The btrfs_ioctl_clone function in fs/btrfs/ioctl.c in the btrfs functionality in the Linux kernel 2.6.29 through 2.6.32, and possibly other versions, does not ensure that a cloned file descriptor has been opened for reading, which allows local users to read sensitive information from a write-only file descriptor.
https://euvd.enisa.europa.eu/vulnerability/EUVD-2010-1657
https://bugzilla.redhat.com/show_bug.cgi?id=593226
https://bugs.launchpad.net/ubuntu/+source/linux/+bug/579585
http://www.openwall.com/lists/oss-security/2010/05/25/8
http://www.openwall.com/lists/oss-security/2010/05/18/2