proc_exit_connector() uses ->real_parent lockless. This is not
safe that its parent can go away at any moment, so use RCU to
protect it, and ensure that this task is not released.

[  747.624551] ==================================================================
[  747.632946] BUG: KASAN: use-after-free in proc_exit_connector+0x1f7/0x310
[  747.640686] Read of size 4 at addr ffff88a0276988e0 by task sshd/2882
[  747.648032]
[  747.649804] CPU: 11 PID: 2882 Comm: sshd Tainted: G            E     4.19.26-rc2 #11
[  747.658629] Hardware name: IBM x3550M4 -[7914OFV]-/00AM544, BIOS -[D7E142BUS-1.71]- 07/31/2014
[  747.668419] Call Trace:
[  747.671269]  dump_stack+0xf0/0x19b
[  747.675186]  ? show_regs_print_info+0x5/0x5
[  747.679988]  ? kmsg_dump_rewind_nolock+0x59/0x59
[  747.685302]  print_address_description+0x6a/0x270
[  747.691162]  kasan_report+0x258/0x380
[  747.695835]  ? proc_exit_connector+0x1f7/0x310
[  747.701402]  proc_exit_connector+0x1f7/0x310
[  747.706767]  ? proc_coredump_connector+0x2d0/0x2d0
[  747.712715]  ? _raw_write_unlock_irq+0x29/0x50
[  747.718270]  ? _raw_write_unlock_irq+0x29/0x50
[  747.723820]  ? ___preempt_schedule+0x16/0x18
[  747.729193]  ? ___preempt_schedule+0x16/0x18
[  747.734574]  do_exit+0xa11/0x14f0
[  747.738880]  ? mm_update_next_owner+0x590/0x590
[  747.744525]  ? debug_show_all_locks+0x3c0/0x3c0
[  747.761448]  ? ktime_get_coarse_real_ts64+0xeb/0x1c0
[  747.767589]  ? lockdep_hardirqs_on+0x1a6/0x290
[  747.773154]  ? check_chain_key+0x139/0x1f0
[  747.778345]  ? check_flags.part.35+0x240/0x240
[  747.783908]  ? __lock_acquire+0x2300/0x2300
[  747.789171]  ? _raw_spin_unlock_irqrestore+0x59/0x70
[  747.795316]  ? _raw_spin_unlock_irqrestore+0x59/0x70
[  747.801457]  ? do_raw_spin_unlock+0x10f/0x1e0
[  747.806914]  ? do_raw_spin_trylock+0x120/0x120
[  747.812481]  ? preempt_count_sub+0x14/0xc0
[  747.817645]  ? _raw_spin_unlock+0x2e/0x50
[  747.822708]  ? __handle_mm_fault+0x12db/0x1fa0
[  747.828367]  ? __pmd_alloc+0x2d0/0x2d0
[  747.833143]  ? check_noncircular+0x50/0x50
[  747.838309]  ? match_held_lock+0x7f/0x340
[  747.843380]  ? check_noncircular+0x50/0x50
[  747.848561]  ? handle_mm_fault+0x21a/0x5f0
[  747.853730]  ? check_flags.part.35+0x240/0x240
[  747.859290]  ? check_chain_key+0x139/0x1f0
[  747.864474]  ? __do_page_fault+0x40f/0x760
[  747.869655]  ? __audit_syscall_entry+0x4b/0x1f0
[  747.875319]  ? syscall_trace_enter+0x1d5/0x7b0
[  747.880877]  ? trace_raw_output_preemptirq_template+0x90/0x90
[  747.887895]  ? trace_raw_output_sys_exit+0x80/0x80
[  747.893860]  ? up_read+0x3b/0x90
[  747.898142]  ? stop_critical_timings+0x260/0x260
[  747.903909]  do_group_exit+0xe0/0x1c0
[  747.908591]  ? __x64_sys_exit+0x30/0x30
[  747.913460]  ? trace_raw_output_preemptirq_template+0x90/0x90
[  747.920485]  ? tracer_hardirqs_on+0x270/0x270
[  747.925956]  __x64_sys_exit_group+0x28/0x30
[  747.931214]  do_syscall_64+0x117/0x400
[  747.935988]  ? syscall_return_slowpath+0x2f0/0x2f0
[  747.941931]  ? trace_hardirqs_off_thunk+0x1a/0x1c
[  747.947788]  ? trace_hardirqs_on_caller+0x1d0/0x1d0
[  747.953838]  ? lockdep_sys_exit+0x16/0x8e
[  747.958915]  ? trace_hardirqs_off_thunk+0x1a/0x1c
[  747.964784]  entry_SYSCALL_64_after_hwframe+0x49/0xbe
[  747.971021] RIP: 0033:0x7f572f154c68
[  747.975606] Code: Bad RIP value.
