[PATCH bpf v5 1/2] bpf: Run generic devmap egress prog on private skb

[Sun Jian <sun.jian.kdev@gmail.com>]

Generic XDP devmap multi redirect uses skb_clone() for intermediate
destinations and sends the last destination with the original skb. This
can leave multiple destinations sharing the same packet data.

This becomes visible after generic devmap egress-program support was
added: a devmap egress program may mutate packet data, and another
destination sharing the same data can observe that mutation.

Native XDP broadcast redirect does not have this issue because
xdpf_clone() copies the frame data for each destination. Generic XDP
should provide the same per-destination isolation before running a
devmap egress program.

Fix this by making cloned skbs private before running the generic devmap
egress program. Use skb_copy() instead of skb_unshare() so allocation
failure does not consume the skb and the existing caller error paths keep
their ownership semantics.

Fixes: 2ea5eabaf04a ("bpf: devmap: Implement devmap prog execution for generic XDP")
Suggested-by: Jiayuan Chen <jiayuan.chen@linux.dev>
Suggested-by: Jakub Kicinski <kuba@kernel.org>
Reviewed-by: Toke Høiland-Jørgensen <toke@redhat.com>
Signed-off-by: Sun Jian <sun.jian.kdev@gmail.com>
---
 kernel/bpf/devmap.c | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/kernel/bpf/devmap.c b/kernel/bpf/devmap.c
index cc0a43ebab6b..28bd44591ce4 100644
--- a/kernel/bpf/devmap.c
+++ b/kernel/bpf/devmap.c
@@ -706,6 +706,18 @@ int dev_map_generic_redirect(struct bpf_dtab_netdev *dst, struct sk_buff *skb,
 	if (unlikely(err))
 		return err;
 
+	if (dst->xdp_prog && skb_cloned(skb)) {
+		struct sk_buff *nskb;
+
+		nskb = skb_copy(skb, GFP_ATOMIC);
+		if (!nskb)
+			return -ENOMEM;
+
+		nskb->mac_len = skb->mac_len;
+		consume_skb(skb);
+		skb = nskb;
+	}
+
 	/* Redirect has already succeeded semantically at this point, so we just
 	 * return 0 even if packet is dropped. Helper below takes care of
 	 * freeing skb.
-- 
2.43.0