Re: [Patch net v2] mlx5: fixup checksum for short ethernet frame padding

[Eric Dumazet <edumazet@google.com>]

On Tue, Nov 27, 2018 at 10:10 PM Cong Wang <xiyou.wangcong@gmail.com> wrote:
>
> When an ethernet frame is padded to meet the minimum ethernet frame
> size, the padding octets are not covered by the hardware checksum.
> Fortunately the padding octets are ususally zero's, which don't affect
> checksum. However, we have a switch which pads non-zero octets, this
> causes kernel hardware checksum fault repeatedly.
>
> Prior to commit 88078d98d1bb ("net: pskb_trim_rcsum() and CHECKSUM_COMPLETE are friends"),
> skb checksum is forced to be CHECKSUM_NONE when padding is detected.
> After it, we need to keep skb->csum updated, like what we do for FCS.
>
> The logic is a bit complicated when dealing with both FCS and padding,
> so I wrap it up in a helper function mlx5e_csum_padding().
>
> I tested this patch with RXFCS on and off, it works fine without any
> warning in both cases.
>
> Fixes: 88078d98d1bb ("net: pskb_trim_rcsum() and CHECKSUM_COMPLETE are friends"),
> Cc: Saeed Mahameed <saeedm@mellanox.com>
> Cc: Eric Dumazet <edumazet@google.com>
> Signed-off-by: Cong Wang <xiyou.wangcong@gmail.com>
> ---
>  .../net/ethernet/mellanox/mlx5/core/en_rx.c   | 39 +++++++++++++++++++
>  1 file changed, 39 insertions(+)
>
> diff --git a/drivers/net/ethernet/mellanox/mlx5/core/en_rx.c b/drivers/net/ethernet/mellanox/mlx5/core/en_rx.c
> index 16985ca3248d..9b6bd2b51556 100644
> --- a/drivers/net/ethernet/mellanox/mlx5/core/en_rx.c
> +++ b/drivers/net/ethernet/mellanox/mlx5/core/en_rx.c
> @@ -732,6 +732,37 @@ static u8 get_ip_proto(struct sk_buff *skb, __be16 proto)
>                                             ((struct ipv6hdr *)ip_p)->nexthdr;
>  }
>
> +static void mlx5e_csum_padding(struct sk_buff *skb, int network_depth,
> +                              __be16 proto, bool has_fcs)
> +{
> +       u32 frame_len = has_fcs ? skb->len - ETH_FCS_LEN : skb->len;
> +       u32 pad_offset, pad_len;
> +       void *pad, *ip_p;
> +
> +       /* We only handle short frames here, and we know they are linear. */
> +       if (likely(frame_len > ETH_ZLEN))
> +               return;
> +
> +       ip_p = skb->data + network_depth;
> +       if (proto == htons(ETH_P_IP)) {
> +               struct iphdr *ipv4 = ip_p;
> +
> +               pad_offset =  network_depth + be16_to_cpu(ipv4->tot_len);
> +       } else { /* Either IPv4 or IPv6 here. */
> +               struct ipv6hdr *ipv6 = ip_p;
> +
> +               pad_offset = network_depth + sizeof(struct ipv6hdr) +
> +                            be16_to_cpu(ipv6->payload_len);
> +       }
> +       pad_len = frame_len - pad_offset;
> +       if (pad_len == 0)
> +               return;

Nice packet of death alert.

pad_len can be 0xFFFFFF67  here, if frame_len is smaller than pad_offset.

Really I suggest you set ip_summed to CHECKSUM_NONE, then remove the
initial test ( if (likely(frame_len > ETH_ZLEN)) ...)

Until the firmware is fixed.

Otherwise frames with a wrong checksum and some non zero padding could
potentially
be seen as correct frames. (Probability of 1/65536)

Do not focus on your immediate problem (small packets being padded by
a non malicious entity)

> +
> +       pad = skb->data + pad_offset;
> +       skb->csum = csum_block_add(skb->csum, csum_partial(pad, pad_len, 0),
> +                                  pad_offset);
> +}
> +
>  static inline void mlx5e_handle_csum(struct net_device *netdev,
>                                      struct mlx5_cqe64 *cqe,
>                                      struct mlx5e_rq *rq,
> @@ -772,6 +803,14 @@ static inline void mlx5e_handle_csum(struct net_device *netdev,
>                         skb->csum = csum_block_add(skb->csum,
>                                                    (__force __wsum)mlx5e_get_fcs(skb),
>                                                    skb->len - ETH_FCS_LEN);
> +
> +               /* CQE csum doesn't cover padding octets in short ethernet
> +                * frames. And the pad field is appended prior to calculating
> +                * and appending the FCS field.
> +                */
> +               mlx5e_csum_padding(skb, network_depth, proto,
> +                                  !!(netdev->features & NETIF_F_RXFCS));
> +
>                 stats->csum_complete++;
>                 return;
>         }
> --
> 2.19.1
>