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A bug in rarely used code path in the refcount table growth code of qcow2 can lead to inconsistencies in the metadata, which can cause real data corruption later on.

This bug can be reproduced like this:

[rhel6/master] kwolf@dhcp-5-188:~/source/qemu-kvm-rhel6$ ./qemu-img create -f qcow2 -ocluster_size=512 /tmp/test.qcow2 1G
Formatting '/tmp/test.qcow2', fmt=qcow2 size=1073741824 encryption=off cluster_size=512 
[rhel6/master] kwolf@dhcp-5-188:~/source/qemu-kvm-rhel6$ ./qemu-io /tmp/test.qcow2 
qemu-io> write -b 0 64M
wrote 67108864/67108864 bytes at offset 0
64 MiB, 1 ops; 0:00:11.00 (5.791 MiB/sec and 0.0905 ops/sec)
qemu-io> [rhel6/master] kwolf@dhcp-5-188:~/source/qemu-kvm-rhel6$ ./qemu-img check /tmp/test.qcow2 
ERROR cluster 768 refcount=0 reference=1
ERROR cluster 773 refcount=2 reference=1
ERROR cluster 774 refcount=2 reference=1
ERROR cluster 1024 refcount=0 reference=1
ERROR cluster 1029 refcount=2 reference=1
ERROR cluster 1030 refcount=2 reference=1
ERROR cluster 1031 refcount=2 reference=1
7 errors were found on the image.

Can reproduce the issue on qemu-kvm-0.12.1.2-2.73.el6, and verified pass on qemu-kvm-0.12.1.2-2.75.el6 with the given steps.

Close it according to Comment 4.