[PATCH] qemuMonitorUnregister: Fix use-after-free of mon->watch

[Peng Liang]

qemuMonitorUnregister will be called in multiple threads (e.g. threads
in rpc worker pool and the vm event thread).  In some cases, it isn't
protected by the monitor lock, which may lead to call g_source_unref
more than one time and a use-after-free problem eventually.

To avoid similar problem in the future, use
g_atomic_pointer_compare_and_exchange instead of adding a lock in the
missing cases.

Signed-off-by: Peng Liang <liangpeng10 at huawei.com>
---
 src/qemu/qemu_monitor.c | 9 +++++----
 1 file changed, 5 insertions(+), 4 deletions(-)

diff --git a/src/qemu/qemu_monitor.c b/src/qemu/qemu_monitor.c
index 0476d606f553..f4d05cd951c2 100644
--- a/src/qemu/qemu_monitor.c
+++ b/src/qemu/qemu_monitor.c
@@ -853,10 +853,11 @@ qemuMonitorRegister(qemuMonitorPtr mon)
 void
 qemuMonitorUnregister(qemuMonitorPtr mon)
 {
-    if (mon->watch) {
-        g_source_destroy(mon->watch);
-        g_source_unref(mon->watch);
-        mon->watch = NULL;
+    GSource *watch = mon->watch;
+
+    if (watch && g_atomic_pointer_compare_and_exchange(&mon->watch, watch, NULL)) {
+        g_source_destroy(watch);
+        g_source_unref(watch);
     }
 }
 
-- 
2.29.2