Re: [PATCH v6 2/2] vhost-user: fix VirtQ notifier cleanup

[Michael S. Tsirkin]

On Tue, Nov 02, 2021 at 06:00:58AM +0000, Xueming(Steven) Li wrote:
> On Mon, 2021-11-01 at 17:00 -0400, Michael S. Tsirkin wrote:
> > On Mon, Nov 01, 2021 at 04:38:13PM +0800, Xueming Li wrote:
> > > When vhost-user device cleanup is executed and un-mmaps notifier
> > > address, VM cpu thread writing the notifier fails by accessing invalid
> > > address error.
> > > 
> > > To avoid this concurrent issue, call RCU and wait for a memory flatview
> > > update, then un-mmap notifiers in callback.
> > > 
> > > Fixes: 44866521bd6e ("vhost-user: support registering external host 
> > > notifiers")
> > > Cc: qemu-stable@nongnu.org
> > > Cc: Yuwei Zhang <zhangyuwei.9149@bytedance.com>
> > > Signed-off-by: Xueming Li <xuemingl@nvidia.com>
> > > ---
> > >  hw/virtio/vhost-user.c         | 50 +++++++++++++++++++++-------------
> > >  include/hw/virtio/vhost-user.h |  2 ++
> > >  2 files changed, 33 insertions(+), 19 deletions(-)
> > > 
> > > diff --git a/hw/virtio/vhost-user.c b/hw/virtio/vhost-user.c
> > > index c671719e9b..5adad4d029 100644
> > > --- a/hw/virtio/vhost-user.c
> > > +++ b/hw/virtio/vhost-user.c
> > > @@ -25,6 +25,7 @@
> > >  #include "migration/migration.h"
> > >  #include "migration/postcopy-ram.h"
> > >  #include "trace.h"
> > > +#include "exec/ramblock.h"
> > >  
> > >  #include <sys/ioctl.h>
> > >  #include <sys/socket.h>
> > > @@ -1143,15 +1144,27 @@ static int vhost_user_set_vring_num(struct 
> > > vhost_dev *dev,
> > >      return vhost_set_vring(dev, VHOST_USER_SET_VRING_NUM, ring);
> > >  }
> > >  
> > > -static void vhost_user_host_notifier_remove(struct vhost_dev *dev,
> > > -                                            int queue_idx)
> > > +static void vhost_user_host_notifier_free(VhostUserHostNotifier *n)
> > >  {
> > > -    struct vhost_user *u = dev->opaque;
> > > -    VhostUserHostNotifier *n = &u->user->notifier[queue_idx];
> > > -    VirtIODevice *vdev = dev->vdev;
> > > +    assert(n && n->old_addr);
> > > +    munmap(n->old_addr, qemu_real_host_page_size);
> > > +    n->old_addr = NULL;
> > > +}
> > > +
> > > +static void vhost_user_host_notifier_remove(VhostUserState *user,
> > > +                                            VirtIODevice *vdev, int 
> > > queue_idx)
> > > +{
> > > +    VhostUserHostNotifier *n = &user->notifier[queue_idx];
> > >  
> > >      if (n->addr) {
> > > -        virtio_queue_set_host_notifier_mr(vdev, queue_idx, &n->mr, 
> > > false);
> > > +        if (vdev) {
> > > +            virtio_queue_set_host_notifier_mr(vdev, queue_idx, &n->mr, 
> > > false);
> > > +        }
> > > +        assert(n->addr);
> > > +        assert(!n->old_addr);
> > > +        n->old_addr = n->addr;
> > > +        n->addr = NULL;
> > > +        call_rcu(n, vhost_user_host_notifier_free, rcu);
> > >      }
> > >  }
> > >  
> > > @@ -1190,8 +1203,9 @@ static int vhost_user_get_vring_base(struct 
> > > vhost_dev *dev,
> > >          .payload.state = *ring,
> > >          .hdr.size = sizeof(msg.payload.state),
> > >      };
> > > +    struct vhost_user *u = dev->opaque;
> > >  
> > > -    vhost_user_host_notifier_remove(dev, ring->index);
> > > +    vhost_user_host_notifier_remove(u->user, dev->vdev, ring->index);
> > >  
> > >      if (vhost_user_write(dev, &msg, NULL, 0) < 0) {
> > >          return -1;
> > > @@ -1486,12 +1500,7 @@ static int 
> > > vhost_user_slave_handle_vring_host_notifier(struct vhost_dev *dev,
> > >  
> > >      n = &user->notifier[queue_idx];
> > >  
> > > -    if (n->addr) {
> > > -        virtio_queue_set_host_notifier_mr(vdev, queue_idx, &n->mr, 
> > > false);
> > > -        object_unparent(OBJECT(&n->mr));
> > > -        munmap(n->addr, page_size);
> > > -        n->addr = NULL;
> > > -    }
> > > +    vhost_user_host_notifier_remove(user, vdev, queue_idx);
> > >  
> > >      if (area->u64 & VHOST_USER_VRING_NOFD_MASK) {
> > >          return 0;
> > > @@ -1510,9 +1519,12 @@ static int 
> > > vhost_user_slave_handle_vring_host_notifier(struct vhost_dev *dev,
> > >  
> > >      name = g_strdup_printf("vhost-user/host-notifier@%p mmaps[%d]",
> > >                             user, queue_idx);
> > > -    if (!n->mr.ram) /* Don't init again after suspend. */
> > > +    if (!n->mr.ram) { /* Don't init again after suspend. */
> > >          memory_region_init_ram_device_ptr(&n->mr, OBJECT(vdev), name,
> > >                                            page_size, addr);
> > > +    } else {
> > > +        n->mr.ram_block->host = addr;
> > > +    }
> > >      g_free(name);
> > >  
> > >      if (virtio_queue_set_host_notifier_mr(vdev, queue_idx, &n->mr, 
> > > true)) {
> > > @@ -2460,17 +2472,17 @@ bool vhost_user_init(VhostUserState *user, 
> > > CharBackend *chr, Error **errp)
> > >  void vhost_user_cleanup(VhostUserState *user)
> > >  {
> > >      int i;
> > > +    VhostUserHostNotifier *n;
> > >  
> > >      if (!user->chr) {
> > >          return;
> > >      }
> > >      memory_region_transaction_begin();
> > >      for (i = 0; i < VIRTIO_QUEUE_MAX; i++) {
> > > -        if (user->notifier[i].addr) {
> > > -            object_unparent(OBJECT(&user->notifier[i].mr));
> > > -            munmap(user->notifier[i].addr, qemu_real_host_page_size);
> > > -            user->notifier[i].addr = NULL;
> > > -        }
> > > +        n = &user->notifier[i];
> > > +        assert(!n->addr);
> > 
> > I'm pretty confused as to why this assert holds.
> > Add a comment?
> 
> Seems notifiers are removed by vhost_user_get_vring_base(), I used this
> assert to catch exception. Will remove it.

