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

[Ilias Apalodimas <ilias.apalodimas@linaro.org>]

On Thu, 15 Jul 2021 at 13:48, Yunsheng Lin <linyunsheng@huawei.com> wrote:
>
> On 2021/7/15 18:38, Ilias Apalodimas wrote:
> > On Thu, 15 Jul 2021 at 13:00, Ilias Apalodimas
> > <ilias.apalodimas@linaro.org> wrote:
> >>
> >> On Thu, 15 Jul 2021 at 07:01, Yunsheng Lin <linyunsheng@huawei.com> wrote:
> >>>
> >>> On 2021/7/9 14:29, Ilias Apalodimas 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;
> >>>
> >>> Is it possible this patch may break the head frag page for the original skb,
> >>> supposing it's head frag page is from the page pool and below change clears
> >>> the pp_recycle for original skb, causing a page leaking for the page pool?
> >>>
> >>
> >> So this would leak eventually dma mapping if the skb is cloned (and
> >> the dataref is now +1) and we are freeing the original skb first?
> >>
> >
> > Apologies for the noise, my description was not complete.
> > The case you are thinking is clone an SKB and then expand the original?
>
> Yes.
> It seems we might need different pp_recycle bit for head frag and data frag.

We could just reset the pp_recycle flag on pskb_carve_inside_header,
pskb_expand_header and pskb_carve_inside_nonlinear which were the
three functions that might trigger the race to begin with.  The point
on adding it on skb_release_data was to have a catch all for all
future cases ...
Let me stare at itt a bit more in case I can come up with something better

Thanks
/Ilias
>
> >
> > thanks
> > /Ilias
> >
> >
> >>>>
> >>>>       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;
> >>>>  }
> >>>>
> >>>>  /*
> >>>>
> > .
> >