On Sat, Mar 13, 2021 at 1:35 AM ishaangandhi <ishaangan...@gmail.com> wrote:
>
> From: Ishaan Gandhi <ishaangan...@gmail.com>
>
> This patch identifies the interface a packet arrived on when sending
> ICMP time exceeded, destination unreachable, and parameter problem
> messages, in accordance with RFC 5837.
>
> It was tested by pinging a machine with a ttl of 1, and observing the
> response in Wireshark.
>
> Changes since v1:
> - Add sysctls, feature is disabled by default
> - Device name is always less than 63, so don't check this
> - MTU is always included in net_device, so don't check its presence
> - Support IPv6 as first class citizen
> - Increment lengths via sizeof operator as opposed to int literals
> - Initialize more local variables with defaults
>
> Change request still unaddressed:
> - Willem pointed out yesterday that `skb_iif` is not guaranteed
> to be the iif of the device the packet arrived on. To get around this
> I propose adding a `skb_orig_iif` field to sk_buff which will remain
> unchanged after rounds. Would this be ok?
There is no space to extend struct sk_buff, unfortunately.
Perhaps it is possible to inspect the skb->dev and infer whether that
is a physical (i.e., orig_iif) device.
> +icmp_errors_identify_if - BOOLEAN
> +
> + If set non-zero, then the kernel will append an extension
"If 1, .." see also comment on proc_dointvec_minmax.
> diff --git a/include/net/netns/ipv4.h b/include/net/netns/ipv4.h
> index 9e36738c1fe1..fd68a47f1130 100644
> --- a/include/net/netns/ipv4.h
> +++ b/include/net/netns/ipv4.h
> @@ -90,6 +90,7 @@ struct netns_ipv4 {
> int sysctl_icmp_ratelimit;
> int sysctl_icmp_ratemask;
> int sysctl_icmp_errors_use_inbound_ifaddr;
> + int sysctl_icmp_errors_identify_if;
per-netns is a prime example of why skb_iif is overwritten in each
__netif_receive_skb_core iteration.
In a containerized setting, the response is probably expected to come
from the container device, not the physical nic.
> +/* Appends interface identification object to ICMP packet to identify
> + * the interface on which the original datagram arrived, per RFC 5837.
> + *
> + * Should only be called on the following messages
> + * - ICMPv4 Time Exceeded
> + * - ICMPv4 Destination Unreachable
> + * - ICMPv4 Parameter Problem
> + * - ICMPv6 Time Exceeded
> + * - ICMPv6 Destination Unreachable
> + */
> +
> +void icmp_identify_arrival_interface(struct sk_buff *skb, struct net *net,
> int room,
> + char *icmph, int ip_version)
> +{
> + unsigned int ext_len, orig_len, word_aligned_orig_len, offset,
> extra_space_needed,
> + if_index, mtu = 0, name_len = 0, name_subobj_len = 0;
> + struct interface_ipv4_addr_sub_obj ip4_addr_subobj = {.addr = 0};
> + struct interface_ipv6_addr_sub_obj ip6_addr_subobj;
> + struct icmp_extobj_hdr *iio_hdr;
> + struct inet6_ifaddr ip6_ifaddr;
> + struct inet6_dev *dev6 = NULL;
> + struct icmp_ext_hdr *ext_hdr;
> + char *name = NULL, ctype;
> + struct net_device *dev;
> + void *subobj_offset;
> +
> + if (ip_version != 4 && ip_version != 6) {
> + pr_debug("ip_version must be 4 or 6\n");
> + return;
> + }
always true
> + skb_linearize(skb);
> + if_index = inet_iif(skb);
> + orig_len = skb->len - skb_network_offset(skb);
> +
> + // IPv4 messages MUST be 32-bit aligned, IPv6 64-bit aligned
> + if (ip_version == 4) {
> + word_aligned_orig_len = (orig_len + 3) & ~0x03;
> + icmph[6] = word_aligned_orig_len / 4;
> + } else {
> + word_aligned_orig_len = (orig_len + 7) & ~0x07;
> + icmph[5] = word_aligned_orig_len / 8;
for alignment, see ALIGN
> + }
> +
> + ctype = ICMP_5837_ARRIVAL_ROLE_CTYPE;
> + ext_len = sizeof(struct icmp_ext_hdr) + sizeof(struct
> icmp_extobj_hdr);
> +
> + // Always add if_index to the IIO
> + ext_len += sizeof(__be32);
> + ctype |= ICMP_5837_IF_INDEX_CTYPE;
> +
> + dev = dev_get_by_index(net, if_index);
> + // Try to append IP address, name, and MTU
> + if (dev) {
I think this is always true, but I see the same check in icmp6_send,
so I might be wrong. Good to have as is, then.
> + if (ip_version == 4) {
> + ip4_addr_subobj.addr = inet_select_addr(dev, 0,
> RT_SCOPE_UNIVERSE);
> + if (ip4_addr_subobj.addr) {
> + ip4_addr_subobj.afi = htons(1);
> + ip4_addr_subobj.reserved = 0;
> + ctype |= ICMP_5837_IP_ADDR_CTYPE;
> + ext_len += sizeof(ip4_addr_subobj);
> + }
> + }
> + if (ip_version == 6) {
> + dev6 = in6_dev_get(dev);
> + if (dev6) {
> + ip6_ifaddr =
> *list_last_entry(&dev6->addr_list,
> + struct
> inet6_ifaddr, if_list);
Why is the last entry the right one? Not criticism, I honestly don't
know the order.
