block-backend: Retain permissions after migration

block-backend: Retain permissions after migration

After migration, the permissions the guest device wants to impose on its
BlockBackend are stored in blk->perm and blk->shared_perm.  In
blk_root_activate(), we take our permissions, but keep all shared
permissions open by calling `blk_set_perm(blk->perm, BLK_PERM_ALL)`.

Only afterwards (immediately or later, depending on the runstate) do we
restrict the shared permissions by calling
`blk_set_perm(blk->perm, blk->shared_perm)`.  Unfortunately, our first
call with shared_perm=BLK_PERM_ALL has overwritten blk->shared_perm to
be BLK_PERM_ALL, so this is a no-op and the set of shared permissions is
not restricted.

Fix this bug by saving the set of shared permissions before invoking
blk_set_perm() with BLK_PERM_ALL and restoring it afterwards.

Fixes: 5f7772c4d0cf32f4e779fcd5a69ae4dae24aeebf
       ("block-backend: Defer shared_perm tightening migration
       completion")
Reported-by: Peng Liang <liangpeng10@huawei.com>
Signed-off-by: Hanna Reitz <hreitz@redhat.com>
Message-Id: <20211125135317.186576-2-hreitz@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Tested-by: Peng Liang <liangpeng10@huawei.com>