On 2026-06-26 12:51:54 [-0400], Jamal Hadi Salim wrote:

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).

I fail to understand this but you and sashiko have an understanding...
If there is TC_ACT_REDIRECT returned by tc_run(), then the skb is NULL
and as such uppon return from sch_handle_egress() the control flow goes
to the out label.
As a fix you move the bpf_net_ctx assigned to before CONFIG_NET_EGRESS
and clear it on exit. What do I miss here?

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

stupid question: how do I get this redirect.o? Just a simply thing to
reproduce this…

  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().

Sebastian