rdma_nl_rcv_msg() checks to see if it should use the .dump() callback
or the .doit() callback.  The check is done with this check:

if (flags & NLM_F_DUMP) ...

The NLM_F_DUMP flag is two bits (NLM_F_ROOT | NLM_F_MATCH).

When an RDMA_NL_LS message (response) is received, the bit used for
indicating an error is the same bit as NLM_F_ROOT.

NLM_F_ROOT == (0x100) == RDMA_NL_LS_F_ERR.

ibacm sends a response with the RDMA_NL_LS_F_ERR bit set if an error
occurs in the service.  The current code then misinterprets the
NLM_F_DUMP bit and trys to call the .dump() callback.

If the .dump() callback for the specified request is not available
(which is true for the RDMA_NL_LS messages) the following Oops occurs:

[ 4555.960256] BUG: unable to handle kernel NULL pointer dereference at
   (null)
[ 4555.969046] IP:           (null)
[ 4555.972664] PGD 10543f1067 P4D 10543f1067 PUD 1033f93067 PMD 0
[ 4555.979287] Oops: 0010 [#1] SMP
[ 4555.982809] Modules linked in: rpcrdma ib_isert iscsi_target_mod
target_core_mod ib_iser libiscsi scsi_transport_iscsi ib_ipoib rdma_ucm ib_ucm
ib_uverbs ib_umad rdma_cm ib_cm iw_cm dm_mirror dm_region_hash dm_log dm_mod
dax sb_edac x86_pkg_temp_thermal intel_powerclamp coretemp kvm irqbypass
crct10dif_pclmul crc32_pclmul ghash_clmulni_intel pcbc aesni_intel crypto_simd
glue_helper cryptd hfi1 rdmavt iTCO_wdt iTCO_vendor_support ib_core mei_me
lpc_ich pcspkr mei ioatdma sg shpchp i2c_i801 mfd_core wmi ipmi_si ipmi_devintf
ipmi_msghandler acpi_power_meter acpi_pad nfsd auth_rpcgss nfs_acl lockd grace
sunrpc ip_tables ext4 mbcache jbd2 sd_mod mgag200 drm_kms_helper syscopyarea
sysfillrect sysimgblt fb_sys_fops ttm igb ahci crc32c_intel ptp libahci
pps_core drm dca libata i2c_algo_bit i2c_core
[ 4556.061190] CPU: 54 PID: 9841 Comm: ibacm Tainted: G          I
4.14.0-rc2+ #6
[ 4556.069667] Hardware name: Intel Corporation S2600WT2/S2600WT2, BIOS
SE5C610.86B.01.01.0008.021120151325 02/11/2015
[ 4556.081339] task: ffff880855f42d00 task.stack: ffffc900246b4000
[ 4556.087967] RIP: 0010:          (null)
[ 4556.092166] RSP: 0018:ffffc900246b7bc8 EFLAGS: 00010246
[ 4556.098018] RAX: ffffffff81dbe9e0 RBX: ffff881058bb1000 RCX:
0000000000000000
[ 4556.105997] RDX: 0000000000001100 RSI: ffff881058bb1320 RDI:
ffff881056362000
[ 4556.113984] RBP: ffffc900246b7bf8 R08: 0000000000000ec0 R09:
0000000000001100
[ 4556.121971] R10: ffff8810573a5000 R11: 0000000000000000 R12:
ffff881056362000
[ 4556.129957] R13: 0000000000000ec0 R14: ffff881058bb1320 R15:
0000000000000ec0
[ 4556.137945] FS:  00007fe0ba5a38c0(0000) GS:ffff88105f080000(0000)
knlGS:0000000000000000
[ 4556.147000] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[ 4556.153433] CR2: 0000000000000000 CR3: 0000001056f5d003 CR4:
00000000001606e0
[ 4556.161419] Call Trace:
[ 4556.164167]  ? netlink_dump+0x12c/0x290
[ 4556.168468]  __netlink_dump_start+0x186/0x1f0
[ 4556.173357]  rdma_nl_rcv_msg+0x193/0x1b0 [ib_core]
[ 4556.178724]  rdma_nl_rcv+0xdc/0x130 [ib_core]
[ 4556.183604]  netlink_unicast+0x181/0x240
[ 4556.187998]  netlink_sendmsg+0x2c2/0x3b0
[ 4556.192392]  sock_sendmsg+0x38/0x50
[ 4556.196299]  SYSC_sendto+0x102/0x190
[ 4556.200308]  ? __audit_syscall_entry+0xaf/0x100
[ 4556.205387]  ? syscall_trace_enter+0x1d0/0x2b0
[ 4556.210366]  ? __audit_syscall_exit+0x209/0x290
[ 4556.215442]  SyS_sendto+0xe/0x10
[ 4556.219060]  do_syscall_64+0x67/0x1b0
[ 4556.223165]  entry_SYSCALL64_slow_path+0x25/0x25
[ 4556.228328] RIP: 0033:0x7fe0b9db2a63
[ 4556.232333] RSP: 002b:00007ffc55edc260 EFLAGS: 00000293 ORIG_RAX:
000000000000002c
[ 4556.240808] RAX: ffffffffffffffda RBX: 0000000000000010 RCX:
00007fe0b9db2a63
[ 4556.248796] RDX: 0000000000000010 RSI: 00007ffc55edc280 RDI:
000000000000000d
[ 4556.256782] RBP: 00007ffc55edc670 R08: 00007ffc55edc270 R09:
000000000000000c
[ 4556.265321] R10: 0000000000000000 R11: 0000000000000293 R12:
00007ffc55edc280
[ 4556.273846] R13: 000000000260b400 R14: 000000000000000d R15:
0000000000000001
[ 4556.282368] Code:  Bad RIP value.
[ 4556.286629] RIP:           (null) RSP: ffffc900246b7bc8
[ 4556.293013] CR2: 0000000000000000
[ 4556.297292] ---[ end trace 8d67abcfd10ec209 ]---
[ 4556.305465] Kernel panic - not syncing: Fatal exception
[ 4556.313786] Kernel Offset: disabled
[ 4556.321563] ---[ end Kernel panic - not syncing: Fatal exception
[ 4556.328960] ------------[ cut here ]------------

Special case RDMA_NL_LS response messages to call the appropriate
callback.

Additionally, make sure that the .dump() callback is not NULL
before calling it.

Fixes: 647c75ac ("RDMA/netlink: Convert LS to doit callback")
Reviewed-by: Mike Marciniszyn <mike.marciniszyn@intel.com>
Reviewed-by: Kaike Wan <kaike.wan@intel.com>
Reviewed-by: Alex Estrin <alex.estrin@intel.com>
Signed-off-by: Michael J. Ruhl <michael.j.ruhl@intel.com>
Reviewed-by: Shiraz Saleem <shiraz.saleem@intel.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>