[PATCH net 1/3] net: Extend bpf_net_context lifetime to cover qdisc enqueue

[Jamal Hadi Salim <jhs@mojatatu.com>]

The bpf_net_context used by sch_handle_egress() is stack-allocated and torn
down in that function returned. By the time tcf_qevent_handle() runs
current->bpf_net_context is NULL.

When a filter attached to a qevent block (e.g. RED's early_drop or mark
qevents, which always use shared blocks) returns TC_ACT_REDIRECT,
tcf_qevent_handle() calls skb_do_redirect(), which in turn calls bpf helper
bpf_net_ctx_get_ri(). That helper unconditionally dereferences
current->bpf_net_context resulting in a NULL pointer dereference.

Note: The same holds for actions that invoke BPF redirect helpers
(e.g. act_bpf running a program that calls bpf_redirect()) during qevent
classification itself. And as a matter of fact the same assumption is
made in the code outside of tc.

Fix:
Move the bpf_net_context lifecycle out of sch_handle_egress() into
__dev_queue_xmit(), so that it spans both the egress TC fast path and the
qdisc enqueue. The setup is placed outside the egress_needed_key static
branch because qevents are independent of clsact/NF egress hooks and
that key may stay disabled when only a qevent-bearing qdisc is
configured. Unfortunately this adds a small unconditional penalty to the
code path _per packet_ only guarded by CONFIG_NET_XGRESS (two writes and
one read for bpf_net_ctx_set, plus one write for bpf_net_ctx_clear).

This keeps all bpf_net_context management in net/core/dev.c i.e the
existing boundary between tc core and BPF without requiring any net/sched/
code to know about BPF plumbing.

Reproducer (see the accompanying tdc test):

  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 qdisc add dev eth0 clsact
  tc filter add block 10 pref 1 bpf obj redirect.o
  tc filter add dev eth0 egress protocol ip prio 1 matchall \
      action gact pass

  traffic through eth0 triggers red_enqueue() -> tcf_qevent_handle() and,
  on a redirect verdict, a NULL deref in skb_do_redirect().

Fixes: 401cb7dae813 ("net: Reference bpf_redirect_info via task_struct on PREEMPT_RT.")
Reported-by: Sashiko <sashiko-bot@kernel.org>
Closes: https://sashiko.dev/#/patchset/20260620130749.226642-1-jhs%40mojatatu.com
Tested-by: Victor Nogueira <victor@mojatatu.com>
Signed-off-by: Jamal Hadi Salim <jhs@mojatatu.com>
---
 net/core/dev.c | 19 ++++++++++++-------
 1 file changed, 12 insertions(+), 7 deletions(-)

diff --git a/net/core/dev.c b/net/core/dev.c
index 4b3d5cfdf6e0..8c214bfff8aa 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -4527,14 +4527,11 @@ sch_handle_egress(struct sk_buff *skb, int *ret, struct net_device *dev)
 {
 	struct bpf_mprog_entry *entry = rcu_dereference_bh(dev->tcx_egress);
 	enum skb_drop_reason drop_reason = SKB_DROP_REASON_TC_EGRESS;
-	struct bpf_net_context __bpf_net_ctx, *bpf_net_ctx;
 	int sch_ret;
 
 	if (!entry)
 		return skb;
 
-	bpf_net_ctx = bpf_net_ctx_set(&__bpf_net_ctx);
-
 	/* qdisc_skb_cb(skb)->pkt_len & tcx_set_ingress() was
 	 * already set by the caller.
 	 */
@@ -4550,12 +4547,10 @@ sch_handle_egress(struct sk_buff *skb, int *ret, struct net_device *dev)
 		/* No need to push/pop skb's mac_header here on egress! */
 		skb_do_redirect(skb);
 		*ret = NET_XMIT_SUCCESS;
-		bpf_net_ctx_clear(bpf_net_ctx);
 		return NULL;
 	case TC_ACT_SHOT:
 		kfree_skb_reason(skb, drop_reason);
 		*ret = NET_XMIT_DROP;
-		bpf_net_ctx_clear(bpf_net_ctx);
 		return NULL;
 	/* used by tc_run */
 	case TC_ACT_STOLEN:
@@ -4565,10 +4560,8 @@ sch_handle_egress(struct sk_buff *skb, int *ret, struct net_device *dev)
 		fallthrough;
 	case TC_ACT_CONSUMED:
 		*ret = NET_XMIT_SUCCESS;
-		bpf_net_ctx_clear(bpf_net_ctx);
 		return NULL;
 	}
-	bpf_net_ctx_clear(bpf_net_ctx);
 
 	return skb;
 }
@@ -4767,6 +4760,9 @@ struct netdev_queue *netdev_core_pick_tx(struct net_device *dev,
  */
 int __dev_queue_xmit(struct sk_buff *skb, struct net_device *sb_dev)
 {
+#ifdef CONFIG_NET_XGRESS
+	struct bpf_net_context __bpf_net_ctx, *bpf_net_ctx = NULL;
+#endif
 	struct net_device *dev = skb->dev;
 	struct netdev_queue *txq = NULL;
 	enum skb_drop_reason reason;
@@ -4795,6 +4791,9 @@ int __dev_queue_xmit(struct sk_buff *skb, struct net_device *sb_dev)
 	skb_update_prio(skb);
 
 	tcx_set_ingress(skb, false);
+#ifdef CONFIG_NET_XGRESS
+	bpf_net_ctx = bpf_net_ctx_set(&__bpf_net_ctx);
+#endif
 #ifdef CONFIG_NET_EGRESS
 	if (static_branch_unlikely(&egress_needed_key)) {
 		if (nf_hook_egress_active()) {
@@ -4898,12 +4897,18 @@ int __dev_queue_xmit(struct sk_buff *skb, struct net_device *sb_dev)
 
 	reason = SKB_DROP_REASON_RECURSION_LIMIT;
 drop:
+#ifdef CONFIG_NET_XGRESS
+	bpf_net_ctx_clear(bpf_net_ctx);
+#endif
 	rcu_read_unlock_bh();
 
 	dev_core_stats_tx_dropped_inc(dev);
 	kfree_skb_list_reason(skb, reason);
 	return rc;
 out:
+#ifdef CONFIG_NET_XGRESS
+	bpf_net_ctx_clear(bpf_net_ctx);
+#endif
 	rcu_read_unlock_bh();
 	return rc;
 }
-- 
2.54.0