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Miller" , Eric Dumazet , Jakub Kicinski , Paolo Abeni , Simon Horman Cc: bpf@vger.kernel.org, linux-kernel@vger.kernel.org, netdev@vger.kernel.org, Peter Oskolkov , Xiang Mei , Weiming Shi Subject: [PATCH] bpf: clear stale IPv4 options after LWT encapsulation Date: Wed, 16 Sep 2026 01:01:48 +0800 Message-ID: <20260915170147.3943392-2-bestswngs@gmail.com> X-Mailer: git-send-email 2.55.0 Precedence: bulk X-Mailing-List: netdev@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit From: Xiang Mei bpf_lwt_push_ip_encap() rebases the network header after prepending an IP header, but leaves IPCB(skb)->opt describing the inner IPv4 header. An ingress LWT route can consequently make an ICMP error interpret an inner-header byte as an option length and copy 255 bytes into 40 bytes of stack storage. The trace decoded with scripts/decode_stacktrace.sh is: BUG: KASAN: stack-out-of-bounds in __ip_options_echo Write of size 255 Call Trace: __asan_memcpy (mm/kasan/shadow.c:106) __ip_options_echo (net/ipv4/ip_options.c:96) __icmp_send (net/ipv4/icmp.c:949) ip_forward (net/ipv4/ip_forward.c:176) lwtunnel_input (net/core/lwtunnel.c:465) ip_rcv (net/ipv4/ip_input.c:612) __netif_receive_skb_one_core (net/core/dev.c:6264) process_backlog (net/core/dev.c:6728) __napi_poll (net/core/dev.c:7787) net_rx_action (net/core/dev.c:8007) handle_softirqs (kernel/softirq.c:645) do_softirq.part.0 (kernel/softirq.c:546) __local_bh_enable_ip (kernel/softirq.c:473) __dev_queue_xmit (net/core/dev.c:4961) packet_sendmsg (net/packet/af_packet.c:3143) __sys_sendto (net/socket.c:2281) __x64_sys_sendto (net/socket.c:2288) do_syscall_64 (arch/x86/entry/syscall_64.c:84) entry_SYSCALL_64_after_hwframe (arch/x86/entry/entry_64.S:121) A helper-only reset can be restored by bpf_prog_run_save_cb(), while a program without ctx->cb[] access can clone-redirect the skb before a return-only reset. Track active LWT runs and their control-block family in the BPF network context. Reset it immediately when the control block is not BPF scratch space, and after bpf_prog_run_save_cb() restores it. Preserve the state across nested runs and retain the ingress interface and L3-slave state when the protocol family changes. Fixes: 52f278774e79 ("bpf: implement BPF_LWT_ENCAP_IP mode in bpf_lwt_push_encap") Reported-by: Weiming Shi Assisted-by: LLM Signed-off-by: Xiang Mei --- include/linux/filter.h | 10 ++++++ net/core/lwt_bpf.c | 81 ++++++++++++++++++++++++++++++++++++++++++ 2 files changed, 91 insertions(+) diff --git a/include/linux/filter.h b/include/linux/filter.h index 39decde7fc730..0edd3e6ce563f 100644 --- a/include/linux/filter.h +++ b/include/linux/filter.h @@ -848,6 +848,15 @@ struct bpf_nh_params { #define BPF_RI_F_CPU_MAP_INIT BIT(2) #define BPF_RI_F_DEV_MAP_INIT BIT(3) #define BPF_RI_F_XSK_MAP_INIT BIT(4) +#define BPF_RI_F_LWT_IP_ENCAP BIT(5) +#define BPF_RI_F_LWT_RUN BIT(6) + +struct bpf_lwt_ip_encap_state { + int iif; + __be16 cb_proto; + bool l3slave; + bool cb_access; +}; struct bpf_redirect_info { u64 tgt_index; @@ -858,6 +867,7 @@ struct bpf_redirect_info { enum bpf_map_type map_type; struct bpf_nh_params nh; u32 kern_flags; + struct bpf_lwt_ip_encap_state lwt_ip_encap; }; struct bpf_net_context { diff --git a/net/core/lwt_bpf.c b/net/core/lwt_bpf.c index da49364ec63de..762d62c959f9f 100644 --- a/net/core/lwt_bpf.c +++ b/net/core/lwt_bpf.c @@ -36,19 +36,88 @@ static inline struct bpf_lwt *bpf_lwt_lwtunnel(struct lwtunnel_state *lwt) #define NO_REDIRECT false #define CAN_REDIRECT true +static void bpf_lwt_reset_ip_cb(struct sk_buff *skb, __be16 orig_proto, + int iif, bool l3slave, bool