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Fun with NULL pointers, part 2

Jul 31, 2009, 09:02 (1 Talkback[s])
(Other stories by Jonathan Corbet)

"One obvious problem is that when the security module mechanism is configured into the kernel, the administrator-specified limits on the lowest valid user-space virtual address are ignored security modules are allowed to override the administrator-specified limit (mmap_min_addr) on the lowest valid user-space address. This behavior is a violation of the understanding by which security modules operate: they are supposed to be able to restrict privileges, but never increase them. In this case, the mere presence of SELinux increased privilege, and the policy enforced by most SELinux deployments failed to close that hole (comments in the exploit code suggest that AppArmor fared no better).

"Additionally, with security modules configured out entirely, mmap_min_addr was not enforced at all. The mainline now has a patch which causes the map_min_addr sysctl knob to always be in effect; this patch has also been put into the 2.6.27.27 and 2.6.30.2 updates (as have many of the others described here).

"Things are also being fixed at the SELinux level. Future versions of Red Hat's SELinux policy will no longer allow unconfined (but otherwise unprivileged) processes to map pages into the bottom of the address space. There are still some open problems, though, especially when programs like WINE are thrown into the mix. It's not yet clear how the system can securely support a small number of programs needing the ability to map the zero page. Ideas like running WINE with root privilege - thus, perhaps, carrying Windows-like behavior a little too far - have garnered little enthusiasm."

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