[PATCH] skbuff: Fix a potential race while recycling page_pool packets

[Ilias Apalodimas <ilias.apalodimas@linaro.org>]

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 that over to cloned SKBs.
When that cloned SKB gets expanded,  we are creating 2 separate instances
accessing the 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.

Fix this by explicitly making the cloned/expanded SKB not recyclable.
If the original SKB is freed first the pages are released.
If it is released after the clone/expended skb then it can still be
recycled.

Fixes: 6a5bcd84e886 ("page_pool: Allow drivers to hint on SKB recycling")
Reported-by: Alexander Duyck <alexander.h.duyck@linux.intel.com>
Signed-off-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
---
 net/core/skbuff.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/net/core/skbuff.c b/net/core/skbuff.c
index 12aabcda6db2..0cb53c05ed76 100644
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c
@@ -1718,6 +1718,13 @@ int pskb_expand_head(struct sk_buff *skb, int nhead, int ntail,
 	}
 	off = (data + nhead) - skb->head;
 
+	/* If it's a cloned skb we expand with frags attached we must prohibit
+	 * the recycling code from running, otherwise we might trigger a race
+	 * while trying to recycle the fragments from the original and cloned
+	 * skb
+	 */
+	if (skb_cloned(skb))
+		skb->pp_recycle = 0;
 	skb->head     = data;
 	skb->head_frag = 0;
 	skb->data    += off;
-- 
2.32.0.rc0