vhost-user: fix VirtQ notifier cleanup

When vhost-user device cleanup, remove notifier MR and munmaps notifier
address in the event-handling thread, VM CPU thread writing the notifier
in concurrent fails with an error of accessing invalid address. It
happens because MR is still being referenced and accessed in another
thread while the underlying notifier mmap address is being freed and
becomes invalid.

This patch calls RCU and munmap notifiers in the callback after the
memory flatview update finish.

Fixes: 4486652 ("vhost-user: support registering external host notifiers")
Cc: qemu-stable@nongnu.org
Signed-off-by: Xueming Li <xuemingl@nvidia.com>
Message-Id: <20220207071929.527149-3-xuemingl@nvidia.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

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>
@@ -1162,15 +1163,26 @@ 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->unmap_addr);
+    munmap(n->unmap_addr, qemu_real_host_page_size);
+    n->unmap_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->unmap_addr);
+        n->unmap_addr = n->addr;
+        n->addr = NULL;
+        call_rcu(n, vhost_user_host_notifier_free, rcu);
     }
 }
 
@@ -1219,8 +1231,9 @@ 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);
 
     ret = vhost_user_write(dev, &msg, NULL, 0);
     if (ret < 0) {
@@ -1506,12 +1519,7 @@ 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;
@@ -1530,9 +1538,12 @@ 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)) {
@@ -2505,17 +2516,16 @@ 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];
+        vhost_user_host_notifier_remove(user, NULL, i);
+        object_unparent(OBJECT(&n->mr));
     }
     memory_region_transaction_commit();
     user->chr = NULL;

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 *unmap_addr;
 } VhostUserHostNotifier;
 
 typedef struct VhostUserState {