net/sched: Handle TC_ACT_REDIRECT from qdisc filter chains

When a TC filter attached to a qdisc filter chain returns
TC_ACT_REDIRECT (ex: via an eBPF program calling bpf_redirect() or an
act_bpf action), the redirect was silently lost i.e no qdisc classify
function handled TC_ACT_REDIRECT, so the packet fell through the
switch and was enqueued normally instead of being redirected.

This has been broken since bpf_redirect() was introduced for TC in
commit 27b29f63058d ("bpf: add bpf_redirect() helper"). We got lucky
for a long time because bpf_net_context was a per-CPU variable that
was always available.

commit 401cb7dae813 ("net: Reference bpf_redirect_info via task_struct
on PREEMPT_RT.") turned bpf_net_context into a task_struct member that
is only set up by explicit callers. Without a caller setting it up,
bpf_redirect() itself crashes with a NULL pointer dereference in
bpf_net_ctx_get_ri(). However, even with bpf_net_context available,
TC_ACT_REDIRECT from qdisc filter chains cannot be honored without
adding skb_do_redirect() calls to every qdisc classify function, which
would require changes across net/sched/. Isolate it to ebpf core where
it belongs.

Instead, add a tcf_classify_qdisc() inline helper in pkt_cls.h, as a
wrapper around tcf_classify() for use by qdisc classify functions and
tcf_qevent_handle(). When the classify verdict is TC_ACT_REDIRECT,
the wrapper converts it to TC_ACT_SHOT, dropping the packet rather
than letting it continue silently. Dropping is preferred over
letting the packet through because the user immediately sees packet
loss. Silently passing the packet through would hide the problem and
leave the user wondering why their redirect is not working.

The clsact fast path, tc_run() continues to call tcf_classify() directly
and is unaffected: TC_ACT_REDIRECT is returned as-is and handled by
sch_handle_egress/ingress() calling skb_do_redirect() as before.

Fixes: 27b29f63058d ("bpf: add bpf_redirect() helper")
Fixes: 401cb7dae813 ("net: Reference bpf_redirect_info via task_struct on PREEMPT_RT.")
Tested-by: Victor Nogueira <victor@mojatatu.com>
Signed-off-by: Jamal Hadi Salim <jhs@mojatatu.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Reviewed-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Link: https://patch.msgid.link/20260706185609.330006-3-daniel@iogearbox.net
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

1 files changed, 13 insertions, 1 deletions

diff --git a/include/net/pkt_cls.h b/include/net/pkt_cls.h
index 3bd08d7f39c1..5f5cb36439fe 100644
--- a/include/net/pkt_cls.h
+++ b/include/net/pkt_cls.h
@@ -156,8 +156,20 @@ static inline int tcf_classify(struct sk_buff *skb,
 {
 	return TC_ACT_UNSPEC;
 }
-
 #endif
+static inline int tcf_classify_qdisc(struct sk_buff *skb,
+				     const struct tcf_proto *tp,
+				     struct tcf_result *res, bool compat_mode)
+{
+	int ret = tcf_classify(skb, NULL, tp, res, compat_mode);
+
+	/* TC_ACT_REDIRECT from qdisc filter chains is not supported.
+	 * Use BPF via tcx or mirred redirect instead.
+	 */
+	if (unlikely(ret == TC_ACT_REDIRECT))
+		ret = TC_ACT_SHOT;
+	return ret;
+}
 
 static inline unsigned long
 __cls_set_class(unsigned long *clp, unsigned long cl)