block/vvfat: Do not unref qcow on closing backing bdrv

Before this commit, BDRVVVFATState.qcow is unrefed in write_target_close
on closing backing bdrv of vvfat. However, qcow bdrv is opend as a child
of vvfat in enable_write_target() so it will be also unrefed on closing
vvfat itself. This causes use-after-free of qcow on freeing vvfat which
has backing bdrv and qcow bdrv as children in this order because
bdrv_close(vvfat) tries to free qcow bdrv after freeing backing bdrv
as QLIST_FOREACH_SAFE() loop keeps next pointer, but BdrvChild of qcow
is already freed in bdrv_close(backing bdrv).

Signed-off-by: Hikaru Nishida <hikarupsp@gmail.com>
Message-Id: <20200209175156.85748-1-hikarupsp@gmail.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

0 files changed, 0 insertions, 0 deletions