Non-IB devices do not have a umad interface and the client_data will be
left set to NULL. In this case calling get_nl_info() will try to kref a
NULL cdev causing a crash:

  general protection fault, probably for non-canonical address 0xdffffc00000000ba: 0000 [#1] PREEMPT SMP KASAN
  KASAN: null-ptr-deref in range [0x00000000000005d0-0x00000000000005d7]
  CPU: 0 PID: 20851 Comm: syz-executor.0 Not tainted 5.6.0-rc2-syzkaller #0
  Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
  RIP: 0010:kobject_get+0x35/0x150 lib/kobject.c:640
  Code: 53 e8 3f b0 8b f9 4d 85 e4 0f 84 a2 00 00 00 e8 31 b0 8b f9 49 8d 7c 24 3c 48 b8 00 00 00 00 00 fc ff df 48 89 fa 48 c1 ea 03 <0f  b6 04 02 48 89 fa
+83 e2 07 38 d0 7f 08 84 c0 0f 85 eb 00 00 00
  RSP: 0018:ffffc9000946f1a0 EFLAGS: 00010203
  RAX: dffffc0000000000 RBX: ffffffff85bdbbb0 RCX: ffffc9000bf22000
  RDX: 00000000000000ba RSI: ffffffff87e9d78f RDI: 00000000000005d4
  RBP: ffffc9000946f1b8 R08: ffff8880581a6440 R09: ffff8880581a6cd0
  R10: fffffbfff154b838 R11: ffffffff8aa5c1c7 R12: 0000000000000598
  R13: 0000000000000000 R14: ffffc9000946f278 R15: ffff88805cb0c4d0
  FS:  00007faa9e8af700(0000) GS:ffff8880ae800000(0000) knlGS:0000000000000000
  CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
  CR2: 0000001b30121000 CR3: 000000004515d000 CR4: 00000000001406f0
  DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
  DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
  Call Trace:
   get_device+0x25/0x40 drivers/base/core.c:2574
   __ib_get_client_nl_info+0x205/0x2e0 drivers/infiniband/core/device.c:1861
   ib_get_client_nl_info+0x35/0x180 drivers/infiniband/core/device.c:1881
   nldev_get_chardev+0x575/0xac0 drivers/infiniband/core/nldev.c:1621
   rdma_nl_rcv_msg drivers/infiniband/core/netlink.c:195 [inline]
   rdma_nl_rcv_skb drivers/infiniband/core/netlink.c:239 [inline]
   rdma_nl_rcv+0x5d9/0x980 drivers/infiniband/core/netlink.c:259
   netlink_unicast_kernel net/netlink/af_netlink.c:1303 [inline]
   netlink_unicast+0x59e/0x7e0 net/netlink/af_netlink.c:1329
   netlink_sendmsg+0x91c/0xea0 net/netlink/af_netlink.c:1918
   sock_sendmsg_nosec net/socket.c:652 [inline]
   sock_sendmsg+0xd7/0x130 net/socket.c:672
   ____sys_sendmsg+0x753/0x880 net/socket.c:2343
   ___sys_sendmsg+0x100/0x170 net/socket.c:2397
   __sys_sendmsg+0x105/0x1d0 net/socket.c:2430
   __do_sys_sendmsg net/socket.c:2439 [inline]
   __se_sys_sendmsg net/socket.c:2437 [inline]
   __x64_sys_sendmsg+0x78/0xb0 net/socket.c:2437
   do_syscall_64+0xfa/0x790 arch/x86/entry/common.c:294
   entry_SYSCALL_64_after_hwframe+0x49/0xbe

Cc: stable@kernel.org
Fixes: 8f71bb00 ("RDMA: Report available cdevs through RDMA_NLDEV_CMD_GET_CHARDEV")
Link: https://lore.kernel.org/r/20200310075339.238090-1-leon@kernel.org
Reported-by: syzbot+46fe08363dbba223dec5@syzkaller.appspotmail.com
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>