Re: [PATCH] perf ordered_events: fix crash in free_dup_event()

[Stephane Eranian]

On Tue, Aug 7, 2018 at 12:20 AM Jiri Olsa <jolsa@xxxxxxxxxx> wrote:
>
> On Mon, Aug 06, 2018 at 06:23:35PM -0700, Stephane Eranian wrote:
> > Depending on memory allocations, it was possible to get a SEGFAULT in
> > free_dup_event() because the event pointer was bogus:
> >
> > perf[1354]: segfault at ffffffff00000006 ip 00000000004b7fc7
>
> is there any reproducer?
>
The cmdline is simple:
$ perf record -e cycles:pp -o - -a sleep 1 | perf inject -b -i - >/dev/null
I was using v4.13 for my tests and it may be sensitive to compiler.
Was using LLVM.

It may be a compiler related issue. You do not allocate the whole struct.
If the compiler was to do a memcpy() behind your back, you'd be in
troubles.

Adding extra padding before *event was also avoiding the problem.
struct ordered_event {
        u64                     timestamp;
        u64                     file_offset;
        char                   pad[32];   <----- extra padding for debugging
        union perf_event        *event;
        struct list_head        list;
};

> >
> > Initially, I thought it was some double free. But it turns out
> > it looked more like a buffer overrun. Adding padding before
> > the union perf_event field in struct ordered_event avoided the
> > problem. But it was not obvious where this could be coming from
> > given the accesses to the struct, i.e.,  no internal array.
> >
> > Then, it struck me that the following was bogus in __dup_event():
> >
> >  __dup_event(struct ordered_events *oe, union perf_event *event)
> > {
> >    ...
> >    union perf_event *new_event;
> >    new_event = memdup(event, event->header.size);
> >    ...
> > }
> >
> > The problem here is that header.size <= sizeof(*new_event). The code
> > was trying to copy only what was necessary, but then, the allocation
>
> hum, and I think we should continue to do so.. we can't allocate ~4000
> bytes space for 30 bytes sample
>
I understand that but it seems the event field gets overwritten under some
conditions.

> > was also only partial. In other words if the event was not the largest
> > possible for the union, it would not be fully backed by memory, likely
> > causing troubles.
>
> how? using that event allocated space for another type of event?
>
> >
> > This patch fixes the problem by passing the size of the union and not
> > that of the actual event.
>
> I think that's just bypassing the real issue, please share more details
> on how to reproduce this
>
> thanks,
> jirka
>
> >
> > Signed-off-by: Stephane Eranian <eranian@xxxxxxxxxx>
> >
> > ---
> >  tools/perf/util/ordered-events.c | 4 ++--
> >  1 file changed, 2 insertions(+), 2 deletions(-)
> >
> > diff --git a/tools/perf/util/ordered-events.c b/tools/perf/util/ordered-events.c
> > index bad9e0296e9a..a90dbe5df019 100644
> > --- a/tools/perf/util/ordered-events.c
> > +++ b/tools/perf/util/ordered-events.c
> > @@ -66,9 +66,9 @@ static union perf_event *__dup_event(struct ordered_events *oe,
> >       union perf_event *new_event = NULL;
> >
> >       if (oe->cur_alloc_size < oe->max_alloc_size) {
> > -             new_event = memdup(event, event->header.size);
> > +             new_event = memdup(event, sizeof(*event));
> >               if (new_event)
> > -                     oe->cur_alloc_size += event->header.size;
> > +                     oe->cur_alloc_size += sizeof(*event);
> >       }
> >
> >       return new_event;
> > --
> > 2.18.0.597.ga71716f1ad-goog
> >