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[Launchpad Bug Tracker <1527643@xxxxxxxxxxxxxxxxxx>]

This bug was fixed in the package linux - 4.4.0-14.30

---------------
linux (4.4.0-14.30) xenial; urgency=low

  [ Tim Gardner ]

  * Release Tracking Bug
    - LP: #1557508

  * Current 4.4 kernel won't boot on powerpc (LP: #1557130)
    - powerpc: Fix dedotify for binutils >= 2.26

  * ZFS: send fails to transmit some holes [corruption] (LP: #1557151)
    - Illumos 6370 - ZFS send fails to transmit some holes

  * Request to cherry-pick uvcvideo patch for Xenial kernel support of RealSense
    camera (LP: #1557138)
    - UVC: Add support for ds4 depth camera

  * use after free of task_struct->numa_faults in task_numa_find_cpu (LP: #1527643)
    - sched/numa: Fix use-after-free bug in the task_numa_compare

  * overlay fs regression: chmod fails with "Operation not permitted" on chowned
    files (LP: #1555997)
    - ovl: copy new uid/gid into overlayfs runtime inode

  * Miscellaneous Ubuntu changes
    - SAUCE: Dump stack when X.509 certificates cannot be loaded

 -- Tim Gardner <tim.gardner@xxxxxxxxxxxxx>  Mon, 14 Mar 2016 07:16:19
-0600

** Changed in: linux (Ubuntu Xenial)
       Status: Fix Committed => Fix Released

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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:
  Fix Released
Status in linux source package in Trusty:
  Fix Committed
Status in linux source package in Vivid:
  Fix Committed
Status in linux source package in Wily:
  Fix Committed
Status in linux source package in Xenial:
  Fix Released

Bug description:
  [Impact]

  The use-after-free invalid read bug, which happens in really tricky
  case, would use the numa_faults data already freed for the NUMA
  balance to make a decision to migrate the exiting process.

  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

  ==================================================================
  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 happening 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.

  [Fix]

  There are 3 patches(renamed to A, B, and C) related to the backport.
  However, not all distribution need all the patches as some are already in the newer version of kernel.

  A: 156654f491dd ("sched/numa: Move task_numa_free() to
   __put_task_struct()"): included in v3.15-rc1~180^2~5.

  Reason: The patch is included because the task_numa_free() should be called inside the __put_task_struct() since the Fix C is based on the
   get_task_struct() to avoid the task_numa_free() being called.

  B: 1effd9f19324 ("sched/numa: Fix unsafe get_task_struct() in
   task_numa_assign()"): included in v3.18-rc3~21^2~5.

  Reason: Add the checking of the PF_EXITING flag to ensure the task has
  not been freed.

  C: 1dff76b92f69 ("sched/numa: Fix use-after-free bug in the
   task_numa_compare"): included in v4.5-rc2~8^2~1.

  Reason: However, as the commit message in B said "rcu_read_lock()
   can't save us from the final put_task_struct() in
   finish_task_switch()" so that's the patch C solved.

  For v3.13 Trusty there are 3 patches needed:
    - A, B, and C.
  For v3.16 Utopic there are 2 patches needed:
    - B and C.
  For v3.19 Vivid/v4.2 Wily there is 1 patch needed:
    - C. <-- clean cherry-pick.

  [Test Case]

  Running the reproducer for about 4 weeks with the backported Trusty
  kernel cannot find the KASan error messages in the dmesg.

  Reproducer:
  https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1527643/+attachment/4595998/+files/kernel_panic_test.sh

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