[PATCH] net: check if protocol extracted by virtio_net_hdr_set_proto is correct

[Balazs Nemeth]

For gso packets, virtio_net_hdr_set_proto sets the protocol (if it isn't
set) based on the type in the virtio net hdr, but the skb could contain
anything since it could come from packet_snd through a raw socket. If
there is a mismatch between what virtio_net_hdr_set_proto sets and
the actual protocol, then the skb could be handled incorrectly later
on by gso.

The network header of gso packets starts at 14 bytes, but a specially
crafted packet could fool the call to skb_flow_dissect_flow_keys_basic
as the network header offset in the skb could be incorrect.
Consequently, EINVAL is not returned.

There are even packets that can cause an infinite loop. For example, a
packet with ethernet type ETH_P_MPLS_UC (which is unnoticed by
virtio_net_hdr_to_skb) that is sent to a geneve interface will be
handled by geneve_build_skb. In turn, it calls
udp_tunnel_handle_offloads which then calls skb_reset_inner_headers.
After that, the packet gets passed to mpls_gso_segment. That function
calculates the mpls header length by taking the difference between
network_header and inner_network_header. Since the two are equal
(due to the earlier call to skb_reset_inner_headers), it will calculate
a header of length 0, and it will not pull any headers. Then, it will
call skb_mac_gso_segment which will again call mpls_gso_segment, etc...
This leads to the infinite loop.

For that reason, address the root cause of the issue: don't blindly
trust the information provided by the virtio net header. Instead,
check if the protocol in the packet actually matches the protocol set by
virtio_net_hdr_set_proto.

Fixes: 9274124f023b ("net: stricter validation of untrusted gso packets")
Signed-off-by: Balazs Nemeth <bnemeth@xxxxxxxxxx>
---
 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 e8a924eeea3d..cf2c53563f22 100644
--- a/include/linux/virtio_net.h
+++ b/include/linux/virtio_net.h
@@ -79,8 +79,13 @@ static inline int virtio_net_hdr_to_skb(struct sk_buff *skb,
 		if (gso_type && skb->network_header) {
 			struct flow_keys_basic keys;
 
-			if (!skb->protocol)
+			if (!skb->protocol) {
+				const struct ethhdr *eth = skb_eth_hdr(skb);
+
 				virtio_net_hdr_set_proto(skb, hdr);
+				if (skb->protocol != eth->h_proto)
+					return -EINVAL;
+			}
 retry:
 			if (!skb_flow_dissect_flow_keys_basic(NULL, skb, &keys,
 							      NULL, 0, 0, 0,
-- 
2.29.2