There is a use-after-free possible: bdrv_unref_child() leaves
bs->backing freed but not NULL. bdrv_attach_child may produce nested
polling loop due to drain, than access of freed pointer is possible.
I've produced the following crash on 30 iotest with modified code. It
does not reproduce on master, but still seems possible:
#0 __strcmp_avx2 () at /lib64/libc.so.6
#1 bdrv_backing_overridden (bs=0x55c9d3cc2060) at block.c:6350
#2 bdrv_refresh_filename (bs=0x55c9d3cc2060) at block.c:6404
#3 bdrv_backing_attach (c=0x55c9d48e5520) at block.c:1063
#4 bdrv_replace_child_noperm
(child=child@entry=0x55c9d48e5520,
new_bs=new_bs@entry=0x55c9d3cc2060) at block.c:2290
#5 bdrv_replace_child
(child=child@entry=0x55c9d48e5520,
new_bs=new_bs@entry=0x55c9d3cc2060) at block.c:2320
#6 bdrv_root_attach_child
(child_bs=child_bs@entry=0x55c9d3cc2060,
child_name=child_name@entry=0x55c9d241d478 "backing",
child_role=child_role@entry=0x55c9d26ecee0 <child_backing>,
ctx=<optimized out>, perm=<optimized out>, shared_perm=21,
opaque=0x55c9d3c5a3d0, errp=0x7ffd117108e0) at block.c:2424
#7 bdrv_attach_child
(parent_bs=parent_bs@entry=0x55c9d3c5a3d0,
child_bs=child_bs@entry=0x55c9d3cc2060,
child_name=child_name@entry=0x55c9d241d478 "backing",
child_role=child_role@entry=0x55c9d26ecee0 <child_backing>,
errp=errp@entry=0x7ffd117108e0) at block.c:5876
#8 in bdrv_set_backing_hd
(bs=bs@entry=0x55c9d3c5a3d0,
backing_hd=backing_hd@entry=0x55c9d3cc2060,
errp=errp@entry=0x7ffd117108e0)
at block.c:2576
#9 stream_prepare (job=0x55c9d49d84a0) at block/stream.c:150
#10 job_prepare (job=0x55c9d49d84a0) at job.c:761
#11 job_txn_apply (txn=<optimized out>, fn=<optimized out>) at
job.c:145
#12 job_do_finalize (job=0x55c9d49d84a0) at job.c:778
#13 job_completed_txn_success (job=0x55c9d49d84a0) at job.c:832
#14 job_completed (job=0x55c9d49d84a0) at job.c:845
#15 job_completed (job=0x55c9d49d84a0) at job.c:836
#16 job_exit (opaque=0x55c9d49d84a0) at job.c:864
#17 aio_bh_call (bh=0x55c9d471a160) at util/async.c:117
#18 aio_bh_poll (ctx=ctx@entry=0x55c9d3c46720) at util/async.c:117
#19 aio_poll (ctx=ctx@entry=0x55c9d3c46720,
blocking=blocking@entry=true)
at util/aio-posix.c:728
#20 bdrv_parent_drained_begin_single (poll=true, c=0x55c9d3d558f0)
at block/io.c:121
#21 bdrv_parent_drained_begin_single (c=c@entry=0x55c9d3d558f0,
poll=poll@entry=true)
at block/io.c:114
#22 bdrv_replace_child_noperm
(child=child@entry=0x55c9d3d558f0,
new_bs=new_bs@entry=0x55c9d3d27300) at block.c:2258
#23 bdrv_replace_child
(child=child@entry=0x55c9d3d558f0,
new_bs=new_bs@entry=0x55c9d3d27300) at block.c:2320
#24 bdrv_root_attach_child
(child_bs=child_bs@entry=0x55c9d3d27300,
child_name=child_name@entry=0x55c9d241d478 "backing",
child_role=child_role@entry=0x55c9d26ecee0 <child_backing>,
ctx=<optimized out>, perm=<optimized out>, shared_perm=21,
opaque=0x55c9d3cc2060, errp=0x7ffd11710c60) at block.c:2424
#25 bdrv_attach_child
(parent_bs=parent_bs@entry=0x55c9d3cc2060,
child_bs=child_bs@entry=0x55c9d3d27300,
child_name=child_name@entry=0x55c9d241d478 "backing",
child_role=child_role@entry=0x55c9d26ecee0 <child_backing>,
errp=errp@entry=0x7ffd11710c60) at block.c:5876
#26 bdrv_set_backing_hd
(bs=bs@entry=0x55c9d3cc2060,
backing_hd=backing_hd@entry=0x55c9d3d27300,
errp=errp@entry=0x7ffd11710c60)
at block.c:2576
#27 stream_prepare (job=0x55c9d495ead0) at block/stream.c:150
...
---
block.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/block.c b/block.c
index 957630b1c5..a862ce4df9 100644
--- a/block.c
+++ b/block.c
@@ -2735,10 +2735,10 @@ void bdrv_set_backing_hd(BlockDriverState *bs,
BlockDriverState *backing_hd,
if (bs->backing) {
bdrv_unref_child(bs, bs->backing);
+ bs->backing = NULL;
}
if (!backing_hd) {
- bs->backing = NULL;
goto out;
}