The btrfs_xattr_set_acl function in fs/btrfs/acl.c in btrfs in the Linux kernel 2.6.34 and earlier does not check file ownership before setting an ACL, which allows local users to bypass file permissions by setting arbitrary ACLs, as demonstrated using setfacl.
http://www.openwall.com/lists/oss-security/2010/06/14/2
http://www.openwall.com/lists/oss-security/2010/06/11/3
http://git.kernel.org/?p=linux/kernel/git/torvalds/linux-2.6.git%3Ba=commitdiff%3Bh=2f26afba