linux-kernel - Re: [PATCH] perf ordered_events: fix crash in free_dup

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Date:   Tue, 7 Aug 2018 09:20:29 +0200
From:   Jiri Olsa <jolsa@...hat.com>
To:     Stephane Eranian <eranian@...gle.com>
Cc:     linux-kernel@...r.kernel.org, acme@...hat.com,
        peterz@...radead.org, mingo@...e.hu
Subject: Re: [PATCH] perf ordered_events: fix crash in free_dup_event()

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?

> 
> 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

> 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@...gle.com>
> 
> ---
>  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
>