[PATCH] bpf: clear stale IPv4 options after LWT encapsulation
From: Weiming Shi <hidden>
Date: 2026-09-15 17:02:11
Also in:
bpf, lkml
Subsystem:
bpf [core], bpf [general] (safe dynamic programs and tools), networking [general], the rest · Maintainers:
Alexei Starovoitov, Daniel Borkmann, Andrii Nakryiko, Eduard Zingerman, Kumar Kartikeya Dwivedi, "David S. Miller", Eric Dumazet, Jakub Kicinski, Paolo Abeni, Linus Torvalds
From: Xiang Mei <redacted>
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:
<IRQ>
__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)
</IRQ>
<TASK>
__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)
</TASK>
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 <redacted>
Assisted-by: LLM
Signed-off-by: Xiang Mei <redacted>
---
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);