netdev - PROBLEM: skb_clone SMP race?

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Date:	Wed, 10 Oct 2007 11:00:19 +0200
From:	"Santiago Font Arquer" <sfa.linux@...il.com>
To:	netdev@...r.kernel.org
Subject: PROBLEM: skb_clone SMP race?

Hello,
   I'm studying the implementation of sk_buff and I think there's a
possible race condition in skb_clone (2.6.22.9)
   The code is:

struct sk_buff *skb_clone(struct sk_buff *skb, gfp_t gfp_mask)
{
 struct sk_buff *n;

 n = skb + 1;
 if (skb->fclone == SKB_FCLONE_ORIG &&
    n->fclone == SKB_FCLONE_UNAVAILABLE) {
 atomic_t *fclone_ref = (atomic_t *) (n + 1);
 n->fclone = SKB_FCLONE_CLONE;
 atomic_inc(fclone_ref);
 } else {
 n = kmem_cache_alloc(skbuff_head_cache, gfp_mask);
 if (!n)
  return NULL;
 n->fclone = SKB_FCLONE_UNAVAILABLE;
 }

   If an skb with fast clone available (first "if" true) has
references in different CPUs (skb->users>1) (I do not find explicit
checks for this to be impossible), if skb_clone is called
simultaneously over that skb, both callers can get the same clone (the
"fast" clone) and different problems follow: wrong "clone_skb->users"
(1 as expected by the caller, but it should be, to be true, 2),
fclone_ref set to 3 involving further problems, ...

  IMO, the same problem arises although the calls to skb_clone are
not simultaneous: there isn´t a memory barrier after the change of
"n->fclone" to guarantee the visibility of that change to other CPUs
(but that barrier will not solve anything; I mentioned this only to
reflect another reason I see for the race to happen).

  Is that correct? Thank you in advance.

       Santiago Font Arquer
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