Fix a potential Use-after-free in test_blockjob_common_drain_node() (v6.2.0).

[wliang]

Hi all,

I find a potential Use-after-free in QEMU 6.2.0, which is in 
test_blockjob_common_drain_node() (./tests/unit/test-bdrv-drain.c).

Specifically, at line 880, the variable 'scr' is released by the bdrv_unref(). 
However, at line 881, it is subsequently used as the 1st parameter of the 
function bdrv_set_backing_hd(). As a result, an UAF bug may be triggered.





880    bdrv_unref(src);


881    bdrv_set_backing_hd(src, src_backing, &error_abort);





I believe that the problem can be fixed by invoking bdrv_unref() after the call 
of bdrv_set_backing_hd() rather than before it.


---    bdrv_unref(src);
881    bdrv_set_backing_hd(src, src_backing, &error_abort);
+++bdrv_unref(src);


I'm looking forward to your confirmation.

Best,
Wentao

--- ./tests/unit/test-bdrv-drain.c	2022-02-23 15:06:32.384786070 +0800
+++ ./tests/unit/test-bdrv-drain-PATCH.c	2022-02-23 21:16:43.444928992 +0800
@@ -877,8 +877,8 @@
                                        BDRV_O_RDWR, &error_abort);
 
     bdrv_set_backing_hd(src_overlay, src, &error_abort);
-    bdrv_unref(src);
     bdrv_set_backing_hd(src, src_backing, &error_abort);
+    bdrv_unref(src);
     bdrv_unref(src_backing);
 
     blk_src = blk_new(qemu_get_aio_context(), BLK_PERM_ALL, BLK_PERM_ALL);