The Linux kernel through 3.17.4 does not properly restrict dropping of supplemental group memberships in certain namespace scenarios, which allows local users to bypass intended file permissions by leveraging a POSIX ACL containing an entry for the group category that is more restrictive than the entry for the other category, aka a "negative groups" issue, related to kernel/groups.c, kernel/uid16.c, and kernel/user_namespace.c.

References

• https://nvd.nist.gov/vuln/detail/CVE-2014-8989
• http://lists.fedoraproject.org/pipermail/package-announce/2015-January/147864.html
• http://lists.fedoraproject.org/pipermail/package-announce/2015-January/147973.html
• http://thread.gmane.org/gmane.linux.man/7385/
• http://www.mandriva.com/security/advisories?name=MDVSA-2015:058
• http://www.openwall.com/lists/oss-security/2014/11/20/4
• http://www.securityfocus.com/bid/71154
• http://www.ubuntu.com/usn/USN-2515-1
• http://www.ubuntu.com/usn/USN-2516-1
• http://www.ubuntu.com/usn/USN-2517-1
• http://www.ubuntu.com/usn/USN-2518-1