block: bdrv_set_backing_bs: fix use-after-free

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
    ...

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-Id: <20200316060631.30052-2-vsementsov@virtuozzo.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: John Snow <jsnow@redhat.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>

1 files changed, 1 insertions, 1 deletions

diff --git a/block.c b/block.c
index 6b984dc883..cccae5add9 100644
--- a/block.c
+++ b/block.c
@@ -2760,10 +2760,10 @@ 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;
     }