[PATCH] chardev: avoid use-after-free when client disconnect

[Hogan Wang]

IOWatchPoll object did not hold the @ioc and @src objects reference,
then io_watch_poll_prepare execute in IO thread, if IOWatchPoll
removed by mian thread, io_watch_poll_prepare may execute at last
chance and access the freed @ioc or @src object.

In IO thread monitor scene, the IO thread used to accept client,
receive qmp request and handle hung-up event. Main thread used to
handle qmp request and send response, it will remove IOWatchPoll
and free @ioc when send response fail, then cause use-after-free
like this:

(gdb) bt
0  0x00007f4d121c8edf in g_source_remove_child_source (source=0x7f4c58003560, 
child_source=0x7f4c58009b10)
1  0x00007f4d11e0705c in io_watch_poll_prepare (source=0x7f4c58003560, 
timeout=timeout@entry=0x7f4c7fffed94
2  0x00007f4d121ca419 in g_main_context_prepare 
(context=context@entry=0x55a1463f8260, priority=priority@entry=0x7f4c7fffee20)
3  0x00007f4d121cadeb in g_main_context_iterate (context=0x55a1463f8260, 
block=block@entry=1, dispatch=dispatch@entry=1, self=self@entry=0x7f4c94002260)
4  0x00007f4d121cb21d in g_main_loop_run (loop=0x55a146c90920)
5  0x00007f4d11de3ea1 in iothread_run (opaque=0x55a146411820)
6  0x00007f4d11d77470 in qemu_thread_start (args=0x55a146b1f3c0)
7  0x00007f4d11f2ef3b in ?? () from /usr/lib64/libpthread.so.0
8  0x00007f4d120ba550 in clone () from /usr/lib64/libc.so.6
(gdb) p iwp
$1 = (IOWatchPoll *) 0x7f4c58003560
(gdb) p *iwp
$2 = {parent = {callback_data = 0x0,
                callback_funcs = 0x0,
                source_funcs = 0x7f4d11f10760 <io_watch_poll_funcs>,
                ref_count = 1,
                context = 0x55a1463f8260,
                priority = 0,
                flags = 0,
                source_id = 544,
                poll_fds = 0x0,
                prev = 0x55a147a47a30,
                next = 0x55a14712fb80,
                name = 0x7f4c580036d0 "chardev-iowatch-charmonitor",
                priv = 0x7f4c58003060},
       ioc = 0x7f4c580033f0,
       src = 0x7f4c58009b10,
       fd_can_read = 0x7f4d11e0a5d0 <tcp_chr_read_poll>,
       fd_read = 0x7f4d11e0a380 <tcp_chr_read>,
       opaque = 0x55a1463aeea0 }
(gdb) p iwp->ioc
$3 = (QIOChannel *) 0x7f4c580033f0
(gdb) p *iwp->ioc
$4 = {parent = {class = 0x55a14707f400,
                free = 0x55a146313010,
                properties = 0x55a147b37b60,
                ref = 0,
                parent = 0x0},
      features = 3,
      name = 0x7f4c580085f0 "\240F",
      ctx = 0x0,
      read_coroutine = 0x0,
      write_coroutine = 0x0}

Solution: IOWatchPoll object hold the @ioc and @src objects reference
and release the reference in GSource finalize callback function.

Signed-off-by: Hogan Wang <hogan.wang@huawei.com>
---
 chardev/char-io.c | 17 ++++++++++++++++-
 1 file changed, 16 insertions(+), 1 deletion(-)

diff --git a/chardev/char-io.c b/chardev/char-io.c
index 8ced184160..3af388eeaa 100644
--- a/chardev/char-io.c
+++ b/chardev/char-io.c
@@ -55,9 +55,9 @@ static gboolean io_watch_poll_prepare(GSource *source,
             iwp->ioc, G_IO_IN | G_IO_ERR | G_IO_HUP | G_IO_NVAL);
         g_source_set_callback(iwp->src, iwp->fd_read, iwp->opaque, NULL);
         g_source_add_child_source(source, iwp->src);
-        g_source_unref(iwp->src);
     } else {
         g_source_remove_child_source(source, iwp->src);
+        g_source_unref(iwp->src);
         iwp->src = NULL;
     }
     return FALSE;
@@ -69,9 +69,23 @@ static gboolean io_watch_poll_dispatch(GSource *source, 
GSourceFunc callback,
     return G_SOURCE_CONTINUE;
 }
 
+static void io_watch_poll_finalize(GSource *source)
+{
+    IOWatchPoll *iwp = io_watch_poll_from_source(source);
+    if (iwp->src) {
+        g_source_unref(iwp->src);
+        iwp->src = NULL;
+    }
+    if (iwp->ioc) {
+        object_unref(OBJECT(iwp->ioc));
+        iwp->ioc = NULL;
+    }
+}
+
 static GSourceFuncs io_watch_poll_funcs = {
     .prepare = io_watch_poll_prepare,
     .dispatch = io_watch_poll_dispatch,
+    .finalize = io_watch_poll_finalize,
 };
 
 GSource *io_add_watch_poll(Chardev *chr,
@@ -88,6 +102,7 @@ GSource *io_add_watch_poll(Chardev *chr,
                                        sizeof(IOWatchPoll));
     iwp->fd_can_read = fd_can_read;
     iwp->opaque = user_data;
+    object_ref(OBJECT(ioc));
     iwp->ioc = ioc;
     iwp->fd_read = (GSourceFunc) fd_read;
     iwp->src = NULL;
-- 
2.33.0