create_field_var_hist() creates a histogram on the matched event to supply a field variable, and records it on the target so it can be torn down later: hist_data = find_compatible_hist(target_hist_data, file); ... /* Save the compatible histogram information */ var_hist->hist_data = hist_data; hist_data is not the histogram it creates. It is one that was already there, whose keys the new one copies. The saved pointer has a single user, unregister_field_var_hists(), which runs when the target is removed: file = hist_data->field_var_hists[i]->hist_data->event_file; find_compatible_hist() matches on keys alone, so it can pick one with no variables of its own. check_var_refs() then returns false and the user can remove it while the target still points at it: BUG: KASAN: slab-use-after-free in event_hist_trigger_free+0x2b2/0x320 Read of size 8 at addr ffff8880093b90e0 by task init/1 Freed by task 1: kfree+0x154/0x420 event_hist_trigger_free+0x1d5/0x320 event_hist_trigger_parse+0x35f3/0x69e0 trigger_process_regex+0x1a6/0x250 Save the event file instead. It is all the dereference ever wanted, and it does not go away when the user removes a trigger. Reported-by: Sashiko Link: https://lore.kernel.org/all/20260914110753.221E61F000FF@smtp.kernel.org/ Cc: stable@vger.kernel.org Fixes: 02205a6752f2 ("tracing: Add support for 'field variables'") Signed-off-by: Donggeun Yoo Assisted-by: Claude:claude-fable-5 --- x86_64 under QEMU/KVM, CONFIG_KASAN=y, 2 CPUs, base 587858367581. One kernel config; one initramfs image per arm, differing only where stated. A compatible histogram on sched_waking, an onmatch() target on sched_switch naming sched_waking's prio so a field variable is forced, then the compatible histogram removed, then the target: echo 'hist:keys=pid' > events/sched/sched_waking/trigger echo 'hist:keys=next_pid:onmatch(sched.sched_waking).my_synth(prio)' \ > events/sched/sched_switch/trigger echo '!hist:keys=pid' > events/sched/sched_waking/trigger echo '!hist:keys=next_pid:onmatch(...)' > events/sched/sched_switch/trigger unfixed 1 KASAN slab-use-after-free, above unfixed, 3rd command omitted 0 patched 0 The second arm is the control: the other three commands are identical, so the report is the removal and not the harness. The read is on freed memory rather than a pointer that is reliably wrong -- whether the '!hist' still reaches the right file depends on what the slab has handed out since. trigger-field-variable-support.tc covers this path, and its removal step goes through the line this patch changes. Run by hand against both arms, since the initramfs carries no ftracetest: inter-event histogram PASS, field variable created PASS, field variable removed PASS, on both, with no KASAN report on either. kernel/trace/trace_events_hist.c | 8 ++++---- 1 file changed, 4 insertions(+), 4 deletions(-) diff --git a/kernel/trace/trace_events_hist.c b/kernel/trace/trace_events_hist.c index 8af97fd4ee2d..cc22bba011b2 100644 --- a/kernel/trace/trace_events_hist.c +++ b/kernel/trace/trace_events_hist.c @@ -549,7 +549,7 @@ struct field_var { }; struct field_var_hist { - struct hist_trigger_data *hist_data; + struct trace_event_file *file; char *cmd; }; @@ -3118,8 +3118,8 @@ create_field_var_hist(struct hist_trigger_data *target_hist_data, return ERR_PTR(-ENOMEM); } - /* Save the compatible histogram information */ - var_hist->hist_data = hist_data; + /* Needed to remove the histogram when the target goes away */ + var_hist->file = file; /* Create the new histogram with our variable */ ret = event_hist_trigger_parse(&trigger_hist_cmd, file, @@ -6344,7 +6344,7 @@ static void unregister_field_var_hists(struct hist_trigger_data *hist_data) int ret; for (i = 0; i < hist_data->n_field_var_hists; i++) { - file = hist_data->field_var_hists[i]->hist_data->event_file; + file = hist_data->field_var_hists[i]->file; cmd = hist_data->field_var_hists[i]->cmd; ret = event_hist_trigger_parse(&trigger_hist_cmd, file, "!hist", "hist", cmd); base-commit: 587858367581b9c55c3690f4e63382ad622719d4 -- 2.53.0