rtnl_fdb_dump() failed to check the result of nlmsg_parse(), which led
to contents of |ifm| being uninitialized because nlh->nlmsglen was too
small to accommodate |ifm|. The uninitialized data may affect some
branches and result in unwanted effects, although kernel data doesn't
seem to leak to the userspace directly.

The bug has been detected with KMSAN and syzkaller.

For the record, here is the KMSAN report:

==================================================================
BUG: KMSAN: use of unitialized memory in rtnl_fdb_dump+0x5dc/0x1000
CPU: 0 PID: 1039 Comm: probe Not tainted 4.11.0-rc5+ #2727
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Bochs 01/01/2011
Call Trace:
 __dump_stack lib/dump_stack.c:16
 dump_stack+0x143/0x1b0 lib/dump_stack.c:52
 kmsan_report+0x12a/0x180 mm/kmsan/kmsan.c:1007
 __kmsan_warning_32+0x66/0xb0 mm/kmsan/kmsan_instr.c:491
 rtnl_fdb_dump+0x5dc/0x1000 net/core/rtnetlink.c:3230
 netlink_dump+0x84f/0x1190 net/netlink/af_netlink.c:2168
 __netlink_dump_start+0xc97/0xe50 net/netlink/af_netlink.c:2258
 netlink_dump_start ./include/linux/netlink.h:165
 rtnetlink_rcv_msg+0xae9/0xb40 net/core/rtnetlink.c:4094
 netlink_rcv_skb+0x339/0x5a0 net/netlink/af_netlink.c:2339
 rtnetlink_rcv+0x83/0xa0 net/core/rtnetlink.c:4110
 netlink_unicast_kernel net/netlink/af_netlink.c:1272
 netlink_unicast+0x13b7/0x1480 net/netlink/af_netlink.c:1298
 netlink_sendmsg+0x10b8/0x10f0 net/netlink/af_netlink.c:1844
 sock_sendmsg_nosec net/socket.c:633
 sock_sendmsg net/socket.c:643
 ___sys_sendmsg+0xd4b/0x10f0 net/socket.c:1997
 __sys_sendmsg net/socket.c:2031
 SYSC_sendmsg+0x2c6/0x3f0 net/socket.c:2042
 SyS_sendmsg+0x87/0xb0 net/socket.c:2038
 do_syscall_64+0x102/0x150 arch/x86/entry/common.c:285
 entry_SYSCALL64_slow_path+0x25/0x25 arch/x86/entry/entry_64.S:246
RIP: 0033:0x401300
RSP: 002b:00007ffc3b0e6d58 EFLAGS: 00000246 ORIG_RAX: 000000000000002e
RAX: ffffffffffffffda RBX: 00000000004002b0 RCX: 0000000000401300
RDX: 0000000000000000 RSI: 00007ffc3b0e6d80 RDI: 0000000000000003
RBP: 00007ffc3b0e6e00 R08: 000000000000000b R09: 0000000000000004
R10: 000000000000000d R11: 0000000000000246 R12: 0000000000000000
R13: 00000000004065a0 R14: 0000000000406630 R15: 0000000000000000
origin: 000000008fe00056
 save_stack_trace+0x59/0x60 arch/x86/kernel/stacktrace.c:59
 kmsan_save_stack_with_flags mm/kmsan/kmsan.c:352
 kmsan_internal_poison_shadow+0xb1/0x1a0 mm/kmsan/kmsan.c:247
 kmsan_poison_shadow+0x6d/0xc0 mm/kmsan/kmsan.c:260
 slab_alloc_node mm/slub.c:2743
 __kmalloc_node_track_caller+0x1f4/0x390 mm/slub.c:4349
 __kmalloc_reserve net/core/skbuff.c:138
 __alloc_skb+0x2cd/0x740 net/core/skbuff.c:231
 alloc_skb ./include/linux/skbuff.h:933
 netlink_alloc_large_skb net/netlink/af_netlink.c:1144
 netlink_sendmsg+0x934/0x10f0 net/netlink/af_netlink.c:1819
 sock_sendmsg_nosec net/socket.c:633
 sock_sendmsg net/socket.c:643
 ___sys_sendmsg+0xd4b/0x10f0 net/socket.c:1997
 __sys_sendmsg net/socket.c:2031
 SYSC_sendmsg+0x2c6/0x3f0 net/socket.c:2042
 SyS_sendmsg+0x87/0xb0 net/socket.c:2038
 do_syscall_64+0x102/0x150 arch/x86/entry/common.c:285
 return_from_SYSCALL_64+0x0/0x6a arch/x86/entry/entry_64.S:246
==================================================================

and the reproducer:

==================================================================
  #include <sys/socket.h>
  #include <net/if_arp.h>
  #include <linux/netlink.h>
  #include <stdint.h>

  int main()
  {
    int sock = socket(PF_NETLINK, SOCK_DGRAM | SOCK_NONBLOCK, 0);
    struct msghdr msg;
    memset(&msg, 0, sizeof(msg));
    char nlmsg_buf[32];
    memset(nlmsg_buf, 0, sizeof(nlmsg_buf));
    struct nlmsghdr *nlmsg = nlmsg_buf;
    nlmsg->nlmsg_len = 0x11;
    nlmsg->nlmsg_type = 0x1e; // RTM_NEWROUTE = RTM_BASE + 0x0e
    // type = 0x0e = 1110b
    // kind = 2
    nlmsg->nlmsg_flags = 0x101; // NLM_F_ROOT | NLM_F_REQUEST
    nlmsg->nlmsg_seq = 0;
    nlmsg->nlmsg_pid = 0;
    nlmsg_buf[16] = (char)7;
    struct iovec iov;
    iov.iov_base = nlmsg_buf;
    iov.iov_len = 17;
    msg.msg_iov = &iov;
    msg.msg_iovlen = 1;
    sendmsg(sock, &msg, 0);
    return 0;
  }
==================================================================
Signed-off-by: Alexander Potapenko <glider@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>