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

[Jesper Dangaard Brouer <jbrouer@redhat.com>]

On 09/07/2021 16.34, Alexander Duyck wrote:
> 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.
> 

The skb->cloned and skb->pp_recycle (bitfields) are on the same 
cache-line (incl. nohdr, destructor, active_extensions).  Thus, we know 
this must be in CPUs cache, regardless of this change.  I do acknowledge 
that it might be in cache coherency "Shared" state, and writing 
skb->pp_recycle=0 the CPU *might* have to change the cache coherency 
state, but I don't expect this to be a performance problem.

> Otherwise this looks good to me.
> 
> Reviewed-by: Alexander Duyck <alexanderduyck@fb.com>
  Acked-by: Jesper Dangaard Brouer <brouer@redhat.com>

I've gone over the code-path, with Ilias on IRC and I've convinced 
myself that this fix is correct, thus ACK.