[PATCH net v2 1/3] bpf: Reject redirect helpers without a bpf_net_context
From: Daniel Borkmann <daniel@iogearbox.net>
Date: 2026-07-06 18:56:12
Also in:
bpf
Subsystem:
bpf [general] (safe dynamic programs and tools), bpf [networking] (tcx & tc bpf, sock_addr), networking [general], tc subsystem, the rest · Maintainers:
Alexei Starovoitov, Daniel Borkmann, Andrii Nakryiko, Eduard Zingerman, Kumar Kartikeya Dwivedi, "David S. Miller", Eric Dumazet, Jakub Kicinski, Paolo Abeni, Jamal Hadi Salim, Jiri Pirko, Linus Torvalds
The bpf_redirect*() helpers and skb_do_redirect() obtain the per-task
bpf_redirect_info via bpf_net_ctx_get_ri(), which dereferences the
current->bpf_net_context unconditionally. That context is established
on the paths that run tc BPF such as sch_handle_{ingress,egress}(),
*except* for the case where {cls,act}_bpf was attached to a proper
qdisc. A program running from there reaches the NULL deref in two ways:
* It calls bpf_redirect() directly, which dereferences the context at
the top of the helper:
tc qdisc add dev eth0 root handle 1: red limit 1MB min 10KB max 20KB \
avpkt 1000 burst 100 qevent early_drop block 10
tc filter add block 10 pref 1 bpf obj redirect.o
* It simply returns TC_ACT_REDIRECT without helper call: tcf_qevent_handle()
then dispatches to skb_do_redirect(), which dereferences the context
Rather than extending bpf_net_context management into the qdisc path,
make the redirect helpers refuse to operate when no context exists, and
have tcf_qevent_handle() drop a TC_ACT_REDIRECT verdict instead of
calling skb_do_redirect(). Previous behaviour was a crash, so nothing
regresses by not supporting it.
Fixes: 401cb7dae813 ("net: Reference bpf_redirect_info via task_struct on PREEMPT_RT.")
Fixes: 3625750f05ec ("net: sched: Introduce helpers for qevent blocks")
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Reviewed-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
---
net/core/filter.c | 17 +++++++++++------
net/sched/cls_api.c | 6 ++----
2 files changed, 13 insertions(+), 10 deletions(-)
diff --git a/net/core/filter.c b/net/core/filter.c
index b446aa8be5c3..11bb0d236822 100644
--- a/net/core/filter.c
+++ b/net/core/filter.c@@ -2552,11 +2552,13 @@ int skb_do_redirect(struct sk_buff *skb) BPF_CALL_2(bpf_redirect, u32, ifindex, u64, flags) { - struct bpf_redirect_info *ri = bpf_net_ctx_get_ri(); + struct bpf_redirect_info *ri; - if (unlikely(flags & (~(BPF_F_INGRESS) | BPF_F_REDIRECT_INTERNAL))) + if (unlikely(!bpf_net_ctx_get() || + (flags & (~(BPF_F_INGRESS) | BPF_F_REDIRECT_INTERNAL)))) return TC_ACT_SHOT; + ri = bpf_net_ctx_get_ri(); ri->flags = flags; ri->tgt_index = ifindex;
@@ -2573,11 +2575,12 @@ static const struct bpf_func_proto bpf_redirect_proto = { BPF_CALL_2(bpf_redirect_peer, u32, ifindex, u64, flags) { - struct bpf_redirect_info *ri = bpf_net_ctx_get_ri(); + struct bpf_redirect_info *ri; - if (unlikely(flags)) + if (unlikely(!bpf_net_ctx_get() || flags)) return TC_ACT_SHOT; + ri = bpf_net_ctx_get_ri(); ri->flags = BPF_F_PEER; ri->tgt_index = ifindex;
@@ -2595,11 +2598,13 @@ static const struct bpf_func_proto bpf_redirect_peer_proto = { BPF_CALL_4(bpf_redirect_neigh, u32, ifindex, struct bpf_redir_neigh *, params, int, plen, u64, flags) { - struct bpf_redirect_info *ri = bpf_net_ctx_get_ri(); + struct bpf_redirect_info *ri; - if (unlikely((plen && plen < sizeof(*params)) || flags)) + if (unlikely((plen && plen < sizeof(*params)) || + !bpf_net_ctx_get() || flags)) return TC_ACT_SHOT; + ri = bpf_net_ctx_get_ri(); ri->flags = BPF_F_NEIGH | (plen ? BPF_F_NEXTHOP : 0); ri->tgt_index = ifindex;
diff --git a/net/sched/cls_api.c b/net/sched/cls_api.c
index ffeea6db8337..523cf2a8bd1d 100644
--- a/net/sched/cls_api.c
+++ b/net/sched/cls_api.c@@ -4046,6 +4046,8 @@ struct sk_buff *tcf_qevent_handle(struct tcf_qevent *qe, struct Qdisc *sch, stru fl = rcu_dereference_bh(qe->filter_chain); switch (tcf_classify(skb, NULL, fl, &cl_res, false)) { + case TC_ACT_REDIRECT: + fallthrough; case TC_ACT_SHOT: qdisc_qstats_drop(sch); __qdisc_drop(skb, to_free);
@@ -4057,10 +4059,6 @@ struct sk_buff *tcf_qevent_handle(struct tcf_qevent *qe, struct Qdisc *sch, stru __qdisc_drop(skb, to_free); *ret = __NET_XMIT_STOLEN; return NULL; - case TC_ACT_REDIRECT: - skb_do_redirect(skb); - *ret = __NET_XMIT_STOLEN; - return NULL; case TC_ACT_CONSUMED: *ret = __NET_XMIT_STOLEN; return NULL;
--
2.43.0