 to 6. Fixes: 3a0af8fd61f9 ("bpf: BPF for lightweight tunnel infrastructure") Signed-off-by: Yuqiang Sun Signed-off-by: Alexei Starovoitov Link: https://patch.msgid.link/20261101182343.17991-1-izaiahsun@gmail.com --- net/core/lwt_bpf.c | 4 +++- 1 file changed, 3 insertions(+), 1 deletion(-) diff --git a/net/core/lwt_bpf.c b/net/core/lwt_bpf.c index 774fad4e9fbf..72c86ff2a847 100644 --- a/net/core/lwt_bpf.c +++ b/net/core/lwt_bpf.c @@ -345,8 +345,10 @@ static int bpf_xmit(struct sk_buff *skb) * made before LWT xmit. */ ret = xmit_check_headroom(skb, hroom); - if (unlikely(ret)) + if (unlikely(ret)) { + kfree_skb(skb); return ret; + } return LWTUNNEL_XMIT_CONTINUE; case BPF_REDIRECT: