[PATCH v2 net] net: virtio_net_hdr_to_skb: count transport header in UFO

virtio_net_hdr_to_skb does not set the skb's gso_size and gso_type
correctly for UFO packets received via virtio-net that are a little over
the GSO size. This can lead to problems elsewhere in the networking
stack, e.g. ovs_vport_send dropping over-sized packets if gso_size is
not set.

This is due to the comparison

  if (skb->len - p_off > gso_size)

not properly accounting for the transport layer header.

p_off includes the size of the transport layer header (thlen), so
skb->len - p_off is the size of the TCP/UDP payload.

gso_size is read from the virtio-net header. For UFO, fragmentation
happens at the IP level so does not need to include the UDP header.

Hence the calculation could be comparing a TCP/UDP payload length with
an IP payload length, causing legitimate virtio-net packets to have
lack gso_type/gso_size information.

Example: a UDP packet with payload size 1473 has IP payload size 1481.
If the guest used UFO, it is not fragmented and the virtio-net header's
flags indicate that it is a GSO frame (VIRTIO_NET_HDR_GSO_UDP), with
gso_size = 1480 for an MTU of 1500.  skb->len will be 1515 and p_off
will be 42, so skb->len - p_off = 1473.  Hence the comparison fails, and
shinfo->gso_size and gso_type are not set as they should be.

Instead, add the UDP header length before comparing to gso_size when
using UFO. In this way, it is the size of the IP payload that is
compared to gso_size.

Fixes: 6dd912f8 ("net: check untrusted gso_size at kernel entry")
Signed-off-by: Jonathan Davies <redacted>

---
Changes in v2:
 - refactor to use variable for readability
---
 include/linux/virtio_net.h | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/include/linux/virtio_net.h b/include/linux/virtio_net.h
index b465f8f..04e87f4b 100644
--- a/include/linux/virtio_net.h
+++ b/include/linux/virtio_net.h

@@ -120,10 +120,15 @@ static inline int virtio_net_hdr_to_skb(struct sk_buff *skb,
 
 	if (hdr->gso_type != VIRTIO_NET_HDR_GSO_NONE) {
 		u16 gso_size = __virtio16_to_cpu(little_endian, hdr->gso_size);
+		unsigned int nh_off = p_off;
 		struct skb_shared_info *shinfo = skb_shinfo(skb);
 
+		/* UFO may not include transport header in gso_size. */
+		if (gso_type & SKB_GSO_UDP)
+			nh_off -= thlen;
+
 		/* Too small packets are not really GSO ones. */
-		if (skb->len - p_off > gso_size) {
+		if (skb->len - nh_off > gso_size) {
 			shinfo->gso_size = gso_size;
 			shinfo->gso_type = gso_type;
 

-- 
2.9.3

On Tue, Nov 16, 2021 at 6:43 PM Jonathan Davies
[off-list ref] wrote:

virtio_net_hdr_to_skb does not set the skb's gso_size and gso_type
correctly for UFO packets received via virtio-net that are a little over
the GSO size. This can lead to problems elsewhere in the networking
stack, e.g. ovs_vport_send dropping over-sized packets if gso_size is
not set.

This is due to the comparison

  if (skb->len - p_off > gso_size)

not properly accounting for the transport layer header.

p_off includes the size of the transport layer header (thlen), so
skb->len - p_off is the size of the TCP/UDP payload.

gso_size is read from the virtio-net header. For UFO, fragmentation
happens at the IP level so does not need to include the UDP header.

Hence the calculation could be comparing a TCP/UDP payload length with
an IP payload length, causing legitimate virtio-net packets to have
lack gso_type/gso_size information.

Example: a UDP packet with payload size 1473 has IP payload size 1481.
If the guest used UFO, it is not fragmented and the virtio-net header's
flags indicate that it is a GSO frame (VIRTIO_NET_HDR_GSO_UDP), with
gso_size = 1480 for an MTU of 1500.  skb->len will be 1515 and p_off
will be 42, so skb->len - p_off = 1473.  Hence the comparison fails, and
shinfo->gso_size and gso_type are not set as they should be.

Instead, add the UDP header length before comparing to gso_size when
using UFO. In this way, it is the size of the IP payload that is
compared to gso_size.

Fixes: 6dd912f8 ("net: check untrusted gso_size at kernel entry")
Signed-off-by: Jonathan Davies <redacted>

Reviewed-by: Willem de Bruijn <willemb@google.com>

Subtracting from an unsigned int always has the chance of negative overflow.

This case is safe, as all three paths that lead here have a p_off >= thlen.

I just noticed a more obscure fourth path:

        if (hdr->gso_type != VIRTIO_NET_HDR_GSO_NONE) {
                switch (hdr->gso_type & ~VIRTIO_NET_HDR_GSO_ECN) {

We do not explicitly check against hdr->gso_type ==
VIRTIO_NET_HDR_GSO_ECN. An obviously bogus value. That leaves p_off 0.
But it also leaves th_len 0, so it is safe.

Negative overflow is also safe in this case.

Hello:

This patch was applied to netdev/net.git (master)
by David S. Miller [off-list ref]:

On Tue, 16 Nov 2021 17:42:42 +0000 you wrote:

virtio_net_hdr_to_skb does not set the skb's gso_size and gso_type
correctly for UFO packets received via virtio-net that are a little over
the GSO size. This can lead to problems elsewhere in the networking
stack, e.g. ovs_vport_send dropping over-sized packets if gso_size is
not set.

This is due to the comparison

[...]

Here is the summary with links:
  - [v2,net] net: virtio_net_hdr_to_skb: count transport header in UFO
    https://git.kernel.org/netdev/net/c/cf9acc90c80e

You are awesome, thank you!
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