Most VM places are using pmd_none but a few are still using pmd_present. The meaning is about the same for the pmd. However pmd_present would return the wrong value on PROT_NONE ranges. When the code using pmd_present gets a false negative, the kernel will crash. An unprivileged local user could use this flaw to crash the system.
This issue did affect the version of Linux kernel as shipped with Red Hat Enterprise Linux 6.
This issue did not affect the version of Linux kernel as shipped with Red Hat Enterprise Linux 5 and Red Hat Enterprise MRG 2.
This issue has been addressed in following products:
Red Hat Enterprise Linux 6
Via RHSA-2013:0496 https://rhn.redhat.com/errata/RHSA-2013-0496.html