On Mon, Oct 19, 2026 at 10:22:02AM +0200, Marcin Knap via B4 Relay wrote:
> From: Marcin Knap <[email protected]>
>
> When an XDP program redirects a zero-copy buffer to an XSK and the
> socket's Rx ring is full, xsk_rcv_zc() frees the buffer and returns
> -ENOBUFS to xdp_do_redirect().
>
> A failed xdp_do_redirect() leaves the buffer owned by the caller, and
> drivers rely on that: mlx5e treats the failure as XDP_DROP and releases
Sashiko report shows that drivers are rather inconsistent here
> the buffer when the Rx WQE is refilled, and i40e and ice free it on
> XDP_CONSUMED. The buffer is therefore freed twice. It gets reused for
> another Rx descriptor, the second free puts an in-use buffer back on
> the pool free list, and the same UMEM frame is later handed to user
> space twice.
Did you verify this or is this rather a speculation? xp_free() protects
against double-free cases.
> On mlx5 with striding RQ this also ended in a NULL pointer
> dereference in mlx5e_post_rx_wqes().
Please more info on this then.
>
> Leave the buffer with the caller on both the single-buffer and the
> multi-buffer -ENOBUFS paths, as xsk_rcv() and xsk_rcv_check() already
> do. i40e and ice relied on the core freeing the buffer on their
> need_wakeup -ENOBUFS (XDP_EXIT) path, so make them free it there.
>
> Seen on Mellanox ConnectX-6 DX MT2892 (FW 22.47.1026) with AF_XDP
> zero-copy on 7.3.0-rc6 when the user-space consumer could not keep up
> with the Rx ring. With this change no duplicate frames were seen with
> striding RQ on or off.
Have you thought about massaging mlx driver instead of intel + core?
Even when that would be correct approach ixgbe would miss being addressed.
>
> Fixes: 269009893146 ("xsk: recycle buffer in case Rx queue was full")
> Signed-off-by: Marcin Knap <[email protected]>
> ---
> Tested on mlx5 (ConnectX-6 DX) in zero-copy single-buffer mode.
> The i40e and ice changes and the multi-buffer path are build-tested
> only, as I don't have that hardware available.
> ---
> drivers/net/ethernet/intel/i40e/i40e_xsk.c | 1 +
> drivers/net/ethernet/intel/ice/ice_xsk.c | 1 +
> net/xdp/xsk.c | 15 +++------------
> 3 files changed, 5 insertions(+), 12 deletions(-)
>
> diff --git a/drivers/net/ethernet/intel/i40e/i40e_xsk.c
> b/drivers/net/ethernet/intel/i40e/i40e_xsk.c
> index 9f47388eaba5..d32c7255f0b3 100644
> --- a/drivers/net/ethernet/intel/i40e/i40e_xsk.c
> +++ b/drivers/net/ethernet/intel/i40e/i40e_xsk.c
> @@ -358,6 +358,7 @@ static void i40e_handle_xdp_result_zc(struct i40e_ring
> *rx_ring,
> return;
>
> if (xdp_res == I40E_XDP_EXIT) {
> + xsk_buff_free(xdp_buff);
> *failure = true;
> return;
> }
> diff --git a/drivers/net/ethernet/intel/ice/ice_xsk.c
> b/drivers/net/ethernet/intel/ice/ice_xsk.c
> index 0643017541c3..7c1dd4c87a71 100644
> --- a/drivers/net/ethernet/intel/ice/ice_xsk.c
> +++ b/drivers/net/ethernet/intel/ice/ice_xsk.c
> @@ -636,6 +636,7 @@ int ice_clean_rx_irq_zc(struct ice_rx_ring *rx_ring,
> if (likely(xdp_res & (ICE_XDP_TX | ICE_XDP_REDIR))) {
> xdp_xmit |= xdp_res;
> } else if (xdp_res == ICE_XDP_EXIT) {
> + xsk_buff_free(first);
> failure = true;
> first = NULL;
> break;
> diff --git a/net/xdp/xsk.c b/net/xdp/xsk.c
> index 33475b180ea6..0edc160c660f 100644
> --- a/net/xdp/xsk.c
> +++ b/net/xdp/xsk.c
> @@ -224,21 +224,15 @@ static int xsk_rcv_zc(struct xdp_sock *xs, struct
> xdp_buff *xdp, u32 len)
> struct list_head *xskb_list;
> u32 contd = 0;
> u32 num_desc;
> - int err;
>
> - if (likely(!frags)) {
> - err = __xsk_rcv_zc(xs, xskb, len, contd);
> - if (err)
> - goto err;
> - return 0;
> - }
> + if (likely(!frags))
> + return __xsk_rcv_zc(xs, xskb, len, contd);
>
> contd = XDP_PKT_CONTD;
> num_desc = xdp_get_shared_info_from_buff(xdp)->nr_frags + 1;
> if (xskq_prod_nb_free(xs->rx, num_desc) < num_desc) {
> xs->rx_queue_full++;
> - err = -ENOBUFS;
> - goto err;
> + return -ENOBUFS;
> }
>
> __xsk_rcv_zc_safe(xs, xskb, len, contd);
> @@ -252,9 +246,6 @@ static int xsk_rcv_zc(struct xdp_sock *xs, struct
> xdp_buff *xdp, u32 len)
> }
>
> return 0;
> -err:
> - xsk_buff_free(xdp);
> - return err;
> }
>
> static void *xsk_copy_xdp_start(struct xdp_buff *from)
>
> ---
> base-commit: ff47652a4b66c067c765a7ad464d930b5a9367cc
> change-id: 20261019-xsk-zc-rx-double-free-1a3b059cd41f
>
> Best regards,
> --
> Marcin Knap <[email protected]>
>
>
