chardev: Fix QIOChannel refcount

The IOWatchPoll holds a reference to the iochannel while the "child"
source (iwp->src) is removed from the context and freed. Freeing the
source leads to the iochannel being also freed at
qio_channel_fd_source_finalize().

Later, io_watch_poll_prepare() tries to create another source with the
same iochannel and hits an use after free:

==8241==ERROR: AddressSanitizer: heap-use-after-free on address 0x514000000040
READ of size 8 at 0x514000000040 thread T2
    #0 0x561c2d272fcd in object_get_class  ../qom/object.c:1043:17
    #1 0x561c2d338f84 in QIO_CHANNEL_GET_CLASS  include/io/channel.h:29:1
    #2 0x561c2d33b26f in qio_channel_create_watch  ../io/channel.c:388:30
    #3 0x561c2d2f0993 in io_watch_poll_prepare  ../chardev/char-io.c:65:20
    ...

0x514000000040 is located 0 bytes inside of 392-byte region [0x514000000040,0x5140000001c8)
freed by thread T2 here:
    #0 0x561c2d2319a5 in free
    #1 0x7fb2c0926638 in g_free
    #2 0x561c2d276507 in object_finalize  ../qom/object.c:734:9
    #3 0x561c2d271d0d in object_unref  ../qom/object.c:1231:9
    #4 0x561c2d32ef1d in qio_channel_fd_source_finalize  ../io/channel-watch.c:95:5
    #5 0x7fb2c091d124 in g_source_unref_internal ../glib/gmain.c:2298
    #6 0x561c2d2f0b6c in io_watch_poll_prepare  ../chardev/char-io.c:71:9
    ...

previously allocated by thread T3 (connect) here:
    #0 0x561c2d231c69 in malloc
    #1 0x7fb2c0926518 in g_malloc
    #2 0x561c2d27246e in object_new_with_type  ../qom/object.c:767:15
    #3 0x561c2d272530 in object_new  ../qom/object.c:789:12
    #4 0x561c2d320193 in qio_channel_socket_new  ../io/channel-socket.c:64:31
    #5 0x561c2d308013 in tcp_chr_connect_client_async  ../chardev/char-socket.c:1181:12
    #6 0x561c2d3002e7 in qmp_chardev_open_socket_client  ../chardev/char-socket.c:1281:9
    ...

Fix the issue by incrementing the iochannel reference count when the
IOWatchPoll takes a reference and decrementing when it is finalized.

Signed-off-by: Fabiano Rosas <farosas@suse.de>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 20260302092225.4088227-6-peter.maydell@linaro.org
[PMM: rebased]
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>

Author: Fabiano Rosas

chardev/char-io.c

@@ -88,6 +88,9 @@ static gboolean io_watch_poll_dispatch(GSource *source, GSourceFunc callback,
 static void io_watch_poll_finalize(GSource *source)
 {
     IOWatchPoll *iwp = io_watch_poll_from_source(source);
+
+    object_unref(OBJECT(iwp->ioc));
+
     if (iwp->src) {
         g_source_destroy(iwp->src);
         g_source_unref(iwp->src);
@@ -117,6 +120,8 @@ GSource *io_add_watch_poll(Chardev *chr,
     iwp->fd_can_read = fd_can_read;
     iwp->opaque = user_data;
     iwp->ioc = ioc;
+    object_ref(OBJECT(iwp->ioc));
+
     iwp->fd_read = (GSourceFunc) fd_read;
     iwp->src = NULL;
     iwp->context = context;