On Thu, 20 Jun 2024 06:19:01 -0400, "Michael S. Tsirkin" [off-list ref] wrote:

On Thu, Jun 20, 2024 at 05:28:48PM +0800, Heng Qi wrote:

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On Thu, 20 Jun 2024 16:33:35 +0800, Jason Wang [off-list ref] wrote:

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On Tue, Jun 18, 2024 at 11:17 AM Heng Qi [off-list ref] wrote:

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On Tue, 18 Jun 2024 11:10:26 +0800, Jason Wang [off-list ref] wrote:

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On Mon, Jun 17, 2024 at 9:15 PM Heng Qi [off-list ref] wrote:

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The XDP program can't correctly handle partially checksummed
packets, but works fine with fully checksummed packets.

Not sure this is ture, if I was not wrong, XDP can try to calculate checksum.

XDP's interface serves a full checksum,

What do you mean by "serve" here? I mean, XDP can calculate the
checksum and fill it in the packet by itself.

Yes, XDP can parse and calculate checksums for all packets.
However, the bpf_csum_diff and bpf_l4_csum_replace APIs provided by XDP assume
that the packets being processed are fully checksumed packets. That is,
after the XDP program modified the packets, the incremental checksum can be
calculated (for example, samples/bpf/tcbpf1_kern.c, samples/bpf/test_lwt_bpf.c).

Therefore, partially checksummed packets cannot be processed normally in these
examples and need to be discarded.

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and this is why we disabled the
offloading of VIRTIO_NET_F_GUEST_CSUM when loading XDP.

If we trust the device to disable VIRTIO_NET_F_GUEST_CSUM, any reason
to check VIRTIO_NET_HDR_F_NEEDS_CSUM again in the receive path?

There doesn't seem to be a mandatory constraint in the spec that devices that
haven't negotiated VIRTIO_NET_F_GUEST_CSUM cannot set NEEDS_CSUM bit, so I check this.

Thanks.

The spec says:

\item If the VIRTIO_NET_F_GUEST_CSUM feature was negotiated, the
  VIRTIO_NET_HDR_F_NEEDS_CSUM bit in \field{flags} can be
  set: if so, the packet checksum at offset \field{csum_offset} 
  from \field{csum_start} and any preceding checksums
  have been validated.  The checksum on the packet is incomplete and
  if bit VIRTIO_NET_HDR_F_RSC_INFO is not set in \field{flags},
  then \field{csum_start} and \field{csum_offset} indicate how to calculate it
  (see Packet Transmission point 1).


So yes, NEEDS_CSUM without VIRTIO_NET_F_GUEST_CSUM is at best undefined.
Please do not try to use it unless VIRTIO_NET_F_GUEST_CSUM is set.

I've seen it before, but thought something like
 "The device MUST NOT set the NEEDS_CSUM bit if GUEST_CSUM has not been negotiated"
would be clearer.

Furthermore, it is still possible for a malicious device to set the bit.

Thanks.

And if you want to be flexible, ignore it unless VIRTIO_NET_F_GUEST_CSUM
has been negotiated.

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Thanks.

Thanks

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Thanks

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If the
device has already validated fully checksummed packets, then
the driver doesn't need to re-validate them, saving CPU resources.

Additionally, the driver does not drop all partially checksummed
packets when VIRTIO_NET_F_GUEST_CSUM is not negotiated. This is
not a bug, as the driver has always done this.

Fixes: 436c9453a1ac ("virtio-net: keep vnet header zeroed after processing XDP")
Signed-off-by: Heng Qi <redacted>
---
 drivers/net/virtio_net.c | 20 +++++++++++++++++++-
 1 file changed, 19 insertions(+), 1 deletion(-)

diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
index aa70a7ed8072..ea10db9a09fa 100644
--- a/drivers/net/virtio_net.c
+++ b/drivers/net/virtio_net.c

@@ -1360,6 +1360,10 @@ static struct sk_buff *receive_small_xdp(struct net_device *dev,
        if (unlikely(hdr->hdr.gso_type))
                goto err_xdp;

+       /* Partially checksummed packets must be dropped. */
+       if (unlikely(hdr->hdr.flags & VIRTIO_NET_HDR_F_NEEDS_CSUM))
+               goto err_xdp;
+
        buflen = SKB_DATA_ALIGN(GOOD_PACKET_LEN + headroom) +
                SKB_DATA_ALIGN(sizeof(struct skb_shared_info));

@@ -1677,6 +1681,10 @@ static void *mergeable_xdp_get_buf(struct virtnet_info *vi,
        if (unlikely(hdr->hdr.gso_type))
                return NULL;

+       /* Partially checksummed packets must be dropped. */
+       if (unlikely(hdr->hdr.flags & VIRTIO_NET_HDR_F_NEEDS_CSUM))
+               return NULL;
+
        /* Now XDP core assumes frag size is PAGE_SIZE, but buffers
         * with headroom may add hole in truesize, which
         * make their length exceed PAGE_SIZE. So we disabled the

@@ -1943,6 +1951,7 @@ static void receive_buf(struct virtnet_info *vi, struct receive_queue *rq,
        struct net_device *dev = vi->dev;
        struct sk_buff *skb;
        struct virtio_net_common_hdr *hdr;
+       u8 flags;

        if (unlikely(len < vi->hdr_len + ETH_HLEN)) {
                pr_debug("%s: short packet %i\n", dev->name, len);

@@ -1951,6 +1960,15 @@ static void receive_buf(struct virtnet_info *vi, struct receive_queue *rq,
                return;
        }

+       /* 1. Save the flags early, as the XDP program might overwrite them.
+        * These flags ensure packets marked as VIRTIO_NET_HDR_F_DATA_VALID
+        * stay valid after XDP processing.
+        * 2. XDP doesn't work with partially checksummed packets (refer to
+        * virtnet_xdp_set()), so packets marked as
+        * VIRTIO_NET_HDR_F_NEEDS_CSUM get dropped during XDP processing.
+        */
+       flags = ((struct virtio_net_common_hdr *)buf)->hdr.flags;
+
        if (vi->mergeable_rx_bufs)
                skb = receive_mergeable(dev, vi, rq, buf, ctx, len, xdp_xmit,
                                        stats);

@@ -1966,7 +1984,7 @@ static void receive_buf(struct virtnet_info *vi, struct receive_queue *rq,
        if (dev->features & NETIF_F_RXHASH && vi->has_rss_hash_report)
                virtio_skb_set_hash(&hdr->hash_v1_hdr, skb);

-       if (hdr->hdr.flags & VIRTIO_NET_HDR_F_DATA_VALID)
+       if (flags & VIRTIO_NET_HDR_F_DATA_VALID)
                skb->ip_summed = CHECKSUM_UNNECESSARY;

        if (virtio_net_hdr_to_skb(skb, &hdr->hdr,
--

2.32.0.3.g01195cf9f