A flaw was found in the way Linux kernel's device-mapper subsystem, under certain conditions, interpreted data written to snapshot block devices. Snapshots are constructed from a single "cow" (copy-on-write) device that contains a mixture of data and metadata, and the bug involves a user writing a data block that is later incorrectly interpreted as metadata controlling how blocks are mapped.
An attacker could construct a mapping to read data from disk blocks in 'free space' that is normally inaccessible.
Please note that apart from having security consequences (data leak), this bug is also a data corruptor.
Acknowledgements:
Red Hat would like to thank Fujitsu for reporting this issue.
Other sources
Interpretation conflict in drivers/md/dm-snap-persistent.c in the Linux kernel through 3.11.6 allows remote authenticated users to obtain sensitive information or modify data via a crafted mapping to a snapshot block device.
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