Re: [PATCH 1/1 v2] skbuff: Fix a potential race while recycling page_pool packets

[Alexander Duyck <alexander.duyck@gmail.com>]

On Thu, Jul 8, 2021 at 11:30 PM Ilias Apalodimas
<ilias.apalodimas@linaro.org> wrote:
>
> As Alexander points out, when we are trying to recycle a cloned/expanded
> SKB we might trigger a race.  The recycling code relies on the
> pp_recycle bit to trigger,  which we carry over to cloned SKBs.
> If that cloned SKB gets expanded or if we get references to the frags,
> call skbb_release_data() and overwrite skb->head, we are creating separate
> instances accessing the same page frags.  Since the skb_release_data()
> will first try to recycle the frags,  there's a potential race between
> the original and cloned SKB, since both will have the pp_recycle bit set.
>
> Fix this by explicitly those SKBs not recyclable.
> The atomic_sub_return effectively limits us to a single release case,
> and when we are calling skb_release_data we are also releasing the
> option to perform the recycling, or releasing the pages from the page pool.
>
> Fixes: 6a5bcd84e886 ("page_pool: Allow drivers to hint on SKB recycling")
> Reported-by: Alexander Duyck <alexanderduyck@fb.com>
> Suggested-by: Alexander Duyck <alexanderduyck@fb.com>
> Signed-off-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
> ---
> Changes since v1:
> - Set the recycle bit to 0 during skb_release_data instead of the
>   individual fucntions triggering the issue, in order to catch all
>   cases
>  net/core/skbuff.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
>
> diff --git a/net/core/skbuff.c b/net/core/skbuff.c
> index 12aabcda6db2..f91f09a824be 100644
> --- a/net/core/skbuff.c
> +++ b/net/core/skbuff.c
> @@ -663,7 +663,7 @@ static void skb_release_data(struct sk_buff *skb)
>         if (skb->cloned &&
>             atomic_sub_return(skb->nohdr ? (1 << SKB_DATAREF_SHIFT) + 1 : 1,
>                               &shinfo->dataref))
> -               return;
> +               goto exit;
>
>         skb_zcopy_clear(skb, true);
>
> @@ -674,6 +674,8 @@ static void skb_release_data(struct sk_buff *skb)
>                 kfree_skb_list(shinfo->frag_list);
>
>         skb_free_head(skb);
> +exit:
> +       skb->pp_recycle = 0;
>  }
>
>  /*
> --
> 2.32.0.rc0
>

This is probably the cleanest approach with the least amount of
change, but one thing I am concerned with in this approach is that we
end up having to dirty a cacheline that I am not sure is otherwise
touched during skb cleanup. I am not sure if that will be an issue or
not. If it is then an alternative or follow-on patch could move the
pp_recycle flag into the skb_shared_info flags itself and then make
certain that we clear it around the same time we are setting
shinfo->dataref to 1.

Otherwise this looks good to me.

Reviewed-by: Alexander Duyck <alexanderduyck@fb.com>