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

On Wed, Jul 14, 2021 at 9:02 PM Yunsheng Lin [off-list ref] wrote:

On 2021/7/9 14:29, Ilias Apalodimas wrote:

quoted

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@fb.com>
Suggested-by: Alexander Duyck <alexanderduyck@fb.com>
Signed-off-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
---
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.

quoted

      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.