[PATCH 1/3] block/nbd: only detach existing iochannel from aio_context

[Roman Kagan]

When the reconnect in NBD client is in progress, the iochannel used for
NBD connection doesn't exist.  Therefore an attempt to detach it from
the aio_context of the parent BlockDriverState results in a NULL pointer
dereference.

The problem is triggerable, in particular, when an outgoing migration is
about to finish, and stopping the dataplane tries to move the
BlockDriverState from the iothread aio_context to the main loop.  If the
NBD connection is lost before this point, and the NBD client has entered
the reconnect procedure, QEMU crashes:

    at /build/qemu-gYtjVn/qemu-5.0.1/io/channel.c:452
    at /build/qemu-gYtjVn/qemu-5.0.1/block.c:6151
    new_context=new_context@entry=0x562a260c9580,
    ignore=ignore@entry=0x7feeadc9b780)
    at /build/qemu-gYtjVn/qemu-5.0.1/block.c:6230
    (bs=bs@entry=0x562a268d6a00, ctx=0x562a260c9580,
    ignore_child=<optimized out>, errp=<optimized out>)
    at /build/qemu-gYtjVn/qemu-5.0.1/block.c:6332
    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
    new_context=<optimized out>, errp=errp@entry=0x0)
    at /build/qemu-gYtjVn/qemu-5.0.1/block/block-backend.c:2010
    out>)
    at /build/qemu-gYtjVn/qemu-5.0.1/hw/block/dataplane/virtio-blk.c:292
    at /build/qemu-gYtjVn/qemu-5.0.1/hw/virtio/virtio-bus.c:245
    running=0, state=<optimized out>)
    at /build/qemu-gYtjVn/qemu-5.0.1/hw/virtio/virtio.c:3220
    state=state@entry=RUN_STATE_FINISH_MIGRATE)
    at /build/qemu-gYtjVn/qemu-5.0.1/softmmu/vl.c:1275
    send_stop=<optimized out>)
    at /build/qemu-gYtjVn/qemu-5.0.1/cpus.c:1032
    at /build/qemu-gYtjVn/qemu-5.0.1/migration/migration.c:2914
    at /build/qemu-gYtjVn/qemu-5.0.1/migration/migration.c:3275
    at /build/qemu-gYtjVn/qemu-5.0.1/migration/migration.c:3439
    at /build/qemu-gYtjVn/qemu-5.0.1/util/qemu-thread-posix.c:519
    at pthread_create.c:333
    oldval=0x562a2452b138, oldlenp=0x0, newval=0x562a2452c5e0
    <__func__.28102>, newlen=0)
    at ../sysdeps/unix/sysv/linux/sysctl.c:30

Fix it by checking that the iochannel is non-null before trying to
detach it from the aio_context.  If it is null, no detaching is needed,
and it will get reattached in the proper aio_context once the connection
is reestablished.

Signed-off-by: Roman Kagan <rvkagan@yandex-team.ru>
---
 block/nbd.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/block/nbd.c b/block/nbd.c
index 42e10c7c93..bcd6641e90 100644
--- a/block/nbd.c
+++ b/block/nbd.c
@@ -235,7 +235,14 @@ static void nbd_client_detach_aio_context(BlockDriverState 
*bs)
 
     /* Timer is deleted in nbd_client_co_drain_begin() */
     assert(!s->reconnect_delay_timer);
-    qio_channel_detach_aio_context(QIO_CHANNEL(s->ioc));
+    /*
+     * If reconnect is in progress we may have no ->ioc.  It will be
+     * re-instantiated in the proper aio context once the connection is
+     * reestablished.
+     */
+    if (s->ioc) {
+        qio_channel_detach_aio_context(QIO_CHANNEL(s->ioc));
+    }
 }
 
 static void nbd_client_attach_aio_context_bh(void *opaque)
-- 
2.29.2