Do not delete BlockDriverState when deleting the drive

When removing a drive from the host-side via drive_del we currently have
the following path:

drive_del
qemu_aio_flush()
bdrv_close()    // zaps bs->drv, which makes any subsequent I/O get
                // dropped.  Works as designed
drive_uninit()
bdrv_delete()   // frees the bs.  Since the device is still connected to
                // bs, any subsequent I/O is a use-after-free.

The value of bs->drv becomes unpredictable on free.  As long as it
remains null, I/O still gets dropped, however it could become non-null
at any point after the free resulting SEGVs or other QEMU state
corruption.

To resolve this issue as simply as possible, we can chose to not
actually delete the BlockDriverState pointer.  Since bdrv_close()
handles setting the drv pointer to NULL, we just need to remove the
BlockDriverState from the QLIST that is used to enumerate the block
devices.  This is currently handled within bdrv_delete, so move this
into its own function, bdrv_make_anon().

The result is that we can now invoke drive_del, this closes the file
descriptors and sets BlockDriverState->drv to NULL which prevents futher
IO to the device, and since we do not free BlockDriverState, we don't
have to worry about the copy retained in the block devices.

We also don't attempt to remove the qdev property since we are no longer
deleting the BlockDriverState on drives with associated drives.  This
also allows for removing Drives with no devices associated either.

Reported-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Ryan Harper <ryanh@us.ibm.com>
Acked-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

1 files changed, 8 insertions, 17 deletions

diff --git a/blockdev.c b/blockdev.c
index bbe92fe19..5429621f0 100644
--- a/blockdev.c
+++ b/blockdev.c
@@ -737,8 +737,6 @@ int do_drive_del(Monitor *mon, const QDict *qdict, QObject **ret_data)
 {
     const char *id = qdict_get_str(qdict, "id");
     BlockDriverState *bs;
-    BlockDriverState **ptr;
-    Property *prop;
 
     bs = bdrv_find(id);
     if (!bs) {
@@ -755,24 +753,17 @@ int do_drive_del(Monitor *mon, const QDict *qdict, QObject **ret_data)
     bdrv_flush(bs);
     bdrv_close(bs);
 
-    /* clean up guest state from pointing to host resource by
-     * finding and removing DeviceState "drive" property */
+    /* if we have a device associated with this BlockDriverState (bs->peer)
+     * then we need to make the drive anonymous until the device
+     * can be removed.  If this is a drive with no device backing
+     * then we can just get rid of the block driver state right here.
+     */
     if (bs->peer) {
-        for (prop = bs->peer->info->props; prop && prop->name; prop++) {
-            if (prop->info->type == PROP_TYPE_DRIVE) {
-                ptr = qdev_get_prop_ptr(bs->peer, prop);
-                if (*ptr == bs) {
-                    bdrv_detach(bs, bs->peer);
-                    *ptr = NULL;
-                    break;
-                }
-            }
-        }
+        bdrv_make_anon(bs);
+    } else {
+        drive_uninit(drive_get_by_blockdev(bs));
     }
 
-    /* clean up host side */
-    drive_uninit(drive_get_by_blockdev(bs));
-
     return 0;
 }