Re: [PATCH] perf: Fix vmalloc ring buffer free function

[Frederic Weisbecker]

2013/2/26 Jiri Olsa <jolsa@xxxxxxxxxx>:
> If we allocate perf ring buffer with the size of single page,
> we will get memory corruption when releasing it. It's caused
> by rb_free_work function (the CONFIG_PERF_USE_VMALLOC option
> variant).
>
> For single page sized ring buffer the page_order is -1 (because
> nr_pages is 0). This needs to be recognized in the rb_free_work
> function and set 'nr' to 0 in this case, so only the user page
> gets freed.
>
> Reported-by: Jan Stancek <jstancek@xxxxxxxxxx>
> Signed-off-by: Jiri Olsa <jolsa@xxxxxxxxxx>
> Cc: Corey Ashford <cjashfor@xxxxxxxxxxxxxxxxxx>
> Cc: Frederic Weisbecker <fweisbec@xxxxxxxxx>
> Cc: Ingo Molnar <mingo@xxxxxxx>
> Cc: Namhyung Kim <namhyung@xxxxxxxxxx>
> Cc: Paul Mackerras <paulus@xxxxxxxxx>
> Cc: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx>
> Cc: Arnaldo Carvalho de Melo <acme@xxxxxxxxxx>
> ---
>  kernel/events/ring_buffer.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/kernel/events/ring_buffer.c b/kernel/events/ring_buffer.c
> index 23cb34f..21159fb 100644
> --- a/kernel/events/ring_buffer.c
> +++ b/kernel/events/ring_buffer.c
> @@ -336,7 +336,8 @@ static void rb_free_work(struct work_struct *work)
>         int i, nr;
>
>         rb = container_of(work, struct ring_buffer, work);
> -       nr = 1 << page_order(rb);
> +       /* -1 if there's only user page */
> +       nr = page_order(rb) == -1 ? 0 : 1 << page_order(rb);

So we allow for 0 sized ring buffer? I'm not entirely convinced this
is a good idea. Besides this above case, perf_output_begin() looks
dangerous in the case of CONFIG_PERF_USE_VMALLOC due to page_order()
being -1. At least rb->nr_pages sould be zero in that case.

>         base = rb->user_page;
>         for (i = 0; i < nr + 1; i++)
> --
> 1.7.11.7
>
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