Re: [PATCH] tracing: guard trace_event_get_offsets() against an empty fields list

On Fri, 16 Oct 2026 10:22:56 -0400
Steven Rostedt [off-list ref] wrote:

On Fri, 16 Oct 2026 17:47:51 +0800
Haoran Wang [off-list ref] wrote:

quoted

From 0000000000000000000000000000000000000000 Mon Sep 17 00:00:00 2001
From: Haoran Wang <redacted>
Date: Fri, 16 Oct 2026 16:30:00 +0800
Subject: [PATCH] tracing: guard trace_event_get_offsets() against an empty fields list

trace_event_get_offsets() dereferences the first entry of the fields list
returned by trace_get_fields() via list_first_entry().  list_first_entry()
then returns the list head itself, and tail->offset + tail->size reads past
it.  The corrupted result is used as the BPF context-size bound in
__perf_event_set_bpf_prog():

    int off = trace_event_get_offsets(event->tp_event);
    if (prog->aux->max_ctx_offset > off)
            return -EACCES;

so a context-reading BPF program that ought to be rejected can instead be
attached to a zero-field tracepoint (which still carries
TRACE_EVENT_FL_TRACEPOINT) and read adjacent kernel memory when the
tracepoint fires, bypassing the verifier's context-size confinement.

This is reachable through user_events: a user can register an event with no
fields (e.g. `echo u:my_event > dynamic_events`), which leaves user->fields
empty while the event remains a BPF-attachable tracepoint,
trace_get_fields() then routes through user_event_get_fields() to that
empty list. The corrupted
result after list_first_entry is then used as the BPF context-size bound in
__perf_event_set_bpf_prog(), which lets context-reading BPF programs attach
to a zero-field tracepoint when they should be rejected with -EACCES.
  

Hmm, so user events can have empty events. :-/

I meant "empty fields".

Beau, Is that a thing?

-- Steve