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

[Yunsheng Lin <linyunsheng@huawei.com>]

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.

> 
> 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;
>>>>  }
>>>>
>>>>  /*
>>>>
> .
>