The Btrfs implementation in the Linux kernel before 3.19 does not ensure that the visible xattr state is consistent with a requested replacement, which allows local users to bypass intended ACL settings and gain privileges via standard filesystem operations (1) during an xattr-replacement time window, related to a race condition, or (2) after an xattr-replacement attempt that fails because the data does not fit.
git.kernel.org/?p=linux/kernel/git/torvalds/linux-2.6.git%3Ba=commit%3Bh=5f5bc6b1e2d5a6f827bc860ef2dc5b6f365d1339
lists.opensuse.org/opensuse-security-announce/2015-07/msg00023.html
lists.opensuse.org/opensuse-security-announce/2015-09/msg00009.html
www.openwall.com/lists/oss-security/2015/03/24/11
www.securitytracker.com/id/1032418
bugzilla.redhat.com/show_bug.cgi?id=1205079
github.com/torvalds/linux/commit/5f5bc6b1e2d5a6f827bc860ef2dc5b6f365d1339