A flaw was found in the way Linux kernel's nested NMI handler and espfix64 functionalities interacted during NMI processing. A local, unprivileged user could use this flaw to crash the system or, potentially, escalate their privileges on the system. Upstream fix: https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=9b6e6a8334d56354853f9c255d1395c2ba570e0a Acknowledgements: Red Hat would like to thank Andy Lutomirski for reporting this issue.
In order to exploit this issue non-root (non-privileged) user needs to make the Linux kernel's NMI handler perform an iret instruction, which re-enables NMIs and thus the nested NMI code path in the NMI handler is exercised. To our knowledge, an unprivileged local user can do that since upstream commit https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=e00b12e64be9a34ef071de7b6052ca9ea29dd460.
Statement: This issue does not affect the Linux kernel packages as shipped with Red Hat Enterprise Linux 5 and 6 since they did not backport the nested NMI handler and espfix64 functionalities. This issue does not affect the Linux kernel packages as shipped with Red Hat Enterprise Linux 7 and Red Hat Enterprise MRG 2 since they did not backport the espfix64 functionality and also did not backport upstream commit e00b12e64be9a3 that allowed an unprivileged local user to re-enable NMIs from the NMI handler.
Created kernel tracking bugs for this issue: Affects: fedora-all [bug 1245936]