Re: [Qemu-devel] [PATCH v3 2/6] vhost-user: introduce shared vhost-user state

[Tiwei Bie <tiwei.bie@intel.com>]

On Thu, May 24, 2018 at 10:24:40AM +0800, Tiwei Bie wrote:
> On Thu, May 24, 2018 at 07:21:01AM +0800, Tiwei Bie wrote:
> > On Wed, May 23, 2018 at 06:43:29PM +0300, Michael S. Tsirkin wrote:
> > > On Wed, May 23, 2018 at 06:36:05PM +0300, Michael S. Tsirkin wrote:
> > > > On Wed, May 23, 2018 at 04:44:51PM +0300, Michael S. Tsirkin wrote:
> > > > > On Thu, Apr 12, 2018 at 11:12:28PM +0800, Tiwei Bie wrote:
> > > > > > When multi queue is enabled e.g. for a virtio-net device,
> > > > > > each queue pair will have a vhost_dev, and the only thing
> > > > > > shared between vhost devs currently is the chardev. This
> > > > > > patch introduces a vhost-user state structure which will
> > > > > > be shared by all vhost devs of the same virtio device.
> > > > > > 
> > > > > > Signed-off-by: Tiwei Bie <tiwei.bie@intel.com>
> > > > > 
> > > > > Unfortunately this patch seems to cause crashes.
> > > > > To reproduce, simply run
> > > > > make check-qtest-x86_64
> > > > > 
> > > > > Sorry that it took me a while to find - it triggers 90% of runs but not
> > > > > 100% which complicates bisection somewhat.
> > 
> > It's my fault to not notice this bug before.
> > I'm very sorry. Thank you so much for finding
> > the root cause!
> > 
> > > > > 
> > > > > > ---
> > > > > >  backends/cryptodev-vhost-user.c     | 20 ++++++++++++++++++-
> > > > > >  hw/block/vhost-user-blk.c           | 22 +++++++++++++++++++-
> > > > > >  hw/scsi/vhost-user-scsi.c           | 20 ++++++++++++++++++-
> > > > > >  hw/virtio/Makefile.objs             |  2 +-
> > > > > >  hw/virtio/vhost-stub.c              | 10 ++++++++++
> > > > > >  hw/virtio/vhost-user.c              | 31 +++++++++++++++++++---------
> > > > > >  include/hw/virtio/vhost-user-blk.h  |  2 ++
> > > > > >  include/hw/virtio/vhost-user-scsi.h |  2 ++
> > > > > >  include/hw/virtio/vhost-user.h      | 20 +++++++++++++++++++
> > > > > >  net/vhost-user.c                    | 40 ++++++++++++++++++++++++++++++-------
> > > > > >  10 files changed, 149 insertions(+), 20 deletions(-)
> > > > > >  create mode 100644 include/hw/virtio/vhost-user.h
> > [...]
> > > > > >          qemu_chr_fe_set_handlers(&s->chr, NULL, NULL,
> > > > > >                                   net_vhost_user_event, NULL, nc0->name, NULL,
> > > > > > @@ -319,6 +336,15 @@ static int net_vhost_user_init(NetClientState *peer, const char *device,
> > > > > >      assert(s->vhost_net);
> > > > > >  
> > > > > >      return 0;
> > > > > > +
> > > > > > +err:
> > > > > > +    if (user) {
> > > > > > +        vhost_user_cleanup(user);
> > > > > > +        g_free(user);
> > > > > > +        s->vhost_user = NULL;
> > > > > > +    }
> > > > > > +
> > > > > > +    return -1;
> > > > > >  }
> > > > > >  
> > > > > >  static Chardev *net_vhost_claim_chardev(
> > > > > > -- 
> > > > > > 2.11.0
> > > > 
> > > > So far I figured out that commenting the free of
> > > > the structure removes the crash, so we seem to
> > > > be dealing with a use-after free here.
> > > > I suspect that in a MQ situation, one queue gets
> > > > closed and attempts to free the structure
> > > > while others still use it.
> > > > 
> > > > diff --git a/net/vhost-user.c b/net/vhost-user.c
> > > > index 525a061..6a1573b 100644
> > > > --- a/net/vhost-user.c
> > > > +++ b/net/vhost-user.c
> > > > @@ -157,8 +157,8 @@ static void net_vhost_user_cleanup(NetClientState *nc)
> > > >          s->vhost_net = NULL;
> > > >      }
> > > >      if (s->vhost_user) {
> > > > -        vhost_user_cleanup(s->vhost_user);
> > > > -        g_free(s->vhost_user);
> > > > +        //vhost_user_cleanup(s->vhost_user);
> > > > +        //g_free(s->vhost_user);
> > > >          s->vhost_user = NULL;
> > > >      }
> > > >      if (nc->queue_index == 0) {
> > > > @@ -339,8 +339,8 @@ static int net_vhost_user_init(NetClientState *peer, const char *device,
> > > >  
> > > >  err:
> > > >      if (user) {
> > > > -        vhost_user_cleanup(user);
> > > > -        g_free(user);
> > > > +        //vhost_user_cleanup(user);
> > > > +        //g_free(user);
> > > >          s->vhost_user = NULL;
> > > >      }
> > > >  
> > > 
> > > 
> > > So the following at least gets rid of the crashes.
> > > I am not sure it does not leak memory though,
> > > and not sure there aren't any configurations where
> > > the 1st queue gets cleaned up first.
> > > 
> > > Thoughts?
> > 
> > Thank you so much for catching it and fixing
> > it! I'll keep your SoB there. Thank you so
> > much! I do appreciate it!
> > 
> > You are right. This structure is freed multiple
> > times when multi-queue is enabled.
> 
> After a deeper digging, I got your point now..
> It could be a use-after-free instead of a double
> free.. As it's safe to deinit the char which is
> shared by all queue pairs when cleanup the 1st
> queue pair, it should be safe to free vhost-user
> structure there too.

One thing worried me is that, I can't reproduce
the crash with `make check-qtest-x86_64`. I tried
to run it a lot of times, but the only output I
got each time was:

        CHK version_gen.h
  GTESTER check-qtest-x86_64

I did a quick glance of the `check-qtest-x86_64`
target in the makefile, it seems that the relevant
test is `tests/vhost-user-test`. So I also tried
to run this test directly:

make tests/vhost-user-test
while true; do
  QTEST_QEMU_BINARY=x86_64-softmmu/qemu-system-x86_64 ./tests/vhost-user-test
done

And the only output in each loop I got was:

/x86_64/vhost-user/migrate: OK
/x86_64/vhost-user/multiqueue: OK
/x86_64/vhost-user/read-guest-mem/memfd: OK
/x86_64/vhost-user/read-guest-mem/memfile: OK

So I'm still not quite sure what caused the crash
on your side. But it does make more sense to free
the vhost-user structure only when cleanup the
1st queue pair (i.e. where the `chr` is deinit-ed).

I have sent a new patch set with above change:

http://lists.gnu.org/archive/html/qemu-devel/2018-05/msg05508.html
https://patchwork.kernel.org/bundle/btw/vhost-user-host-notifiers/

Because the above change is got from your diff
and also based on your analysis, I kept your SoB
in that patch (if you have any concern about it,
please let me know).

In this patch set, I also introduced a protocol
feature to allow slave to send fds to master via
the slave channel.

If you still see crashes with the new patch set,
please provide me a bit more details, e.g. the
crash message. Thanks a lot!

Best regards,
Tiwei Bie