[  747.979791] RSP: 002b:00007ffed2dfaa58 EFLAGS: 00000246 ORIG_RAX: 00000000000000e7
[  747.989324] RAX: ffffffffffffffda RBX: 00007f572f431840 RCX: 00007f572f154c68
[  747.997910] RDX: 0000000000000001 RSI: 000000000000003c RDI: 0000000000000001
[  748.006495] RBP: 0000000000000001 R08: 00000000000000e7 R09: fffffffffffffee0
[  748.015079] R10: 00007f572f4387e8 R11: 0000000000000246 R12: 00007f572f431840
[  748.023664] R13: 000055a7f90f2c50 R14: 000055a7f96e2310 R15: 000055a7f96e2310
[  748.032287]
[  748.034509] Allocated by task 2300:
[  748.038982]  kasan_kmalloc+0xa0/0xd0
[  748.043562]  kmem_cache_alloc_node+0xf5/0x2e0
[  748.049018]  copy_process+0x1781/0x4790
[  748.053884]  _do_fork+0x166/0x9a0
[  748.058163]  do_syscall_64+0x117/0x400
[  748.062943]  entry_SYSCALL_64_after_hwframe+0x49/0xbe
[  748.069180]
[  748.071405] Freed by task 15395:
[  748.075591]  __kasan_slab_free+0x130/0x180
[  748.080752]  kmem_cache_free+0xc2/0x310
[  748.085619]  free_task+0xea/0x130
[  748.089901]  __put_task_struct+0x177/0x230
[  748.095063]  finish_task_switch+0x51b/0x5d0
[  748.100315]  __schedule+0x506/0xfa0
[  748.104791]  schedule+0xca/0x260
[  748.108978]  futex_wait_queue_me+0x27e/0x420
[  748.114333]  futex_wait+0x251/0x550
[  748.118814]  do_futex+0x75b/0xf80
[  748.123097]  __x64_sys_futex+0x231/0x2a0
[  748.128065]  do_syscall_64+0x117/0x400
[  748.132835]  entry_SYSCALL_64_after_hwframe+0x49/0xbe
[  748.139066]
[  748.141289] The buggy address belongs to the object at ffff88a027698000
[  748.141289]  which belongs to the cache task_struct of size 12160
[  748.156589] The buggy address is located 2272 bytes inside of
[  748.156589]  12160-byte region [ffff88a027698000, ffff88a02769af80)
[  748.171114] The buggy address belongs to the page:
[  748.177055] page:ffffea00809da600 count:1 mapcount:0 mapping:ffff888107d01e00 index:0x0 compound_mapcount: 0
[  748.189136] flags: 0x57ffffc0008100(slab|head)
[  748.194688] raw: 0057ffffc0008100 ffffea00809a3200 0000000300000003 ffff888107d01e00
[  748.204424] raw: 0000000000000000 0000000000020002 00000001ffffffff 0000000000000000
[  748.214146] page dumped because: kasan: bad access detected
[  748.220976]
[  748.223197] Memory state around the buggy address:
[  748.229128]  ffff88a027698780: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
[  748.238271]  ffff88a027698800: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
[  748.247414] >ffff88a027698880: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
[  748.256564]                                                        ^
[  748.264267]  ffff88a027698900: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
[  748.273493]  ffff88a027698980: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
[  748.282630] ==================================================================

Fixes: b086ff87 ("connector: add parent pid and tgid to coredump and exit events")
Signed-off-by: Zhang Yu <zhangyu31@baidu.com>
Signed-off-by: Li RongQing <lirongqing@baidu.com>
Acked-by: Evgeniy Polyakov <zbr@ioremap.net>
Signed-off-by: David S. Miller <davem@davemloft.net>