block: bdrv_set_backing_bs: fix use-after-free

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>
(cherry picked from commit 6e57963a77df1e275a73dab4c6a7ec9a9d3468d4)
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>