Willem de Bruijn
2021-Feb-23 14:22 UTC
[PATCH] net: check if protocol extracted by virtio_net_hdr_set_proto is correct
On Tue, Feb 23, 2021 at 8:48 AM Balazs Nemeth <bnemeth at redhat.com> wrote:> > On Mon, 2021-02-22 at 11:39 +0800, Jason Wang wrote: > > > > On 2021/2/19 10:55 ??, Willem de Bruijn wrote: > > > On Fri, Feb 19, 2021 at 3:53 AM Jason Wang <jasowang at redhat.com> > > > wrote: > > > > > > > > On 2021/2/18 11:50 ??, Willem de Bruijn wrote: > > > > > On Thu, Feb 18, 2021 at 10:01 AM Balazs Nemeth < > > > > > bnemeth at redhat.com> wrote: > > > > > > 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. > > > > > > > > I remember kernel will validate dodgy gso packets in gso ops. I > > > > wonder > > > > why not do the check there? The reason is that virtio/TUN is not > > > > the > > > > only source for those packets. > > > It is? All other GSO packets are generated by the stack itself, > > > either > > > locally or through GRO. > > > > > > Something like what has been done in tcp_tso_segment()? > > > > if (skb_gso_ok(skb, features | NETIF_F_GSO_ROBUST)) { > > /* Packet is from an untrusted source, reset > > gso_segs. */ > > > > skb_shinfo(skb)->gso_segs = DIV_ROUND_UP(skb->len, mss); > > > > segs = NULL; > > goto out; > > } > > > > My understanding of the header check logic is that it tries to dealy > > the > > check as much as possible, so for device that has GRO_ROBUST, there's > > even no need to do that. > > > > > > > > > > But indeed some checks are better performed in the GSO layer. Such > > > as > > > likely the 0-byte mpls header length. > > > > > > If we cannot trust virtio_net_hdr.gso_type passed from userspace, > > > then > > > we can also not trust the eth.h_proto coming from the same source. > > > > > > I agree. > > > I'll add a check in the GSO layer as well. > > > > > But > > > it makes sense to require them to be consistent. There is a > > > dev_parse_header_protocol that may return the link layer type in a > > > more generic fashion than casting to skb_eth_hdr. > > > > > > Question remains what to do for the link layer types that do not > > > implement > > > header_ops->parse_protocol, and so we cannot validate the packet's > > > network protocol. Drop will cause false positives, accepts will > > > leave a > > > potential path, just closes it for Ethernet. > > > > > > This might call for multiple fixes, both on first ingest and inside > > > the stack? > > > Given that this is related to dodgy packets and that we can't trust > eth.h_proto, wouldn't it make sense to always drop packets (with > potential false positives), erring on the side of caution, if > header_ops->parse_protocol isn't implemented for the dev in question?Unfortunately, that might break applications somewhere out there.