net: tighten bad gso csum offset check in virtio_net_hdr

The referenced commit drops bad input, but has false positives.
Tighten the check to avoid these.

The check detects illegal checksum offload requests, which produce
csum_start/csum_off beyond end of packet after segmentation.

But it is based on two incorrect assumptions:

1. virtio_net_hdr_to_skb with VIRTIO_NET_HDR_GSO_TCP[46] implies GSO.
True in callers that inject into the tx path, such as tap.
But false in callers that inject into rx, like virtio-net.
Here, the flags indicate GRO, and CHECKSUM_UNNECESSARY or
CHECKSUM_NONE without VIRTIO_NET_HDR_F_NEEDS_CSUM is normal.

2. TSO requires checksum offload, i.e., ip_summed == CHECKSUM_PARTIAL.
False, as tcp[46]_gso_segment will fix up csum_start and offset for
all other ip_summed by calling __tcp_v4_send_check.

Because of 2, we can limit the scope of the fix to virtio_net_hdr
that do try to set these fields, with a bogus value.

Link: https://lore.kernel.org/netdev/20240909094527.GA3048202@port70.net/
Fixes: 89add40066 ("net: drop bad gso csum_start and offset in virtio_net_hdr")
Signed-off-by: Willem de Bruijn <willemb@google.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Cc: stable@vger.kernel.org
Link: https://patch.msgid.link/20240910213553.839926-1-willemdebruijn.kernel@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

include/linux/virtio_net.h

@@ -173,7 +173,8 @@ retry:
 			break;
 		case SKB_GSO_TCPV4:
 		case SKB_GSO_TCPV6:
-			if (skb->csum_offset != offsetof(struct tcphdr, check))
+			if (skb->ip_summed == CHECKSUM_PARTIAL &&
+			    skb->csum_offset != offsetof(struct tcphdr, check))
 				return -EINVAL;
 			break;
 		}