block/nbd: only enter connection coroutine if it's present

When an NBD block driver state is moved from one aio_context to another
(e.g. when doing a drain in a migration thread),
nbd_client_attach_aio_context_bh is executed that enters the connection
coroutine.

However, the assumption that ->connection_co is always present here
appears incorrect: the connection may have encountered an error other
than -EIO in the underlying transport, and thus may have decided to quit
rather than keep trying to reconnect, and therefore it may have
terminated the connection coroutine.  As a result an attempt to reassign
the client in this state (NBD_CLIENT_QUIT) to a different aio_context
leads to a null pointer dereference:

  #0  qio_channel_detach_aio_context (ioc=0x0)
      at /build/qemu-gYtjVn/qemu-5.0.1/io/channel.c:452
  AFLplusplus#1  0x0000562a242824b3 in bdrv_detach_aio_context (bs=0x562a268d6a00)
      at /build/qemu-gYtjVn/qemu-5.0.1/block.c:6151
  AFLplusplus#2  bdrv_set_aio_context_ignore (bs=bs@entry=0x562a268d6a00,
      new_context=new_context@entry=0x562a260c9580,
      ignore=ignore@entry=0x7feeadc9b780)
      at /build/qemu-gYtjVn/qemu-5.0.1/block.c:6230
  AFLplusplus#3  0x0000562a24282969 in bdrv_child_try_set_aio_context
      (bs=bs@entry=0x562a268d6a00, ctx=0x562a260c9580,
      ignore_child=<optimized out>, errp=<optimized out>)
      at /build/qemu-gYtjVn/qemu-5.0.1/block.c:6332
  AFLplusplus#4  0x0000562a242bb7db in blk_do_set_aio_context (blk=0x562a2735d0d0,
      new_context=0x562a260c9580,
      update_root_node=update_root_node@entry=true, errp=errp@entry=0x0)
      at /build/qemu-gYtjVn/qemu-5.0.1/block/block-backend.c:1989
  AFLplusplus#5  0x0000562a242be0bd in blk_set_aio_context (blk=<optimized out>,
      new_context=<optimized out>, errp=errp@entry=0x0)
      at /build/qemu-gYtjVn/qemu-5.0.1/block/block-backend.c:2010
  AFLplusplus#6  0x0000562a23fbd953 in virtio_blk_data_plane_stop (vdev=<optimized
      out>)
      at /build/qemu-gYtjVn/qemu-5.0.1/hw/block/dataplane/virtio-blk.c:292
  AFLplusplus#7  0x0000562a241fc7bf in virtio_bus_stop_ioeventfd (bus=0x562a260dbf08)
      at /build/qemu-gYtjVn/qemu-5.0.1/hw/virtio/virtio-bus.c:245
  AFLplusplus#8  0x0000562a23fefb2e in virtio_vmstate_change (opaque=0x562a260dbf90,
      running=0, state=<optimized out>)
      at /build/qemu-gYtjVn/qemu-5.0.1/hw/virtio/virtio.c:3220
  AFLplusplus#9  0x0000562a2402ebfd in vm_state_notify (running=running@entry=0,
      state=state@entry=RUN_STATE_FINISH_MIGRATE)
      at /build/qemu-gYtjVn/qemu-5.0.1/softmmu/vl.c:1275
  AFLplusplus#10 0x0000562a23f7bc02 in do_vm_stop (state=RUN_STATE_FINISH_MIGRATE,
      send_stop=<optimized out>)
      at /build/qemu-gYtjVn/qemu-5.0.1/cpus.c:1032
  AFLplusplus#11 0x0000562a24209765 in migration_completion (s=0x562a260e83a0)
      at /build/qemu-gYtjVn/qemu-5.0.1/migration/migration.c:2914
  AFLplusplus#12 migration_iteration_run (s=0x562a260e83a0)
      at /build/qemu-gYtjVn/qemu-5.0.1/migration/migration.c:3275
  AFLplusplus#13 migration_thread (opaque=opaque@entry=0x562a260e83a0)
      at /build/qemu-gYtjVn/qemu-5.0.1/migration/migration.c:3439
  AFLplusplus#14 0x0000562a2435ca96 in qemu_thread_start (args=<optimized out>)
      at /build/qemu-gYtjVn/qemu-5.0.1/util/qemu-thread-posix.c:519
  AFLplusplus#15 0x00007feed31466ba in start_thread (arg=0x7feeadc9c700)
      at pthread_create.c:333
  AFLplusplus#16 0x00007feed2e7c41d in __GI___sysctl (name=0x0, nlen=608471908,
      oldval=0x562a2452b138, oldlenp=0x0, newval=0x562a2452c5e0
      <__func__.28102>, newlen=0)
      at ../sysdeps/unix/sysv/linux/sysctl.c:30
  AFLplusplus#17 0x0000000000000000 in ?? ()

Fix it by checking that the connection coroutine is non-null before
trying to enter it.  If it is null, no entering is needed, as the
connection is probably going down anyway.

Signed-off-by: Roman Kagan <rvkagan@yandex-team.ru>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-Id: <20210129073859.683063-3-rvkagan@yandex-team.ru>
Signed-off-by: Eric Blake <eblake@redhat.com>

block/nbd.c

@@ -250,13 +250,15 @@ static void nbd_client_attach_aio_context_bh(void *opaque)
     BlockDriverState *bs = opaque;
     BDRVNBDState *s = (BDRVNBDState *)bs->opaque;
 
-    /*
-     * The node is still drained, so we know the coroutine has yielded in
-     * nbd_read_eof(), the only place where bs->in_flight can reach 0, or it is
-     * entered for the first time. Both places are safe for entering the
-     * coroutine.
-     */
-    qemu_aio_coroutine_enter(bs->aio_context, s->connection_co);
+    if (s->connection_co) {
+        /*
+         * The node is still drained, so we know the coroutine has yielded in
+         * nbd_read_eof(), the only place where bs->in_flight can reach 0, or
+         * it is entered for the first time. Both places are safe for entering
+         * the coroutine.
+         */
+        qemu_aio_coroutine_enter(bs->aio_context, s->connection_co);
+    }
     bdrv_dec_in_flight(bs);
 }