[Qemu-devel] Possible corruption on qcow2_snapshot_goto

[Frediano Ziglio]

This function seems to be not that safe.

The first problem is that the first thing it does if free refcount
calling qcow2_update_snapshot_refcount(bs, s->l1_table_offset,
s->l1_size, -1). Now if something goes wrong (crash, disk errors) this
could lead to refcount == 0 (depending on L1 size, cache, open flags).

If table grow and qcow2_grow_l1_table needs to resize on disk it
probably will found some reference counter set to 0 (cause we already
decremented it), fail than qcow2_snapshot_goto return -EIO leaving
refcounts deallocated.

Frediano