[PATCH net 1/3] bpf: Reject redirect helpers without a bpf_net_context

[Daniel Borkmann <daniel@iogearbox.net>]

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>
Cc: 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 3e67600a4a1a..ac49ca6d9a0c 100644
--- a/net/sched/cls_api.c
+++ b/net/sched/cls_api.c
@@ -4034,6 +4034,8 @@ 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);
@@ -4045,10 +4047,6 @@ 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