On 26/07/2025 21.56, Chenyuan Yang wrote:
The xdp_convert_buff_to_frame() function can return NULL when there is
insufficient headroom in the buffer to store the xdp_frame structure
or when the driver didn't reserve enough tailroom for skb_shared_info.
Currently, the sfc driver does not check for this NULL return value
in the XDP_TX case within efx_do_xdp(). While the efx_xdp_tx_buffers()
function has some defensive checks, passing a NULL xdpf can still lead
to undefined behavior when the function tries to access xdpf->len and
xdpf->data.
The XDP_TX case is only for driver local frames/packets. And Edward says
the sfc driver reserves both enough headroom and tailroom.
Fix by adding a proper NULL check in the XDP_TX case, following the
suggestions of the developers.
In [V1] reply I question if this is possible
- [v1]
https://lore.kernel.org/all/a13646af-78f7-4ba7-9767-41d598222b1d@xxxxxxxxxx/
Hmm... have you actually tested that XDP/BPF can adjust headroom so much
that xdp_convert_buff_to_frame() function fails?
I really doubt this possible for BPF-progs to violate this.
The XDP BPF-prog can only adjust the headroom via the helpers
bpf_xdp_adjust_head() and bpf_xdp_adjust_meta(). These helpers reserve
room for sizeof(struct xdp_frame).
The tailroom can be adjusted via helper bpf_xdp_adjust_tail() and it
also reserve room for sizeof(struct skb_shared_info) such that BPF-progs
cannot get access to this area. See define for xdp_data_hard_end.
Fixes: 1b698fa5d8ef ("xdp: Rename convert_to_xdp_frame in xdp_convert_buff_to_frame")
Signed-off-by: Chenyuan Yang <chenyuan0y@xxxxxxxxx>
Cc: Kunwu Chan <kunwu.chan@xxxxxxxxx>
Cc: Edward Cree <ecree@xxxxxxx>
---
drivers/net/ethernet/sfc/rx.c | 12 +++++++++---
1 file changed, 9 insertions(+), 3 deletions(-)
diff --git a/drivers/net/ethernet/sfc/rx.c b/drivers/net/ethernet/sfc/rx.c
index ffca82207e47..b56457c23f66 100644
--- a/drivers/net/ethernet/sfc/rx.c
+++ b/drivers/net/ethernet/sfc/rx.c
@@ -308,14 +308,20 @@ static bool efx_do_xdp(struct efx_nic *efx, struct efx_channel *channel,
case XDP_TX:
/* Buffer ownership passes to tx on success. */
xdpf = xdp_convert_buff_to_frame(&xdp);
- err = efx_xdp_tx_buffers(efx, 1, &xdpf, true);
+ if (unlikely(!xdpf))
+ err = -ENOBUFS;
+ else
+ err = efx_xdp_tx_buffers(efx, 1, &xdpf, true);
+
if (unlikely(err != 1)) {
efx_free_rx_buffers(rx_queue, rx_buf, 1);
if (net_ratelimit())
netif_err(efx, rx_err, efx->net_dev,
- "XDP TX failed (%d)\n", err);
+ "XDP TX failed (%d)%s\n", err,
+ err == -ENOBUFS ? " [frame conversion]" : "");
channel->n_rx_xdp_bad_drops++;
- trace_xdp_exception(efx->net_dev, xdp_prog, xdp_act);
+ if (err != -ENOBUFS)
+ trace_xdp_exception(efx->net_dev, xdp_prog, xdp_act);
} else {
channel->n_rx_xdp_tx++;
}