use_new_proto) +{ + __be16 cb_proto = orig_proto; + + if (use_new_proto) + cb_proto = skb->protocol; + + if (cb_proto == htons(ETH_P_IP)) { + if (orig_proto == htons(ETH_P_IP)) { + memset(&IPCB(skb)->opt, 0, sizeof(IPCB(skb)->opt)); + } else { + memset(IPCB(skb), 0, sizeof(*IPCB(skb))); + IPCB(skb)->iif = iif; + if (l3slave) + IPCB(skb)->flags |= IPSKB_L3SLAVE; + } + } else if (cb_proto == htons(ETH_P_IPV6)) { + memset(IP6CB(skb), 0, sizeof(*IP6CB(skb))); + IP6CB(skb)->iif = iif; + IP6CB(skb)->nhoff = offsetof(struct ipv6hdr, nexthdr); + if (l3slave) + IP6CB(skb)->flags |= IP6SKB_L3SLAVE; + } else if (orig_proto == htons(ETH_P_IP)) { + memset(&IPCB(skb)->opt, 0, sizeof(IPCB(skb)->opt)); + } +} + static int run_lwt_bpf(struct sk_buff *skb, struct bpf_lwt_prog *lwt, struct dst_entry *dst, bool can_redirect) { struct bpf_net_context __bpf_net_ctx, *bpf_net_ctx; + struct bpf_lwt_ip_encap_state nested_lwt_ip_encap_state; + struct bpf_redirect_info *ri; + bool lwt_ip_encap, nested_lwt_ip_encap, nested_lwt_run; + __be16 orig_proto = skb->protocol; + bool l3slave = false; + int iif = 0; int ret; + if (orig_proto == htons(ETH_P_IP)) { + iif = IPCB(skb)->iif; + l3slave = ipv4_l3mdev_skb(IPCB(skb)->flags); + } else if (orig_proto == htons(ETH_P_IPV6)) { + iif = IP6CB(skb)->iif; + l3slave = ipv6_l3mdev_skb(IP6CB(skb)->flags); + } + /* Disabling BH is needed to protect per-CPU bpf_redirect_info between * BPF prog and skb_do_redirect(). */ local_bh_disable(); bpf_net_ctx = bpf_net_ctx_set(&__bpf_net_ctx); + ri = bpf_net_ctx_get_ri(); + nested_lwt_run = ri->kern_flags & BPF_RI_F_LWT_RUN; + nested_lwt_ip_encap = ri->kern_flags & BPF_RI_F_LWT_IP_ENCAP; + if (nested_lwt_run) + nested_lwt_ip_encap_state = ri->lwt_ip_encap; + ri->kern_flags &= ~BPF_RI_F_LWT_IP_ENCAP; + ri->kern_flags |= BPF_RI_F_LWT_RUN; + ri->lwt_ip_encap.iif = iif; + ri->lwt_ip_encap.cb_proto = orig_proto; + ri->lwt_ip_encap.l3slave = l3slave; + ri->lwt_ip_encap.cb_access = lwt->prog->cb_access; bpf_compute_data_pointers(skb); ret = bpf_prog_run_save_cb(lwt->prog, skb); + lwt_ip_encap = ri->kern_flags & BPF_RI_F_LWT_IP_ENCAP; + ri->kern_flags &= ~BPF_RI_F_LWT_IP_ENCAP; + + if (lwt_ip_encap && ri->lwt_ip_encap.cb_access) + bpf_lwt_reset_ip_cb(skb, ri->lwt_ip_encap.cb_proto, + ri->lwt_ip_encap.iif, + ri->lwt_ip_encap.l3slave, + (ret == BPF_LWT_REROUTE && + lwt->prog->type != BPF_PROG_TYPE_LWT_OUT) || + (ret == BPF_REDIRECT && can_redirect)); + if (nested_lwt_run) + ri->lwt_ip_encap = nested_lwt_ip_encap_state; + else + ri->kern_flags &= ~BPF_RI_F_LWT_RUN; + if (nested_lwt_ip_encap) + ri->kern_flags |= BPF_RI_F_LWT_IP_ENCAP; switch (ret) { case BPF_OK: @@ -604,6 +673,7 @@ static int handle_gso_encap(struct sk_buff *skb, bool ipv4, int encap_len) int bpf_lwt_push_ip_encap(struct sk_buff *skb, void *hdr, u32 len, bool ingress) { + struct bpf_redirect_info *ri; bool is_udp_tunnel; struct iphdr *iph; bool ipv4; @@ -657,6 +727,7 @@ int bpf_lwt_push_ip_encap(struct sk_buff *skb, void *hdr, u32 len, bool ingress) memcpy(skb_network_header(skb), hdr, len); bpf_compute_data_pointers(skb); skb_clear_hash(skb); + ri = bpf_net_ctx_get_ri(); if (ipv4) { skb->protocol = htons(ETH_P_IP); @@ -669,6 +740,16 @@ int bpf_lwt_push_ip_encap(struct sk_buff *skb, void *hdr, u32 len, bool ingress) skb->protocol = htons(ETH_P_IPV6); } + if (ri->kern_flags & BPF_RI_F_LWT_RUN) { + ri->kern_flags |= BPF_RI_F_LWT_IP_ENCAP; + if (!ri->lwt_ip_encap.cb_access) { + bpf_lwt_reset_ip_cb(skb, ri->lwt_ip_encap.cb_proto, + ri->lwt_ip_encap.iif, + ri->lwt_ip_encap.l3slave, true); + ri->lwt_ip_encap.cb_proto = skb->protocol; + } + } + if (skb_is_gso(skb)) return handle_gso_encap(skb, ipv4, len);