[PATCH net v2] net/mlx5e: Prevent stale XSK buffer release on refill retry

When an XDP redirect to an AF_XDP socket fails because its RX ring is
full, the XSK core frees the buffer. During the subsequent batched refill
of a legacy cyclic RQ, mlx5e also releases the WQE's XSK buffer before
allocating a replacement. If that refill succeeds only partially, a WQE
left without a replacement retains its old buffer pointer.

The buffer can meanwhile be allocated to another WQE. A later refill
retry can then free the live buffer through the stale pointer and publish
the same UMEM frame twice.

Mark the WQE as released immediately after the driver-side free. The flag
is already cleared when a replacement buffer is assigned, so refill
retries no longer release stale pointers.

The failure is silent and produces no kernel warning or splat. A
standalone legacy cyclic-RQ zero-copy libxsk reproducer, using 64-byte UDP
traffic offered at 12 Mpps, detected it: stock stopped after 2,854,914
packets in 4.094 seconds, with 4,542 xdp_rx_ring_full events and 64
ownership/double-publication errors. With this change it processed
356,904,225 packets in 30 seconds despite 571,405 xdp_rx_ring_full events,
with no ownership or data errors.

Fixes: 3f93f82988bc ("net/mlx5e: RX, Defer page release in legacy rq for better recycling")
Cc: [email protected]
Suggested-by: Daniel Borkmann <[email protected]>
Reviewed-by: Dragos Tatulea <[email protected]>
Signed-off-by: Jerome Tollet <[email protected]>
---
Changes in v2:
- Reorder and tighten the problem statement.
- State explicitly that the failure produces no warning or splat.
- Add Dragos' Reviewed-by tag.

v1: https://lore.kernel.org/netdev/[email protected]/

 drivers/net/ethernet/mellanox/mlx5/core/en_rx.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/drivers/net/ethernet/mellanox/mlx5/core/en_rx.c b/drivers/net/ethernet/mellanox/mlx5/core/en_rx.c
index 6fc6605d2..7967a2737 100644
--- a/drivers/net/ethernet/mellanox/mlx5/core/en_rx.c
+++ b/drivers/net/ethernet/mellanox/mlx5/core/en_rx.c
@@ -410,8 +410,11 @@ static inline void mlx5e_free_rx_wqe(struct mlx5e_rq *rq,
 
 static void mlx5e_xsk_free_rx_wqe(struct mlx5e_wqe_frag_info *wi)
 {
-	if (!(wi->flags & BIT(MLX5E_WQE_FRAG_SKIP_RELEASE)))
-		xsk_buff_free(*wi->xskp);
+	if (wi->flags & BIT(MLX5E_WQE_FRAG_SKIP_RELEASE))
+		return;
+
+	xsk_buff_free(*wi->xskp);
+	wi->flags |= BIT(MLX5E_WQE_FRAG_SKIP_RELEASE);
 }
 
 static void mlx5e_dealloc_rx_wqe(struct mlx5e_rq *rq, u16 ix)
-- 
2.55.0