rtnetlink: fix rtnl_fdb_dump() for ndmsg header

Currently, rtnl_fdb_dump() assumes the family header is 'struct ifinfomsg', which is not always true -- 'struct ndmsg' is used by iproute2 ('ip neigh').

The problem is, the function bails out early if nlmsg_parse() fails, which does occur for iproute2 usage of 'struct ndmsg' because the payload length is shorter than the family header alone (as 'struct ifinfomsg' is assumed).

This breaks backward compatibility with userspace -- nothing is sent back.

Some examples with iproute2 and netlink library for go [1]:

1) $ bridge fdb show

    33:33:00:00:00:01 dev ens3 self permanent
    01:00:5e:00:00:01 dev ens3 self permanent
    33:33:ff:15:98:30 dev ens3 self permanent

      This one works, as it uses 'struct ifinfomsg'.

      fdb_show() @ iproute2/bridge/fdb.c
        """
        .n.nlmsg_len = NLMSG_LENGTH(sizeof(struct ifinfomsg)),
        ...
        if (rtnl_dump_request(&rth, RTM_GETNEIGH, [...]
        """

2) $ ip --family bridge neigh
RTNETLINK answers: Invalid argument
Dump terminated

This one fails, as it uses 'struct ndmsg'.

do_show_or_flush() @ iproute2/ip/ipneigh.c
"""
.n.nlmsg_type = RTM_GETNEIGH, .n.nlmsg_len = NLMSG_LENGTH(sizeof(struct ndmsg)), """

3) $ ./neighlist
< no output >

This one fails, as it uses 'struct ndmsg'-based.

neighList() @ netlink/neigh_linux.go
"""
req := h.newNetlinkRequest(unix.RTM_GETNEIGH, [...] msg := Ndmsg{
"""

The actual breakage was introduced by commit 0ff50e83b512 ("net: rtnetlink:
bail out from rtnl_fdb_dump() on parse error"), because nlmsg_parse() fails if the payload length (with the _actual_ family header) is less than the family header length alone (which is assumed, in parameter 'hdrlen'). This is true in the examples above with struct ndmsg, with size and payload length shorter than struct ifinfomsg.

However, that commit just intends to fix something under the assumption the family header is indeed an 'struct ifinfomsg' - by preventing access to the payload as such (via 'ifm' pointer) if the payload length is not sufficient to actually contain it.

The assumption was introduced by commit 5e6d24358799 ("bridge: netlink dump interface at par with brctl"), to support iproute2's 'bridge fdb' command (not 'ip neigh') which indeed uses 'struct ifinfomsg', thus is not broken.

So, in order to unbreak the 'struct ndmsg' family headers and still allow 'struct ifinfomsg' to continue to work, check for the known message sizes used with 'struct ndmsg' in iproute2 (with zero or one attribute which is not used in this function anyway) then do not parse the data as ifinfomsg.

Same examples with this patch applied (or revert/before the original fix):

    $ bridge fdb show
    33:33:00:00:00:01 dev ens3 self permanent
    01:00:5e:00:00:01 dev ens3 self permanent
    33:33:ff:15:98:30 dev ens3 self permanent

    $ ip --family bridge neigh
    dev ens3 lladdr 33:33:00:00:00:01 PERMANENT
    dev ens3 lladdr 01:00:5e:00:00:01 PERMANENT
    dev ens3 lladdr 33:33:ff:15:98:30 PERMANENT

    $ ./neighlist
    netlink.Neigh{LinkIndex:2, Family:7, State:128, Type:0, Flags:2, IP:net.IP(nil), HardwareAddr:net.HardwareAddr{0x33, 0x33, 0x0, 0x0, 0x0, 0x1}, LLIPAddr:net.IP(nil), Vlan:0, VNI:0}
    netlink.Neigh{LinkIndex:2, Family:7, State:128, Type:0, Flags:2, IP:net.IP(nil), HardwareAddr:net.HardwareAddr{0x1, 0x0, 0x5e, 0x0, 0x0, 0x1}, LLIPAddr:net.IP(nil), Vlan:0, VNI:0}
    netlink.Neigh{LinkIndex:2, Family:7, State:128, Type:0, Flags:2, IP:net.IP(nil), HardwareAddr:net.HardwareAddr{0x33, 0x33, 0xff, 0x15, 0x98, 0x30}, LLIPAddr:net.IP(nil), Vlan:0, VNI:0}

Tested on mainline (v4.19-rc6) and net-next (3bd09b05b068).

References:

[1] netlink library for go (test-case)
https://github.com/vishvananda/netlink

$ cat ~/go/src/neighlist/main.go
package main
import ("fmt"; "syscall"; "github.com/vishvananda/netlink") func main() {
neighs, _ := netlink.NeighList(0, syscall.AF_BRIDGE) for _, neigh := range neighs { fmt.Printf("%#v\n", neigh) }
}

$ export GOPATH=~/go
$ go get github.com/vishvananda/netlink
$ go build neighlist
$ ~/go/src/neighlist/neighlist

Thanks to David Ahern for suggestions to improve this patch.

---
v2: Change logic to check msg size for ndmsg with optional attribute.
Thanks: David Ahern <[email protected]>

 net/core/rtnetlink.c | 29 ++++++++++++++++++++---------
 1 file changed, 20 insertions(+), 9 deletions(-)

index 60c928894a78..6633f245fce5 100644
@@ -3744,16 +3744,27 @@ static int rtnl_fdb_dump(struct sk_buff *skb, struct netlink_callback *cb)
 	int err = 0;
 	int fidx = 0;
 
-	err = nlmsg_parse(cb->nlh, sizeof(struct ifinfomsg), tb,
-			  IFLA_MAX, ifla_policy, NULL);
-	if (err < 0) {
-		return -EINVAL;
-	} else if (err == 0) {
-		if (tb[IFLA_MASTER])
-			br_idx = nla_get_u32(tb[IFLA_MASTER]);
-	}
+	/* A hack to preserve kernel<->userspace interface.
+	 * Before Linux v4.12 this code accepted ndmsg since iproute2 v3.3.0.
+	 * However, ndmsg is shorter than ifinfomsg thus nlmsg_parse() bails.
+	 * So, check for ndmsg with an optional u32 attribute (not used here).
+	 * Fortunately these sizes don't conflict with the size of ifinfomsg
+	 * with an optional attribute.
+	 */
+	if (nlmsg_len(cb->nlh) != sizeof(struct ndmsg) &&
+	    (nlmsg_len(cb->nlh) != sizeof(struct ndmsg) +
+	     nla_attr_size(sizeof(u32)))) {
+		err = nlmsg_parse(cb->nlh, sizeof(struct ifinfomsg), tb,
+				  IFLA_MAX, ifla_policy, NULL);
+		if (err < 0) {
+			return -EINVAL;
+		} else if (err == 0) {
+			if (tb[IFLA_MASTER])
+				br_idx = nla_get_u32(tb[IFLA_MASTER]);
+		}
 
-	brport_idx = ifm->ifi_index;
+		brport_idx = ifm->ifi_index;
+	}
 
 	if (br_idx) {
 		br_dev = __dev_get_by_index(net, br_idx);

2.17.1

...

Seems reasonable for 4.19 and back