After a successful allocation of path_rec, num_paths is set to 1, but any
error after such allocation will leave num_paths uncleared.

This causes to de-referencing a NULL pointer later on. Hence, num_paths
needs to be set back to 0 if such an error occurs.

The following crash from syzkaller revealed it.

  kasan: CONFIG_KASAN_INLINE enabled
  kasan: GPF could be caused by NULL-ptr deref or user memory access
  general protection fault: 0000 [#1] SMP DEBUG_PAGEALLOC KASAN PTI
  CPU: 0 PID: 357 Comm: syz-executor060 Not tainted 4.18.0+ #311
  Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS
  rel-1.11.0-0-g63451fca13-prebuilt.qemu-project.org 04/01/2014
  RIP: 0010:ib_copy_path_rec_to_user+0x94/0x3e0
  Code: f1 f1 f1 f1 c7 40 0c 00 00 f4 f4 65 48 8b 04 25 28 00 00 00 48 89
  45 c8 31 c0 e8 d7 60 24 ff 48 8d 7b 4c 48 89 f8 48 c1 e8 03 <42> 0f b6
  14 30 48 89 f8 83 e0 07 83 c0 03 38 d0 7c 08 84 d2 0f 85
  RSP: 0018:ffff88006586f980 EFLAGS: 00010207
  RAX: 0000000000000009 RBX: 0000000000000000 RCX: 1ffff1000d5fe475
  RDX: ffff8800621e17c0 RSI: ffffffff820d45f9 RDI: 000000000000004c
  RBP: ffff88006586fa50 R08: ffffed000cb0df73 R09: ffffed000cb0df72
  R10: ffff88006586fa70 R11: ffffed000cb0df73 R12: 1ffff1000cb0df30
  R13: ffff88006586fae8 R14: dffffc0000000000 R15: ffff88006aff2200
  FS: 00000000016fc880(0000) GS:ffff88006d000000(0000)
  knlGS:0000000000000000
  CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
  CR2: 0000000020000040 CR3: 0000000063fec000 CR4: 00000000000006b0
  DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
  DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
  Call Trace:
  ? ib_copy_path_rec_from_user+0xcc0/0xcc0
  ? __mutex_unlock_slowpath+0xfc/0x670
  ? wait_for_completion+0x3b0/0x3b0
  ? ucma_query_route+0x818/0xc60
  ucma_query_route+0x818/0xc60
  ? ucma_listen+0x1b0/0x1b0
  ? sched_clock_cpu+0x18/0x1d0
  ? sched_clock_cpu+0x18/0x1d0
  ? ucma_listen+0x1b0/0x1b0
  ? ucma_write+0x292/0x460
  ucma_write+0x292/0x460
  ? ucma_close_id+0x60/0x60
  ? sched_clock_cpu+0x18/0x1d0
  ? sched_clock_cpu+0x18/0x1d0
  __vfs_write+0xf7/0x620
  ? ucma_close_id+0x60/0x60
  ? kernel_read+0x110/0x110
  ? time_hardirqs_on+0x19/0x580
  ? lock_acquire+0x18b/0x3a0
  ? finish_task_switch+0xf3/0x5d0
  ? _raw_spin_unlock_irq+0x29/0x40
  ? _raw_spin_unlock_irq+0x29/0x40
  ? finish_task_switch+0x1be/0x5d0
  ? __switch_to_asm+0x34/0x70
  ? __switch_to_asm+0x40/0x70
  ? security_file_permission+0x172/0x1e0
  vfs_write+0x192/0x460
  ksys_write+0xc6/0x1a0
  ? __ia32_sys_read+0xb0/0xb0
  ? entry_SYSCALL_64_after_hwframe+0x3e/0xbe
  ? do_syscall_64+0x1d/0x470
  do_syscall_64+0x9e/0x470
  entry_SYSCALL_64_after_hwframe+0x49/0xbe

Fixes: 3c86aa70 ("RDMA/cm: Add RDMA CM support for IBoE devices")
Link: https://lore.kernel.org/r/20200318101741.47211-1-leon@kernel.orgSigned-off-by: Avihai Horon <avihaih@mellanox.com>
Reviewed-by: Maor Gottlieb <maorg@mellanox.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>