skbuff: Fix a potential race while recycling page_pool packets

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 skb_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: 6a5bcd8 ("page_pool: Allow drivers to hint on SKB recycling")
Reported-by: Alexander Duyck <alexanderduyck@fb.com>
Suggested-by: Alexander Duyck <alexanderduyck@fb.com>
Reviewed-by: Alexander Duyck <alexanderduyck@fb.com>
Acked-by: Jesper Dangaard Brouer <brouer@redhat.com>
Signed-off-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

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 +674,17 @@ static void skb_release_data(struct sk_buff *skb)
 		kfree_skb_list(shinfo->frag_list);
 
 	skb_free_head(skb);
+exit:
+	/* When we clone an SKB we copy the reycling bit. The pp_recycle
+	 * bit is only set on the head though, so in order to avoid races
+	 * while trying to recycle fragments on __skb_frag_unref() we need
+	 * to make one SKB responsible for triggering the recycle path.
+	 * So disable the recycling bit if an SKB is cloned and we have
+	 * additional references to to the fragmented part of the SKB.
+	 * Eventually the last SKB will have the recycling bit set and it's
+	 * dataref set to 0, which will trigger the recycling
+	 */
+	skb->pp_recycle = 0;
 }
 
 /*