> + memcpy(ip6_addr_subobj.addr,
> ip6_ifaddr.addr.s6_addr32,
> + sizeof(ip6_addr_subobj.addr));
> + ip6_addr_subobj.afi = htons(2);
> + ip6_addr_subobj.reserved = 0;
> + ctype |= ICMP_5837_IP_ADDR_CTYPE;
> + ext_len += sizeof(ip6_addr_subobj);
> + }
> + }
> +
> + name = dev->name;
> + if (name) {
always true
> + name_len = strlen(name);
> + // 1 extra byte for the length field
> + name_subobj_len = name_len + 1;
> + // Sub-objects must be aligned to 32-bits
> + name_subobj_len = (name_subobj_len + 3) & ~0x03;
> + ctype |= ICMP_5837_NAME_CTYPE;
> + ext_len += name_subobj_len;
> + }
> +
> + mtu = dev->mtu;
> + ctype |= ICMP_5837_MTU_CTYPE;
> + ext_len += sizeof(__be32);
> + }
> +
> + if (word_aligned_orig_len + ext_len > room) {
> + offset = room - ext_len;
> + extra_space_needed = room - orig_len;
> + } else if (orig_len < ICMP_5837_MIN_ORIG_LEN) {
> + // Original packet must be zero padded to 128 bytes
> + offset = ICMP_5837_MIN_ORIG_LEN;
> + extra_space_needed = offset + ext_len - orig_len;
> + } else {
> + // There is enough room to just add to the end of the packet
> + offset = word_aligned_orig_len;
> + extra_space_needed = ext_len;
> + }
> +
> + if (skb_tailroom(skb) < extra_space_needed) {
> + if (pskb_expand_head(skb, 0, extra_space_needed -
> skb_tailroom(skb), GFP_ATOMIC))
> + return;
> + }
> +
> + // Zero-pad from the end of the original message to the beginning of
> the header
> + if (orig_len < ICMP_5837_MIN_ORIG_LEN) {
> + // Original packet must be zero-padded to 128 bytes
> + memset(skb_network_header(skb) + orig_len, 0,
> ICMP_5837_MIN_ORIG_LEN - orig_len);
> + } else {
> + // Just zero-pad so the original packet is word aligned
> + memset(skb_network_header(skb) + orig_len, 0,
> word_aligned_orig_len - orig_len);
> + }
> +
> + skb_put(skb, extra_space_needed);
> + ext_hdr = (struct icmp_ext_hdr *)(skb_network_header(skb) + offset);
> + iio_hdr = (struct icmp_extobj_hdr *)(ext_hdr + 1);
> + subobj_offset = (void *)(iio_hdr + 1);
> +
> + ext_hdr->reserved1 = 0;
> + ext_hdr->reserved2 = 0;
> + ext_hdr->version = 2;
> + ext_hdr->checksum = 0;
> +
> + iio_hdr->length = htons(ext_len - sizeof(struct icmp_ext_hdr));
> + iio_hdr->class_num = 2;
> + iio_hdr->class_type = ctype;
> +
> + *(__be32 *)subobj_offset = htonl(if_index);
> + subobj_offset += sizeof(__be32);
> +
> + if (ip4_addr_subobj.addr) {
> + *(struct interface_ipv4_addr_sub_obj *)subobj_offset =
> ip4_addr_subobj;
> + subobj_offset += sizeof(ip4_addr_subobj);
> + }
> +
> + if (dev6) {
> + *(struct interface_ipv6_addr_sub_obj *)subobj_offset =
> ip6_addr_subobj;
> + subobj_offset += sizeof(ip6_addr_subobj);
> + }
> +
> + if (name) {
> + *(__u8 *)subobj_offset = name_subobj_len;
> + subobj_offset += sizeof(__u8);
> + memcpy(subobj_offset, name, name_len);
> + memset(subobj_offset + name_len, 0, name_subobj_len -
> (name_len + sizeof(__u8)));
> + subobj_offset += name_subobj_len - sizeof(__u8);
> + }
> +
> + if (mtu) {
> + *(__be32 *)subobj_offset = htonl(mtu);
> + subobj_offset += sizeof(__be32);
> + }
> @@ -628,6 +628,13 @@ static struct ctl_table ipv4_net_table[] = {
> .mode = 0644,
> .proc_handler = proc_dointvec
> },
> + {
> + .procname = "icmp_errors_identify_if",
> + .data =
> &init_net.ipv4.sysctl_icmp_errors_identify_if,
> + .maxlen = sizeof(int),
> + .mode = 0644,
> + .proc_handler = proc_dointvec
procdointvec_minmax with zero and one. This allows future refinements,
if necessary.
See also another ICMP patchset in review:
https://patchwork.kernel.org/project/netdevbpf/list/?series=447825&archive=both&state=*
