Commit 2cc3aeb5 authored Jul 16, 2021 by Ilias Apalodimas Committed by David S. Miller Jul 16, 2021

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: 6a5bcd84 ("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>

parent 20192d9c

net/core/skbuff.c

+12 −1

Original line number	Diff line number	Diff line
		@@ -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;
		}

		/*