The __tun_chr_ioctl function in drivers/net/tun.c in the Linux kernel before 3.6 does not initialize a certain structure, which allows local users to obtain sensitive information from kernel stack memory via a crafted application.
https://www.kernel.org/pub/linux/kernel/v3.x/patch-3.6.bz2
https://github.com/torvalds/linux/commit/a117dacde0288f3ec60b6e5bcedae8fa37ee0dfc
http://www.openwall.com/lists/oss-security/2013/03/05/13
http://www.mandriva.com/security/advisories?name=MDVSA-2013:176