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 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.

> 
> I think it's safe to let the first queue pair
> free the vhost-user structure, because it won't
> be touched by other queue pairs during cleanup.
> 
> Best regards,
> Tiwei Bie
> 
> 
> > 
> > Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
> > 
> > ---
> > 
> > diff --git a/net/vhost-user.c b/net/vhost-user.c
> > index 525a061..7549d25 100644
> > --- a/net/vhost-user.c
> > +++ b/net/vhost-user.c
> > @@ -156,19 +156,20 @@ static void net_vhost_user_cleanup(NetClientState *nc)
> >          g_free(s->vhost_net);
> >          s->vhost_net = NULL;
> >      }
> > -    if (s->vhost_user) {
> > -        vhost_user_cleanup(s->vhost_user);
> > -        g_free(s->vhost_user);
> > -        s->vhost_user = NULL;
> > -    }
> >      if (nc->queue_index == 0) {
> >          if (s->watch) {
> >              g_source_remove(s->watch);
> >              s->watch = 0;
> >          }
> >          qemu_chr_fe_deinit(&s->chr, true);
> > +        if (s->vhost_user) {
> > +            vhost_user_cleanup(s->vhost_user);
> > +            g_free(s->vhost_user);
> > +        }
> >      }
> >  
> > +    s->vhost_user = NULL;

Maybe we should move above line, like:

     if (nc->queue_index == 0) {
         if (s->watch) {
             g_source_remove(s->watch);
             s->watch = 0;
         }
         qemu_chr_fe_deinit(&s->chr, true);
+        if (s->vhost_user) {
+            vhost_user_cleanup(s->vhost_user);
+            g_free(s->vhost_user);
+            s->vhost_user = NULL;
+        }
     }

otherwise s->vhost_user may not be freed.

> > +
> >      qemu_purge_queued_packets(nc);
> >  }
> >  
> > @@ -341,7 +342,6 @@ err:
> >      if (user) {
> >          vhost_user_cleanup(user);
> >          g_free(user);
> > -        s->vhost_user = NULL;

I don't get why cannot zero it in this case.

> >      }
> >  
> >      return -1;

Best regards,
Tiwei Bie