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

[Jesper Dangaard Brouer]

On 09/07/2021 16.34, Alexander Duyck wrote:
> On Thu, Jul 8, 2021 at 11:30 PM Ilias Apalodimas
> <ilias.apalodimas@xxxxxxxxxx> 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;
> >
> >          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@xxxxxx>
 Acked-by: Jesper Dangaard Brouer <brouer@xxxxxxxxxx>

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