linux-kernel - [PATCH 1/1 v2] skbuff: Fix a potential race while recycling page

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Date:   Fri,  9 Jul 2021 09:29:39 +0300
From:   Ilias Apalodimas <ilias.apalodimas@...aro.org>
To:     netdev@...r.kernel.org
Cc:     Ilias Apalodimas <ilias.apalodimas@...aro.org>,
        Alexander Duyck <alexanderduyck@...com>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Alexander Lobakin <alobakin@...me>,
        Jonathan Lemon <jonathan.lemon@...il.com>,
        Willem de Bruijn <willemb@...gle.com>,
        Miaohe Lin <linmiaohe@...wei.com>,
        Guillaume Nault <gnault@...hat.com>,
        Cong Wang <cong.wang@...edance.com>,
        Jesper Dangaard Brouer <brouer@...hat.com>,
        Matteo Croce <mcroce@...rosoft.com>,
        linux-kernel@...r.kernel.org
Subject: [PATCH 1/1 v2] 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 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@...com>
Suggested-by: Alexander Duyck <alexanderduyck@...com>
Signed-off-by: Ilias Apalodimas <ilias.apalodimas@...aro.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