A use-after-free flaw was found in fs/userfaultfd.c in the Linux kernel before 4.13.6. The issue is related to the handling of fork failure when dealing with event messages. Failure to fork correctly can lead to a situation where a fork event will be removed from an already freed list of events with userfaultfd_ctx_put().
https://www.kernel.org/pub/linux/kernel/v4.x/ChangeLog-4.13.6
https://github.com/torvalds/linux/commit/384632e67e0829deb8015ee6ad916b180049d252
https://bugzilla.redhat.com/show_bug.cgi?id=1523481
https://access.redhat.com/security/cve/CVE-2017-15126
https://access.redhat.com/errata/RHSA-2018:1062
https://access.redhat.com/errata/RHSA-2018:0676