Re: [PATCH] tracing: Fix use-after-free on field name/type of dynamic probe events

[Steven Rostedt <rostedt@goodmis.org>]

On Tue, 25 Aug 2026 19:03:03 +0800
henry martin <bsdhenrymartin@gmail.com> wrote:

> > >
> > > When several probes are appended to the same event, they share the
> > > trace_event_call and its field list, which stays the one defined by
> > > the primary probe. Deleting just the primary probe with  
> >
> > What do you mean by "appended to the same event"? Do you mean eprobes?
> >  
> 
> Not eprobes -- I mean the kprobe multi-probe-per-event feature from the
> Fixes: commit (append_trace_kprobe()): two probes registered under one
> event name with identical arg names/types but different symbols, so they
> share a single trace_event_call. eprobe/uprobe/fprobe are affected too
> only because they all define their fields through the same helper
> (traceprobe_define_arg_fields()), but the reproducer below is plain
> kprobe.
> 
> > Can you post a reproducer for this?  
> 
> Run as root with KASAN, inside the guest:

Does it matter being inside a guest?

> 
>   cd /sys/kernel/tracing
>   # 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
>   # field lookup -> strcmp() on the freed name
>   echo 'a1 == 1' > events/kprobes/ev/filter
> 
>   BUG: KASAN: slab-use-after-free in strcmp+0xa7/0xb0
>    trace_find_event_field
>    parse_pred
>    process_preds
>    create_filter
>    apply_event_filter
>    event_filter_write
> 
>   Freed by: traceprobe_free_probe_arg / trace_probe_cleanup /
>             free_trace_kprobe / ... / dyn_event_release

The above is useful information to include in the change log.

> 
> Deleting A runs trace_probe_cleanup(A), which frees A's args, then
> trace_probe_unlink(A) keeps the trace_probe_event because B is still on
> the probe list. The event survives via B while its fields still point at
> A's freed arg->name (and, for array args, arg->fmt). Both the delete and
> the filter write hold event_mutex, so it is a dangling reference after
> removal, not a race -- it triggers every time.
> 
> >  
> > > "-:group/event symbol" frees the trace_probe and its argument
> > > strings, while the event call is kept registered by the remaining
> > > sibling probes. field->name and field->type are left dangling, and
> > > any field lookup - e.g. writing to events/<grp>/<ev>/filter - reads
> > > freed memory:
> > >
> > >   BUG: KASAN: slab-use-after-free in strcmp+0xa7/0xb0
> > >   Call trace:
> > >    trace_find_event_field+0xd6/0x220
> > >    parse_pred
> > >    process_preds
> > >    create_filter
> > >    apply_event_filter
> > >    event_filter_write
> > >
> > > Make the field own its strings: duplicate name and type with
> > > kstrdup_const() in __trace_define_field() and release them with
> > > kfree_const() in trace_destroy_fields(). Fields of static trace
> > > events still reference their kernel/module rodata string literals
> > > directly, as kstrdup_const()/kfree_const() only touch memory that
> > > was actually allocated.  
> >
> > Wrong fix.
> >  
> > >
> > > The issue was found by the autokbug dynamic kernel fuzzer at Tencent
> > > Yunding Lab.
> > >
> > > Fixes: ca89bc071d5e4 ("tracing/kprobe: Add multi-probe per event  
> support")
> > > Signed-off-by: Henry Martin <bsdhenrymartin@gmail.com>
> > > ---
> > >  kernel/trace/trace_events.c | 14 ++++++++++++--
> > >  1 file changed, 12 insertions(+), 2 deletions(-)
> > >
> > > diff --git a/kernel/trace/trace_events.c b/kernel/trace/trace_events.c
> > > index c01b10b99f67e..ee3b93fa09ee8 100644
> > > --- a/kernel/trace/trace_events.c
> > > +++ b/kernel/trace/trace_events.c  
> >
> > This is a bug with trace_probes.c and not trace_events.c. This should be
> > fixed without touching trace_events.c. That is, the trace_probes.c code
> >  
> (or
> > trace_eprobes.c if it's only affects eprobes) should handle this issue.
> >  
> 
> Agreed, that was the wrong place. v3 keeps trace_define_field() and all
> static events untouched and fixes it where the borrowing happens:
> traceprobe_define_arg_fields() now kstrdup()s the name/type, and the
> copies are owned by the trace_probe_event (which embeds the event call
> and outlives every individual probe), freed in trace_probe_event_free().
> It is one helper plus its teardown, both in trace_probe.c, plus two
> fields on struct trace_probe_event.
> 
> v3 is posted as a reply in this thread, tested with KASAN +

Please post new versions as a separate thread. It helps with tooling.

> kasan_multi_shot. The reproducer above triggers the UAF reliably on the
> unpatched tree; with the patch, deleting the primary probe leaves
> format/filter intact on the surviving event and the report is gone. I
> also checked the array-arg
> case (arr=+0($arg1):u64[2]), where field->type borrows the kmalloc'd
> parg->fmt: the type string survives the primary delete and full teardown
> afterwards shows no double-free.
> 
> Thanks for the review.

I'll look at your other patch.

Thanks,

-- Steve