Virtualization - Apr 2015 - [Qemu-devel] [PATCH 00/18] virtio-blk: Support "VIRTIO_CONFIG_S_NEEDS_RESET"

On Fri, Apr 17, 2015 at 03:59:15PM +0800, Fam Zheng
wrote:
> Currently, virtio code chooses to kill QEMU if the guest passes any invalid
> data with vring.
> That has drawbacks such as losing unsaved data (e.g. when
> guest user is writing a very long email), or possible denial of service in
> a nested vm use case where virtio device is passed through.
> 
> virtio-1 has introduced a new status bit "NEEDS RESET" which
could be used to
> improve this by communicating the error state between virtio devices and
> drivers. The device notifies guest upon setting the bit, then the guest
driver
> should detect this bit and report to userspace, or recover the device by
> resetting it.
Unfortunately, virtio 1 spec does not have a conformance statement
that requires driver to recover. We merely have a non-normative looking
text:
	Note: For example, the driver can?t assume requests in flight
	will be completed if DEVICE_NEEDS_RESET is set, nor can it assume that
	they have not been completed. A good implementation will try to recover
	by issuing a reset.

Implementing this reset for all devices in a race-free manner might also
be far from trivial.  I think we'd need a feature bit for this.
OTOH as long as we make this a new feature, would an ability to
reset a single VQ be a better match for what you are trying to
achieve?
> This series makes necessary changes in virtio core code, based on which
> virtio-blk is converted. Other devices now keep the existing behavior by
> passing in "error_abort". They will be converted in following
series. The Linux
> driver part will also be worked on.
> 
> One concern with this behavior change is that it's now harder to notice
the
> actual driver bug that caused the error, as the guest continues to run.  To
> address that, we could probably add a new error action option to virtio
> devices,  similar to the "read/write werror" in block layer, so
the vm could be
> paused and the management will get an event in QMP like pvpanic.  This work
can
> be done on top.
At the architectural level, that's only one concern. Others would be
- workloads such as openstack handle guest crash better than
  a guest that's e.g. slow because of a memory leak
- it's easier for guests to probe host for security issues
  if guest isn't killed
- guest can flood host log with guest-triggered errors


At the implementation level, there's one big issue you seem to have
missed: DMA to invalid memory addresses causes a crash in memory core.
I'm not sure whether it makes sense to recover from virtio core bugs
when we can't recover from device bugs.

> 
> 
> Fam Zheng (18):
>   virtio: Return error from virtqueue_map_sg
>   virtio: Return error from virtqueue_num_heads
>   virtio: Return error from virtqueue_get_head
>   virtio: Return error from virtqueue_next_desc
>   virtio: Return error from virtqueue_get_avail_bytes
>   virtio: Return error from virtqueue_pop
>   virtio: Return error from virtqueue_avail_bytes
>   virtio: Return error from virtio_add_queue
>   virtio: Return error from virtio_del_queue
>   virtio: Add macro for VIRTIO_CONFIG_S_NEEDS_RESET
>   virtio: Add "needs_reset" flag to virtio device
>   virtio: Return -EINVAL if the vdev needs reset in virtqueue_pop
>   virtio-blk: Graceful error handling of virtqueue_pop
>   qtest: Add "QTEST_FILTER" to filter test cases
>   qtest: virtio-blk: Extract "setup" for future reuse
>   libqos: Add qvirtio_needs_reset
>   qtest: Add test case for "needs reset" of virtio-blk
>   qtest: virtio-blk: Suppress virtio error messages in "make
check"
> 
>  hw/9pfs/virtio-9p-device.c                     |   2 +-
>  hw/9pfs/virtio-9p.c                            |   2 +-
>  hw/block/dataplane/virtio-blk.c                |   9 +-
>  hw/block/virtio-blk.c                          |  62 +++++--
>  hw/char/virtio-serial-bus.c                    |  30 ++--
>  hw/net/virtio-net.c                            |  36 +++--
>  hw/scsi/virtio-scsi.c                          |   8 +-
>  hw/virtio/virtio-balloon.c                     |  13 +-
>  hw/virtio/virtio-rng.c                         |   6 +-
>  hw/virtio/virtio.c                             | 214
++++++++++++++++++-------
>  include/hw/virtio/virtio-blk.h                 |   3 +-
>  include/hw/virtio/virtio.h                     |  17 +-
>  include/standard-headers/linux/virtio_config.h |   2 +
>  tests/Makefile                                 |   6 +-
>  tests/libqos/virtio.c                          |   5 +
>  tests/libqos/virtio.h                          |   2 +
>  tests/virtio-blk-test.c                        | 196
++++++++++++++++++++--
>  17 files changed, 482 insertions(+), 131 deletions(-)
> 
> -- 
> 1.9.3
>

