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

On 09/07/2021 16.34, Alexander Duyck wrote:

On Thu, Jul 8, 2021 at 11:30 PM Ilias Apalodimas
[off-list ref] 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;

         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@fb.com>

  Acked-by: Jesper Dangaard Brouer [off-list ref]

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