[Qemu-devel] [Bug 1626972] Re: QEMU memfd_create fallback mechanism change for security drivers

[Rafael David Tinoco]

Hello Antonio (@arcimboldo)

The fix only makes sense for newer QEMUs (>= Xenial, like the one from
Mitaka Ubuntu Cloud Archive).

OBS:

The "migration check" is done in VHOST initialization functions when the
devices are virtually attached to the virtual machine. If you are using
kernel 3.13 and have apparmor enabled, then all the running instances
have the "migration blocker" ON - because of this buggy migration check
- and won't be able to live migration.

Unfortunately there is a "in-memory" linked list telling qemu that is
has a blocker (with the reason). This blocker was added during instance
startup and will be checked/used only when instance is live-migrated.

Check this: http://pastebin.ubuntu.com/23517175/

If you started the instance in a host not running apparmor (or not
having libvirt profile loaded, for example) it won't block the creation
of /tmp/memfd-XXX files during instance initialization. That won't
trigger the "blocker flag" inside the running program and, if/when
needed, the live migration will be able to occur.

This means that, after installing the new package, if you're using
apparmor, yes, you would have to RESTART running instances that were
affected by this bug in order to live migrating them. Sorry for the bad
news!  Even if you remove the apparmor rules, the migration blocker is
already set.

Hacking your process virtual memory would jeopardize the contents of the
virtual memory (could be catastrophic specially for a virtual machine).

-- 
You received this bug notification because you are a member of qemu-
devel-ml, which is subscribed to QEMU.
https://bugs.launchpad.net/bugs/1626972

Title:
  QEMU memfd_create fallback mechanism change for security drivers

Status in Ubuntu Cloud Archive:
  Invalid
Status in Ubuntu Cloud Archive mitaka series:
  Fix Committed
Status in QEMU:
  Fix Committed
Status in qemu package in Ubuntu:
  Fix Released
Status in qemu source package in Xenial:
  Fix Committed
Status in qemu source package in Yakkety:
  Fix Committed
Status in qemu source package in Zesty:
  Fix Released

Bug description:
  [Impact]

   * Updated QEMU (from UCA) live migration doesn't work with 3.13 kernels.
   * QEMU code checks if it can create /tmp/memfd-XXX files wrongly.
   * Apparmor will block access to /tmp/ and QEMU will fail migrating.

  [Test Case]

   * Install 2 Ubuntu Trusty (3.13) + UCA Mitaka + apparmor rules.
   * Try to live-migration from one to another. 
   * Apparmor will block creation of /tmp/memfd-XXX files.

  [Regression Potential]

   Pros:
   * Exhaustively tested this.
   * Worked with upstream on this fix. 
   * I'm implementing new vhost log mechanism for upstream.
   * One line change to a blocker that is already broken.

   Cons:
   * To break live migration in other circumstances. 

  [Other Info]

   * Christian Ehrhardt has been following this.

  ORIGINAL DESCRIPTION:

  When libvirt starts using apparmor, and creating apparmor profiles for
  every virtual machine created in the compute nodes, mitaka qemu (2.5 -
  and upstream also) uses a fallback mechanism for creating shared
  memory for live-migrations. This fall back mechanism, on kernels 3.13
  - that don't have memfd_create() system-call, try to create files on
  /tmp/ directory and fails.. causing live-migration not to work.

  Trusty with kernel 3.13 + Mitaka with qemu 2.5 + apparmor capability =
  can't live migrate.

  From qemu 2.5, logic is on :

  void *qemu_memfd_alloc(const char *name, size_t size, unsigned int seals, int 
*fd)
  {
      if (memfd_create)... ### only works with HWE kernels

      else ### 3.13 kernels, gets blocked by apparmor
         tmpdir = g_get_tmp_dir
         ...
         mfd = mkstemp(fname)
  }

  And you can see the errors:

  From the host trying to send the virtual machine:

  2016-08-15 16:36:26.160 1974 ERROR nova.virt.libvirt.driver 
[req-0cac612b-8d53-4610-b773-d07ad6bacb91 691a581cfa7046278380ce82b1c38ddd 
133ebc3585c041aebaead8c062cd6511 - - -] [instance: 
2afa1131-bc8c-43d2-9c4a-962c1bf7723e] Migration operation has aborted
  2016-08-15 16:36:26.248 1974 ERROR nova.virt.libvirt.driver 
[req-0cac612b-8d53-4610-b773-d07ad6bacb91 691a581cfa7046278380ce82b1c38ddd 
133ebc3585c041aebaead8c062cd6511 - - -] [instance: 
2afa1131-bc8c-43d2-9c4a-962c1bf7723e] Live Migration failure: internal error: 
unable to execute QEMU command 'migrate': Migration disabled: failed to 
allocate shared memory

  From the host trying to receive the virtual machine:

  Aug 15 16:36:19 tkcompute01 kernel: [ 1194.356794] type=1400 
audit(1471289779.791:72): apparmor="STATUS" operation="profile_load" 
profile="unconfined" name="libvirt-2afa1131-bc8c-43d2-9c4a-962c1bf7723e" 
pid=12565 comm="apparmor_parser"
  Aug 15 16:36:19 tkcompute01 kernel: [ 1194.357048] type=1400 
audit(1471289779.791:73): apparmor="STATUS" operation="profile_load" 
profile="unconfined" name="qemu_bridge_helper" pid=12565 comm="apparmor_parser"
  Aug 15 16:36:20 tkcompute01 kernel: [ 1194.877027] type=1400 
audit(1471289780.311:74): apparmor="STATUS" operation="profile_replace" 
profile="unconfined" name="libvirt-2afa1131-bc8c-43d2-9c4a-962c1bf7723e" 
pid=12613 comm="apparmor_parser"
  Aug 15 16:36:20 tkcompute01 kernel: [ 1194.904407] type=1400 
audit(1471289780.343:75): apparmor="STATUS" operation="profile_replace" 
profile="unconfined" name="qemu_bridge_helper" pid=12613 comm="apparmor_parser"
  Aug 15 16:36:20 tkcompute01 kernel: [ 1194.973064] type=1400 
audit(1471289780.407:76): apparmor="DENIED" operation="mknod" 
profile="libvirt-2afa1131-bc8c-43d2-9c4a-962c1bf7723e" name="/tmp/memfd-tNpKSj" 
pid=12625 comm="qemu-system-x86" requested_mask="c" denied_mask="c" fsuid=107 
ouid=107
  Aug 15 16:36:20 tkcompute01 kernel: [ 1194.979871] type=1400 
audit(1471289780.411:77): apparmor="DENIED" operation="open" 
profile="libvirt-2afa1131-bc8c-43d2-9c4a-962c1bf7723e" name="/tmp/" pid=12625 
comm="qemu-system-x86" requested_mask="r" denied_mask="r" fsuid=107 ouid=0
  Aug 15 16:36:20 tkcompute01 kernel: [ 1194.979881] type=1400 
audit(1471289780.411:78): apparmor="DENIED" operation="open" 
profile="libvirt-2afa1131-bc8c-43d2-9c4a-962c1bf7723e" name="/var/tmp/" 
pid=12625 comm="qemu-system-x86" requested_mask="r" denied_mask="r" fsuid=107 
ouid=0

  When leaving libvirt without apparmor capabilities (thus not confining
  virtual machines on compute nodes, the live migration works as
  expected, so, clearly, apparmor is stepping into the live migration).
  I'm sure that virtual machines have to be confined and that this isn't
  the desired behaviour...

To manage notifications about this bug go to:
https://bugs.launchpad.net/cloud-archive/+bug/1626972/+subscriptions