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

From: Ilias Apalodimas
Date: Fri Jul 09 2021 - 02:29:59 EST

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@xxxxxx>
Suggested-by: Alexander Duyck <alexanderduyck@xxxxxx>
Signed-off-by: Ilias Apalodimas <ilias.apalodimas@xxxxxxxxxx>
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
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,
- return;
+ goto exit;

skb_zcopy_clear(skb, true);

@@ -674,6 +674,8 @@ static void skb_release_data(struct sk_buff *skb)

+ skb->pp_recycle = 0;