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[Gavin Guo <gavin.guo@xxxxxxxxxxxxx>]

Reproducer for the bug.

** Attachment added: "kernel_panic_test.sh"
   https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1527643/+attachment/4595998/+files/kernel_panic_test.sh

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https://bugs.launchpad.net/bugs/1527643

Title:
  use after free of task_struct->numa_faults in task_numa_find_cpu

Status in linux package in Ubuntu:
  Triaged

Bug description:
  ==================================================================
  BUG: KASan: use after free in task_numa_find_cpu+0x64c/0x890 at addr ffff880dd393ecd8
  Read of size 8 by task qemu-system-x86/3998900
  =============================================================================
  BUG kmalloc-128 (Tainted: G    B        ): kasan: bad access detected
  -----------------------------------------------------------------------------

  INFO: Allocated in task_numa_fault+0xc1b/0xed0 age=41980 cpu=18 pid=3998890
          __slab_alloc+0x4f8/0x560
          __kmalloc+0x1eb/0x280
          task_numa_fault+0xc1b/0xed0
          do_numa_page+0x192/0x200
          handle_mm_fault+0x808/0x1160
          __do_page_fault+0x218/0x750
          do_page_fault+0x1a/0x70
          page_fault+0x28/0x30
          SyS_poll+0x66/0x1a0
          system_call_fastpath+0x1a/0x1f
  INFO: Freed in task_numa_free+0x1d2/0x200 age=62 cpu=18 pid=0
          __slab_free+0x2ab/0x3f0
          kfree+0x161/0x170
          task_numa_free+0x1d2/0x200
          finish_task_switch+0x1d2/0x210
          __schedule+0x5d4/0xc60
          schedule_preempt_disabled+0x40/0xc0
          cpu_startup_entry+0x2da/0x340
          start_secondary+0x28f/0x360
  INFO: Slab 0xffffea00374e4f00 objects=37 used=17 fp=0xffff880dd393ecb0 flags=0x6ffff0000004080
  INFO: Object 0xffff880dd393ecb0 @offset=11440 fp=0xffff880dd393f700

