Overview

In the Linux kernel, the following vulnerability has been resolved:

tracing: Free histogram the field rejected for a bad modifier

Writing a hist trigger whose value or variable carries a modifier that is not allowed there leaks the fields that were built for it.

__create_val_field() takes the field from parse_expr() and stores it in hist_data->fields[] only after the modifier checks have run:

hist_field = parse_expr(hist_data, file, field_str, flags, var_name,
			&n_subexprs);
...
if (hist_field->flags & HIST_FIELD_FL_VAR) {
	if (hist_field->flags & (...))
		goto err;
} else {
	if (hist_field->flags & (...))
		goto err;
}

hist_data->fields[val_idx] = hist_field;

Both checks jump past that store, and the err label returns without freeing anything. The error unwinds to create_hist_data(), which calls destroy_hist_data() -> destroy_hist_fields(), and that reaches a field only by walking fields[]. A field that never got there is unreachable.

commit e0213434fe3e ("tracing: Do not let histogram values have some modifiers") set ret to -EINVAL and fell through to the store, which left the field owned by fields[] and freed along with the rest of hist_data. Splitting the check into a value case and a variable case replaced that fall-through with a goto that skips it.

With CONFIG_DEBUG_KMEMLEAK, 200 writes of

echo 'hist:keys=prev_pid:vals=next_pid.log2' > \

 events/sched/sched_switch/trigger

each correctly rejected with -EINVAL, leave 332 unreferenced objects (63744 bytes) reported at create_hist_field(); 200 install and remove cycles of a valid trigger leave none. A '.log2' field is two allocations, since create_hist_field() puts the plain field in operands[0] of the log2 field, and both are reported.

Use destroy_hist_field() rather than __destroy_hist_field() so that operands[0] is freed as well. It returns early for HIST_FIELD_FL_VAR_REF, which is what an operand owned by hist_data->var_refs[] needs; the rejected field itself is never a var ref, because a var ref never carries a modifier flag.

Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.