Prior to access UCMA commands, the context should be initialized
and connected to CM_ID with ucma_create_id(). In case user skips
this step, he can provide non-valid ctx without CM_ID and cause
to multiple NULL dereferences.

Also there are situations where the create_id can be raced with
other user access, ensure that the context is only shared to
other threads once it is fully initialized to avoid the races.

[  109.088108] BUG: unable to handle kernel NULL pointer dereference at 0000000000000020
[  109.090315] IP: ucma_connect+0x138/0x1d0
[  109.092595] PGD 80000001dc02d067 P4D 80000001dc02d067 PUD 1da9ef067 PMD 0
[  109.095384] Oops: 0000 [#1] SMP KASAN PTI
[  109.097834] CPU: 0 PID: 663 Comm: uclose Tainted: G    B 4.16.0-rc1-00062-g2975d5de #45
[  109.100816] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.11.0-0-g63451fca13-prebuilt.qemu-project.org 04/01/2014
[  109.105943] RIP: 0010:ucma_connect+0x138/0x1d0
[  109.108850] RSP: 0018:ffff8801c8567a80 EFLAGS: 00010246
[  109.111484] RAX: 0000000000000000 RBX: 1ffff100390acf50 RCX: ffffffff9d7812e2
[  109.114496] RDX: 1ffffffff3f507a5 RSI: 0000000000000297 RDI: 0000000000000297
[  109.117490] RBP: ffff8801daa15600 R08: 0000000000000000 R09: ffffed00390aceeb
[  109.120429] R10: 0000000000000001 R11: ffffed00390aceea R12: 0000000000000000
[  109.123318] R13: 0000000000000120 R14: ffff8801de6459c0 R15: 0000000000000118
[  109.126221] FS:  00007fabb68d6700(0000) GS:ffff8801e5c00000(0000) knlGS:0000000000000000
[  109.129468] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[  109.132523] CR2: 0000000000000020 CR3: 00000001d45d8003 CR4: 00000000003606b0
[  109.135573] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
[  109.138716] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
[  109.142057] Call Trace:
[  109.144160]  ? ucma_listen+0x110/0x110
[  109.146386]  ? wake_up_q+0x59/0x90
[  109.148853]  ? futex_wake+0x10b/0x2a0
[  109.151297]  ? save_stack+0x89/0xb0
[  109.153489]  ? _copy_from_user+0x5e/0x90
[  109.155500]  ucma_write+0x174/0x1f0
[  109.157933]  ? ucma_resolve_route+0xf0/0xf0
[  109.160389]  ? __mod_node_page_state+0x1d/0x80
[  109.162706]  __vfs_write+0xc4/0x350
[  109.164911]  ? kernel_read+0xa0/0xa0
[  109.167121]  ? path_openat+0x1b10/0x1b10
[  109.169355]  ? fsnotify+0x899/0x8f0
[  109.171567]  ? fsnotify_unmount_inodes+0x170/0x170
[  109.174145]  ? __fget+0xa8/0xf0
[  109.177110]  vfs_write+0xf7/0x280
[  109.179532]  SyS_write+0xa1/0x120
[  109.181885]  ? SyS_read+0x120/0x120
[  109.184482]  ? compat_start_thread+0x60/0x60
[  109.187124]  ? SyS_read+0x120/0x120
[  109.189548]  do_syscall_64+0xeb/0x250
[  109.192178]  entry_SYSCALL_64_after_hwframe+0x21/0x86
[  109.194725] RIP: 0033:0x7fabb61ebe99
[  109.197040] RSP: 002b:00007fabb68d5e98 EFLAGS: 00000202 ORIG_RAX: 0000000000000001
[  109.200294] RAX: ffffffffffffffda RBX: 0000000000000000 RCX: 00007fabb61ebe99
[  109.203399] RDX: 0000000000000120 RSI: 00000000200001c0 RDI: 0000000000000004
[  109.206548] RBP: 00007fabb68d5ec0 R08: 0000000000000000 R09: 0000000000000000
[  109.209902] R10: 0000000000000000 R11: 0000000000000202 R12: 00007fabb68d5fc0
[  109.213327] R13: 0000000000000000 R14: 00007fff40ab2430 R15: 00007fabb68d69c0
[  109.216613] Code: 88 44 24 2c 0f b6 84 24 6e 01 00 00 88 44 24 2d 0f
b6 84 24 69 01 00 00 88 44 24 2e 8b 44 24 60 89 44 24 30 e8 da f6 06 ff
31 c0 <66> 41 83 7c 24 20 1b 75 04 8b 44 24 64 48 8d 74 24 20 4c 89 e7
[  109.223602] RIP: ucma_connect+0x138/0x1d0 RSP: ffff8801c8567a80
[  109.226256] CR2: 0000000000000020

Fixes: 75216638 ("RDMA/cma: Export rdma cm interface to userspace")
Reported-by: <syzbot+36712f50b0552615bf59@syzkaller.appspotmail.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>