ipv6: Fix out-of-bound access in fib6_add_rt2node().

In the Linux kernel, the following vulnerability has been resolved:

ipv6: Fix out-of-bound access in fib6_add_rt2node().

syzbot reported out-of-bound read in fib6_add_rt2node(). [0]

When IPv6 route is created with RTA_NH_ID, struct fib6_info
does not have the trailing struct fib6_nh.

The cited commit started to check !iter->fib6_nh->fib_nh_gw_family
to ensure that rt6_qualify_for_ecmp() will return false for iter.

If iter->nh is not NULL, rt6_qualify_for_ecmp() returns false anyway.

Let's check iter->nh before reading iter->fib6_nh and avoid OOB read.

[0]:
BUG: KASAN: slab-out-of-bounds in fib6_add_rt2node+0x349c/0x3500 net/ipv6/ip6_fib.c:1142
Read of size 1 at addr ffff8880384ba6de by task syz.0.18/5500

CPU: 0 UID: 0 PID: 5500 Comm: syz.0.18 Not tainted syzkaller #0 PREEMPT(full)
Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.3-debian-1.16.3-2 04/01/2014
Call Trace:
<TASK>
dump_stack_lvl+0xe8/0x150 lib/dump_stack.c:120
print_address_description mm/kasan/report.c:378 [inline]
print_report+0xba/0x230 mm/kasan/report.c:482
kasan_report+0x117/0x150 mm/kasan/report.c:595
fib6_add_rt2node+0x349c/0x3500 net/ipv6/ip6_fib.c:1142
fib6_add_rt2node_nh net/ipv6/ip6_fib.c:1363 [inline]
fib6_add+0x910/0x18c0 net/ipv6/ip6_fib.c:1531
__ip6_ins_rt net/ipv6/route.c:1351 [inline]
ip6_route_add+0xde/0x1b0 net/ipv6/route.c:3957
inet6_rtm_newroute+0x268/0x19e0 net/ipv6/route.c:5660
rtnetlink_rcv_msg+0x7d5/0xbe0 net/core/rtnetlink.c:6958
netlink_rcv_skb+0x232/0x4b0 net/netlink/af_netlink.c:2550
netlink_unicast_kernel net/netlink/af_netlink.c:1318 [inline]
netlink_unicast+0x80f/0x9b0 net/netlink/af_netlink.c:1344
netlink_sendmsg+0x813/0xb40 net/netlink/af_netlink.c:1894
sock_sendmsg_nosec net/socket.c:727 [inline]
__sock_sendmsg net/socket.c:742 [inline]
____sys_sendmsg+0xa68/0xad0 net/socket.c:2592
___sys_sendmsg+0x2a5/0x360 net/socket.c:2646
__sys_sendmsg net/socket.c:2678 [inline]
__do_sys_sendmsg net/socket.c:2683 [inline]
__se_sys_sendmsg net/socket.c:2681 [inline]
__x64_sys_sendmsg+0x1bd/0x2a0 net/socket.c:2681
do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
do_syscall_64+0xe2/0xf80 arch/x86/entry/syscall_64.c:94
entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7f9316b9aeb9
Code: ff c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 44 00 00 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 c7 c1 e8 ff ff ff f7 d8 64 89 01 48
RSP: 002b:00007ffd8809b678 EFLAGS: 00000246 ORIG_RAX: 000000000000002e
RAX: ffffffffffffffda RBX: 00007f9316e15fa0 RCX: 00007f9316b9aeb9
RDX: 0000000000000000 RSI: 0000200000004380 RDI: 0000000000000003
RBP: 00007f9316c08c1f R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
R13: 00007f9316e15fac R14: 00007f9316e15fa0 R15: 00007f9316e15fa0
</TASK>

Allocated by task 5499:
kasan_save_stack mm/kasan/common.c:57 [inline]
kasan_save_track+0x3e/0x80 mm/kasan/common.c:78
poison_kmalloc_redzone mm/kasan/common.c:398 [inline]
__kasan_kmalloc+0x93/0xb0 mm/kasan/common.c:415
kasan_kmalloc include/linux/kasan.h:263 [inline]
__do_kmalloc_node mm/slub.c:5657 [inline]
__kmalloc_noprof+0x40c/0x7e0 mm/slub.c:5669
kmalloc_noprof include/linux/slab.h:961 [inline]
kzalloc_noprof include/linux/slab.h:1094 [inline]
fib6_info_alloc+0x30/0xf0 net/ipv6/ip6_fib.c:155
ip6_route_info_create+0x142/0x860 net/ipv6/route.c:3820
ip6_route_add+0x49/0x1b0 net/ipv6/route.c:3949
inet6_rtm_newroute+0x268/0x19e0 net/ipv6/route.c:5660
rtnetlink_rcv_msg+0x7d5/0xbe0 net/core/rtnetlink.c:6958
netlink_rcv_skb+0x232/0x4b0 net/netlink/af_netlink.c:2550
netlink_unicast_kernel net/netlink/af_netlink.c:1318 [inline]
netlink_unicast+0x80f/0x9b0 net/netlink/af_netlink.c:1344
netlink_sendmsg+0x813/0xb40 net/netlink/af_netlink.c:1894
sock_sendmsg_nosec net/socket.c:727 [inline]
__sock_sendmsg net/socket.c:742 [inline]
____sys_sendmsg+0xa68/0xad0 net/socket.c:2592
___sys_s
---truncated---