  Bytes b4 ffff880dd393eca0: 0c 00 00 00 18 00 00 00 af 63 3a 04 01 00 00 00  .........c:.....
  Object ffff880dd393ecb0: 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b  kkkkkkkkkkkkkkkk
  Object ffff880dd393ecc0: 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b  kkkkkkkkkkkkkkkk
  Object ffff880dd393ecd0: 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b  kkkkkkkkkkkkkkkk
  Object ffff880dd393ece0: 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b  kkkkkkkkkkkkkkkk
  Object ffff880dd393ecf0: 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b  kkkkkkkkkkkkkkkk
  Object ffff880dd393ed00: 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b  kkkkkkkkkkkkkkkk
  Object ffff880dd393ed10: 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b  kkkkkkkkkkkkkkkk
  Object ffff880dd393ed20: 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b a5  kkkkkkkkkkkkkkk.
  CPU: 61 PID: 3998900 Comm: qemu-system-x86 Tainted: G    B         3.13.0-65-generic #105
  Hardware name: Supermicro X8QB6/X8QB6, BIOS 2.0c    06/11/2
   ffffea00374e4f00 ffff8816c572b420 ffffffff81a6ce35 ffff88045f00f500
   ffff8816c572b450 ffffffff81244aed ffff88045f00f500 ffffea00374e4f00
   ffff880dd393ecb0 0000000000000012 ffff8816c572b478 ffffffff8124ac36
  Call Trace:
   [<ffffffff81a6ce35>] dump_stack+0x45/0x56
   [<ffffffff81244aed>] print_trailer+0xfd/0x170
   [<ffffffff8124ac36>] object_err+0x36/0x40
   [<ffffffff8124cbf9>] kasan_report_error+0x1e9/0x3a0
   [<ffffffff8124d260>] kasan_report+0x40/0x50
   [<ffffffff810dda7c>] ? task_numa_find_cpu+0x64c/0x890
   [<ffffffff8124bee9>] __asan_load8+0x69/0xa0
   [<ffffffff814f5c38>] ? find_next_bit+0xd8/0x120
   [<ffffffff810dda7c>] task_numa_find_cpu+0x64c/0x890
   [<ffffffff810de16c>] task_numa_migrate+0x4ac/0x7b0
   [<ffffffff810de523>] numa_migrate_preferred+0xb3/0xc0
   [<ffffffff810e0b88>] task_numa_fault+0xb88/0xed0
   [<ffffffff8120ef02>] do_numa_page+0x192/0x200
   [<ffffffff81211038>] handle_mm_fault+0x808/0x1160
   [<ffffffff810d7dbd>] ? sched_clock_cpu+0x10d/0x160
   [<ffffffff81068c52>] ? native_load_tls+0x82/0xa0
   [<ffffffff81a7bd68>] __do_page_fault+0x218/0x750
   [<ffffffff810c2186>] ? hrtimer_try_to_cancel+0x76/0x160
   [<ffffffff81a6f5e7>] ? schedule_hrtimeout_range_clock.part.24+0xf7/0x1c0
   [<ffffffff81a7c2ba>] do_page_fault+0x1a/0x70
   [<ffffffff81a772e8>] page_fault+0x28/0x30
   [<ffffffff8128cbd4>] ? do_sys_poll+0x1c4/0x6d0
   [<ffffffff810e64f6>] ? enqueue_task_fair+0x4b6/0xaa0
   [<ffffffff810233c9>] ? sched_clock+0x9/0x10
   [<ffffffff810cf70a>] ? resched_task+0x7a/0xc0
   [<ffffffff810d0663>] ? check_preempt_curr+0xb3/0x130
   [<ffffffff8128b5c0>] ? poll_select_copy_remaining+0x170/0x170
   [<ffffffff810d3bc0>] ? wake_up_state+0x10/0x20
   [<ffffffff8112a28f>] ? drop_futex_key_refs.isra.14+0x1f/0x90
   [<ffffffff8112d40e>] ? futex_requeue+0x3de/0xba0
   [<ffffffff8112e49e>] ? do_futex+0xbe/0x8f0
   [<ffffffff81022c89>] ? read_tsc+0x9/0x20
   [<ffffffff8111bd9d>] ? ktime_get_ts+0x12d/0x170
   [<ffffffff8108f699>] ? timespec_add_safe+0x59/0xe0
   [<ffffffff8128d1f6>] SyS_poll+0x66/0x1a0
   [<ffffffff81a830dd>] system_call_fastpath+0x1a/0x1f
  Memory state around the buggy address:
   ffff880dd393eb80: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
   ffff880dd393ec00: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
  >ffff880dd393ec80: fc fc fc fc fc fc fb fb fb fb fb fb fb fb fb fb
                                                      ^
   ffff880dd393ed00: fb fb fb fb fb fb fc fc fc fc fc fc fc fc fc fc
   ffff880dd393ed80: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
  ==================================================================

  --------------------------8<--------------------------
  $ addr2line 0xffffffff810dda7c -e usr/lib/debug/boot/vmlinux-3.13.0-65-generic -f -i
  task_numa_compare
  /home/gavin/os/ubuntu-trusty-amd64/kernel/sched/fair.c:1084
  task_numa_find_cpu
  /home/gavin/os/ubuntu-trusty-amd64/kernel/sched/fair.c:1170

  1083                 if (cur->numa_group == env->p->numa_group) {
  1084                         imp = taskimp + task_weight(cur, env->src_nid) -
  1085                               task_weight(cur, env->dst_nid);

  In short, this is the use-after-free bug happens on the
  task_struct->numa_faults which is freed by the task_numa_free called
  by the finish_task_switch when the process is exiting. While the numa
  balance mechanism is triggering the do_numa_page fault and need to
  read the task_struct->numa_faults to determine if the current exiting
  process is needed to migrate to the other CPU for better memory access
  performance because of shorter distance to access memory on the other
  node.

  The bug was found by the Ubuntu-3.13.0-65 with Kasan backported
  binary package: http://kernel.ubuntu.com/~gavinguo/kasan/Ubuntu-3.13.0-65.105/ 

  source code: http://kernel.ubuntu.com/git/gavinguo/ubuntu-trusty-
  amd64.git/log/?h=Ubuntu-3.13.0-65-kasan

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