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

[Alexander Duyck]

On Wed, Jul 14, 2021 at 9:02 PM Yunsheng Lin <linyunsheng@xxxxxxxxxx> 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@xxxxxx>
> > Suggested-by: Alexander Duyck <alexanderduyck@xxxxxx>
> > Signed-off-by: Ilias Apalodimas <ilias.apalodimas@xxxxxxxxxx>
> > ---
> > 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?

I don't see how. The assumption here is that when atomic_sub_return
gets down to 0 we will still have an skb with skb->pp_recycle set and
it will flow down and encounter skb_free_head below. All we are doing
is skipping those steps and clearing skb->pp_recycle for all but the
last buffer and the last one to free it will trigger the recycling.

> >
> >       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;

Note the path here. We don't clear skb->pp_recycle for the last buffer
where "dataref == 0" until *AFTER* the head has been freed, and all
clones will have skb->pp_recycle = 1 as long as they are a clone of
the original skb that had it set.