It was found that if an NMI occurred immediately after a SYSCALL or before a SYSRET with the user RSP pointing to the NMI IST stack, the kernel could skip that NMI.
Red Hat would like to thank Andy Lutomirski for reporting this issue.
This issue does not affect the Linux kernel packages as shipped with Red Hat Enterprise Linux 5 and 6 as they did not backport the nested NMI handler functionality.
This issue does affect the Linux kernel packages as shipped with Red Hat Enterprise Linux 7 and Red Hat Enterprise MRG 2. This has been rated as having Low security impact and is not currently planned to be addressed in future updates.
Created kernel tracking bugs for this issue:
Affects: fedora-all [bug 1245927]