On Fri, 16 Oct 2026 10:22:56 -0400 Steven Rostedt <[email protected]> wrote:
> On Fri, 16 Oct 2026 17:47:51 +0800 > Haoran Wang <[email protected]> wrote: > > > From 0000000000000000000000000000000000000000 Mon Sep 17 00:00:00 2001 > > From: Haoran Wang <[email protected]> > > 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
