The ext4_fill_super function in fs/ext4/super.c in the Linux kernel before 2.6.39 does not properly initialize a certain error-report data structure, which allows local users to cause a denial of service (OOPS) by attempting to mount a crafted ext4 filesystem.
http://www.openwall.com/lists/oss-security/2011/06/24/4
http://ftp.osuosl.org/pub/linux/kernel/v2.6/ChangeLog-2.6.39