On 20/04/2015 19:36, Michael S. Tsirkin wrote:
> At the implementation level, there's one big issue you seem to have
> missed: DMA to invalid memory addresses causes a crash in memory core.
> I'm not sure whether it makes sense to recover from virtio core bugs
> when we can't recover from device bugs.
What do you mean exactly?  DMA to invalid memory addresses causes
address_space_map to return a "short read".

Paolo

On Mon, Apr 20, 2015 at 09:10:02PM +0200, Paolo Bonzini
wrote:
> 
> 
> On 20/04/2015 19:36, Michael S. Tsirkin wrote:
> > At the implementation level, there's one big issue you seem to
have
> > missed: DMA to invalid memory addresses causes a crash in memory core.
> > I'm not sure whether it makes sense to recover from virtio core
bugs
> > when we can't recover from device bugs.
> 
> What do you mean exactly?  DMA to invalid memory addresses causes
> address_space_map to return a "short read".
> 
> Paolo
I mean, first of all, a bunch of virtio_XXX_phys calls.
These eventually call qemu_get_ram_ptr, which internally calls
qemu_get_ram_block and ramblock_ptr.
Both abort on errors.

-- 
MST

On Mon, 04/20 19:36, Michael S. Tsirkin wrote:
> On Fri, Apr 17, 2015 at 03:59:15PM +0800, Fam Zheng wrote:
> > Currently, virtio code chooses to kill QEMU if the guest passes any
invalid
> > data with vring.
> > That has drawbacks such as losing unsaved data (e.g. when
> > guest user is writing a very long email), or possible denial of
service in
> > a nested vm use case where virtio device is passed through.
> > 
> > virtio-1 has introduced a new status bit "NEEDS RESET" which
could be used to
> > improve this by communicating the error state between virtio devices
and
> > drivers. The device notifies guest upon setting the bit, then the
guest driver
> > should detect this bit and report to userspace, or recover the device
by
> > resetting it.
> 
> Unfortunately, virtio 1 spec does not have a conformance statement
> that requires driver to recover. We merely have a non-normative looking
> text:
> 	Note: For example, the driver can?t assume requests in flight
> 	will be completed if DEVICE_NEEDS_RESET is set, nor can it assume that
> 	they have not been completed. A good implementation will try to recover
> 	by issuing a reset.
> 
> Implementing this reset for all devices in a race-free manner might also
> be far from trivial.  I think we'd need a feature bit for this.
> OTOH as long as we make this a new feature, would an ability to
> reset a single VQ be a better match for what you are trying to
> achieve?
I think that is too complicated as a recovery measure, a device level resetting
will be better to get to a deterministic state, at least.
> 
> > This series makes necessary changes in virtio core code, based on
which
> > virtio-blk is converted. Other devices now keep the existing behavior
by
> > passing in "error_abort". They will be converted in
following series. The Linux
> > driver part will also be worked on.
> > 
> > One concern with this behavior change is that it's now harder to
notice the
> > actual driver bug that caused the error, as the guest continues to
run.  To
> > address that, we could probably add a new error action option to
virtio
> > devices,  similar to the "read/write werror" in block layer,
so the vm could be
> > paused and the management will get an event in QMP like pvpanic.  This
work can
> > be done on top.
> 
> At the architectural level, that's only one concern. Others would be
> - workloads such as openstack handle guest crash better than
>   a guest that's e.g. slow because of a memory leak
What memory leak are you referring to?
> - it's easier for guests to probe host for security issues
>   if guest isn't killed
> - guest can flood host log with guest-triggered errors
We can still abort() if guest is triggering error too quickly.

