zebra: reset nexthop pointer in zread of nexthops

We were not resetting the nexthop pointer to NULL for each
new read of a nexthop from the zapi route. On the chance we
get a nexthop that does not have a proper type, we will not
create a new nexthop and update that pointer, thus it still
has the last valid one and will create a group with two
pointers to the same nexthop.

Then when it enters any code that iterates the group, it loops
endlessly.

This was found with zapi fuzzing.

```
0x00007f728891f1c3 in jhash2 (k=<optimized out>, length=<optimized out>, initval=12183506) at lib/jhash.c:138
0x00007f728896d92c in nexthop_hash (nexthop=<optimized out>) at lib/nexthop.c:563
0x00007f7288979ece in nexthop_group_hash (nhg=<optimized out>) at lib/nexthop_group.c:394
0x0000000000621036 in zebra_nhg_hash_key (arg=<optimized out>) at zebra/zebra_nhg.c:356
0x00007f72888ec0e1 in hash_get (hash=<optimized out>, data=0x7ffffb94aef0, alloc_func=0x0) at lib/hash.c:138
0x00007f72888ee118 in hash_lookup (hash=0x7f7288de2f10, data=0x7f728908e7fc) at lib/hash.c:183
0x0000000000626613 in zebra_nhg_find (nhe=0x7ffffb94b080, id=0, nhg=0x6020000032d0, nhg_depends=0x0, vrf_id=<optimized out>,
    afi=<optimized out>, type=<optimized out>) at zebra/zebra_nhg.c:541
0x0000000000625f39 in zebra_nhg_rib_find (id=0, nhg=<optimized out>, rt_afi=AFI_IP) at zebra/zebra_nhg.c:1126
0x000000000065f953 in rib_add_multipath (afi=AFI_IP, safi=<optimized out>, p=0x7ffffb94b370, src_p=0x0, re=0x6070000013d0,
    ng=0x7f728908e7fc) at zebra/zebra_rib.c:2616
0x0000000000768f90 in zread_route_add (client=0x61f000000080, hdr=<optimized out>, msg=<optimized out>, zvrf=<optimized out>)
    at zebra/zapi_msg.c:1596
0x000000000077c135 in zserv_handle_commands (client=<optimized out>, msg=0x61b000000780) at zebra/zapi_msg.c:2636
0x0000000000575e1f in main (argc=<optimized out>, argv=<optimized out>) at zebra/main.c:309
```

```
(gdb) p *nhg->nexthop
$4 = {next = 0x5488e0, prev = 0x5488e0, vrf_id = 16843009, ifindex = 16843009, type = NEXTHOP_TYPE_IFINDEX, flags = 8 '\b', {gate = {ipv4 = {s_addr = 0},
      ipv6 = {__in6_u = {__u6_addr8 = '\000' <repeats 15 times>, __u6_addr16 = {0, 0, 0, 0, 0, 0, 0, 0}, __u6_addr32 = {0, 0, 0, 0}}}},
    bh_type = BLACKHOLE_UNSPEC}, src = {ipv4 = {s_addr = 0}, ipv6 = {__in6_u = {__u6_addr8 = '\000' <repeats 15 times>, __u6_addr16 = {0, 0, 0, 0, 0, 0, 0,
          0}, __u6_addr32 = {0, 0, 0, 0}}}}, rmap_src = {ipv4 = {s_addr = 0}, ipv6 = {__in6_u = {__u6_addr8 = '\000' <repeats 15 times>, __u6_addr16 = {0, 0,
          0, 0, 0, 0, 0, 0}, __u6_addr32 = {0, 0, 0, 0}}}}, resolved = 0x0, rparent = 0x0, nh_label_type = ZEBRA_LSP_NONE, nh_label = 0x0, weight = 1 '\001'}
(gdb) quit

```

Signed-off-by: Stephen Worley <sworley@cumulusnetworks.com>

diff --git a/zebra/zapi_msg.c b/zebra/zapi_msg.c
index c21d00bbe652cd2d44faa0c7e0faffeba2076baa..4fa7a3c164e83285d58da417f1aea45ba73725eb 100644 (file)
--- a/zebra/zapi_msg.c
+++ b/zebra/zapi_msg.c
@@ -1475,6 +1475,8 @@ static void zread_route_add(ZAPI_HANDLER_ARGS)
                api_nh = &api.nexthops[i];
                ifindex_t ifindex = 0;
 
+               nexthop = NULL;
+
                if (IS_ZEBRA_DEBUG_RECV)
                        zlog_debug("nh type %d", api_nh->type);