From secunia security advisory at $URL: Description: An error within the implementation of the ext4 file system when splitting extents can be exploited to cause a "BUG_ON()". The vulnerability is reported in version 2.6.39.4. Other versions may also be affected. Solution: Fixed in the GIT repository. http://git.kernel.org/?p=linux/kernel/git/torvalds/linux-2.6.git;a=commitdiff;h=667eff35a1f56fa74ce98a0c7c29a40adc1ba4e3
CVE-2011-3638 (http://nvd.nist.gov/nvd.cfm?cvename=CVE-2011-3638): fs/ext4/extents.c in the Linux kernel before 3.0 does not mark a modified extent as dirty in certain cases of extent splitting, which allows local users to cause a denial of service (system crash) via vectors involving ext4 umount and mount operations.
There are no longer any 2.x kernels available in the repository with the exception of sys-kernel/xbox-sources which is unsupported by security.