Fam

On Tue, Apr 21, 2015 at 10:37:00AM +0800, Fam Zheng
wrote:
> On Mon, 04/20 19:36, Michael S. Tsirkin wrote:
> > On Fri, Apr 17, 2015 at 03:59:15PM +0800, Fam Zheng wrote:
> > > Currently, virtio code chooses to kill QEMU if the guest passes
any invalid
> > > data with vring.
> > > That has drawbacks such as losing unsaved data (e.g. when
> > > guest user is writing a very long email), or possible denial of
service in
> > > a nested vm use case where virtio device is passed through.
> > > 
> > > virtio-1 has introduced a new status bit "NEEDS RESET"
which could be used to
> > > improve this by communicating the error state between virtio
devices and
> > > drivers. The device notifies guest upon setting the bit, then the
guest driver
> > > should detect this bit and report to userspace, or recover the
device by
> > > resetting it.
> > 
> > Unfortunately, virtio 1 spec does not have a conformance statement
> > that requires driver to recover. We merely have a non-normative
looking
> > text:
> > 	Note: For example, the driver can?t assume requests in flight
> > 	will be completed if DEVICE_NEEDS_RESET is set, nor can it assume
that
> > 	they have not been completed. A good implementation will try to
recover
> > 	by issuing a reset.
> > 
> > Implementing this reset for all devices in a race-free manner might
also
> > be far from trivial.  I think we'd need a feature bit for this.
> > OTOH as long as we make this a new feature, would an ability to
> > reset a single VQ be a better match for what you are trying to
> > achieve?
> 
> I think that is too complicated as a recovery measure, a device level
resetting
> will be better to get to a deterministic state, at least.
Question would be, how hard is it to stop host from using all queues,
retrieve all host OS state and re-program it into the device.
If we need to shadow all OS state within the driver, then that's a lot
of not well tested code with a possibility of introducing more bugs.
> > 
> > > This series makes necessary changes in virtio core code, based on
which
> > > virtio-blk is converted. Other devices now keep the existing
behavior by
> > > passing in "error_abort". They will be converted in
following series. The Linux
> > > driver part will also be worked on.
> > > 
> > > One concern with this behavior change is that it's now harder
to notice the
> > > actual driver bug that caused the error, as the guest continues
to run.  To
> > > address that, we could probably add a new error action option to
virtio
> > > devices,  similar to the "read/write werror" in block
layer, so the vm could be
> > > paused and the management will get an event in QMP like pvpanic. 
This work can
> > > be done on top.
> > 
> > At the architectural level, that's only one concern. Others would
be
> > - workloads such as openstack handle guest crash better than
> >   a guest that's e.g. slow because of a memory leak
> 
> What memory leak are you referring to?
That was just an example.  If host detects a malformed ring, it will
crash.  But often it doesn't, result is buffers not being used, so guest
can't free them up.
> > - it's easier for guests to probe host for security issues
> >   if guest isn't killed
> > - guest can flood host log with guest-triggered errors
> 
> We can still abort() if guest is triggering error too quickly.
> 
> Fam

Absolutely, and if it looked like I'm against error detection and
recovery, this was not my intent.

I am merely saying we can't apply this patchset as is, deferring
addressing the issues to patches on top.

But I have an idea: refactor the code to use error_abort. This way we
can apply the patchset without making functional changes, and you can
make progress to complete this, on top.



-- 
MST