Um I'm not actually asking about that. asserts are good but
how do we know this one holds?

> > 
> > > +        vhost_user_host_notifier_remove(user, NULL, i);
> > > +        object_unparent(OBJECT(&n->mr));
> > >      }
> > >      memory_region_transaction_commit();
> > >      user->chr = NULL;
> > 
> > I'm also confused on why we can do unparent for notifiers which have
> > never been set up. Won't n->mr be invalid then?
> 
> There is a parent check in object_unparent().

It does not seem to be idempotent though in that it does not
set parent to NULL. What if this is called twice?

> > 
> > 
> > > diff --git a/include/hw/virtio/vhost-user.h 
> > > b/include/hw/virtio/vhost-user.h
> > > index f6012b2078..03aa22d450 100644
> > > --- a/include/hw/virtio/vhost-user.h
> > > +++ b/include/hw/virtio/vhost-user.h
> > > @@ -12,8 +12,10 @@
> > >  #include "hw/virtio/virtio.h"
> > >  
> > >  typedef struct VhostUserHostNotifier {
> > > +    struct rcu_head rcu;
> > >      MemoryRegion mr;
> > >      void *addr;
> > > +    void *old_addr;
> > 
> > That's not a very clear name. Is this literally just
> > "address for the rcu callback to unmap"?
> > Maybe unmap_addr then?
> 
> LGTM, thanks!
> 
> 
> > 
> > >  } VhostUserHostNotifier;
> > >  
> > >  typedef struct VhostUserState {
> > > -- 
> > > 2.33.0
> > 
>