tracing/probes: Fix use-after-free on field name/type of events with multiple probes

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

tracing/probes: Fix use-after-free on field name/type of events with multiple probes

The fields of a probe-based dynamic event (kprobe, uprobe, eprobe and
fprobe events) are created in traceprobe_define_arg_fields() by handing
the probe_arg name/type strings to trace_define_field(), which only
stores the pointers without copying. Those strings are owned by the
trace_probe and are freed when that probe is removed.

An event can have several probes attached. The field list is defined
only once, by the first probe that registers the event, but it is kept
alive by any surviving sibling probe. Deleting just that first probe by
symbol -

# primary A: fields are defined from A's args
echo 'p:kprobes/ev vfs_read a1=$arg1' > kprobe_events
# append B: shares A's event call
echo 'p:kprobes/ev vfs_write a1=$arg1' >> kprobe_events
# delete only A (matched by symbol), B survives
echo '-:kprobes/ev vfs_read' >> kprobe_events

frees A's args (trace_probe_cleanup() -> traceprobe_free_probe_arg()),
but trace_probe_unlink() keeps the trace_probe_event because the probe
list is not empty. The event call stays registered via B while its
fields now reference freed memory. Any field lookup then reads it, e.g.

echo 'a1 == 1' > events/kprobes/ev/filter

BUG: KASAN: slab-use-after-free in strcmp+0xa7/0xb0
Call Trace:
strcmp
trace_find_event_field
parse_pred
process_preds
create_filter
apply_event_filter
event_filter_write

field->name references parg->name (kstrdup'd, freed with the probe) and,
for array arguments, field->type references parg->fmt (kmalloc'd, freed
with the probe) - the scalar type otherwise points at the static
fmttype rodata, which is safe.

Have traceprobe_define_arg_fields() duplicate the name and type strings
and anchor the copies on the trace_probe_event, which embeds the event
call and outlives every individual probe; trace_probe_event_free()
releases them.

The reproducer above triggers reliably; the field lookup and the delete
both run under event_mutex, so this is a dangling reference after
removal rather than a race.

The issue was found by the autokbug dynamic kernel fuzzer at Tencent
Yunding Lab.