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Subject[PATCH 4.20 169/183] net: avoid false positives in untrusted gso validation
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4.20-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Willem de Bruijn <willemb@google.com>

commit 9e8db5913264d3967b93c765a6a9e464d9c473db upstream.

GSO packets with vnet_hdr must conform to a small set of gso_types.
The below commit uses flow dissection to drop packets that do not.

But it has false positives when the skb is not fully initialized.
Dissection needs skb->protocol and skb->network_header.

Infer skb->protocol from gso_type as the two must agree.
SKB_GSO_UDP can use both ipv4 and ipv6, so try both.

Exclude callers for which network header offset is not known.

Fixes: d5be7f632bad ("net: validate untrusted gso packets without csum offload")
Signed-off-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
include/linux/virtio_net.h | 14 ++++++++++++--
1 file changed, 12 insertions(+), 2 deletions(-)

--- a/include/linux/virtio_net.h
+++ b/include/linux/virtio_net.h
@@ -61,10 +61,20 @@ static inline int virtio_net_hdr_to_skb(
/* gso packets without NEEDS_CSUM do not set transport_offset.
* probe and drop if does not match one of the above types.
*/
- if (gso_type) {
+ if (gso_type && skb->network_header) {
+ if (!skb->protocol)
+ virtio_net_hdr_set_proto(skb, hdr);
+retry:
skb_probe_transport_header(skb, -1);
- if (!skb_transport_header_was_set(skb))
+ if (!skb_transport_header_was_set(skb)) {
+ /* UFO does not specify ipv4 or 6: try both */
+ if (gso_type & SKB_GSO_UDP &&
+ skb->protocol == htons(ETH_P_IP)) {
+ skb->protocol = htons(ETH_P_IPV6);
+ goto retry;
+ }
return -EINVAL;
+ }
}
}


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