The inode_init_owner function in fs/inode.c in the Linux kernel through 3.16 allows local users to create files with an unintended group ownership, in a scenario where a directory is SGID to a certain group and is writable by a user who is not a member of that group. Here, the non-member can trigger creation of a plain file whose group ownership is that group. The intended behavior was that the non-member can trigger creation of a directory (but not a plain file) whose group ownership is that group. The non-member can escalate privileges by making the plain file executable and SGID.
git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=0fa3ecd87848c9c93c2c828ef4c3a8ca36ce46c7
openwall.com/lists/oss-security/2018/07/13/2
www.securityfocus.com/bid/106503
access.redhat.com/errata/RHSA-2018:2948
access.redhat.com/errata/RHSA-2018:3083
access.redhat.com/errata/RHSA-2018:3096
access.redhat.com/errata/RHSA-2019:0717
access.redhat.com/errata/RHSA-2019:2476
access.redhat.com/errata/RHSA-2019:2566
access.redhat.com/errata/RHSA-2019:2696
access.redhat.com/errata/RHSA-2019:2730
access.redhat.com/errata/RHSA-2019:4159
access.redhat.com/errata/RHSA-2019:4164
git.kernel.org/pub/scm/linux/kernel/git/tip/tip.git/commit/?id=0b3369840cd61c23e2b9241093737b4c395cb406
github.com/torvalds/linux/commit/0fa3ecd87848c9c93c2c828ef4c3a8ca36ce46c7
lists.debian.org/debian-lts-announce/2018/08/msg00014.html
lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/HRBNBX73SAFKQWBOX76SLMWPTKJPVGEJ/
lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/MTKKIAUMR5FAYLZ7HLEPOXMKAAE3BYBQ/
support.f5.com/csp/article/K00854051
twitter.com/grsecurity/status/1015082951204327425
usn.ubuntu.com/3752-1/
usn.ubuntu.com/3752-2/
usn.ubuntu.com/3752-3/
usn.ubuntu.com/3753-1/
usn.ubuntu.com/3753-2/
usn.ubuntu.com/3754-1/
www.debian.org/security/2018/dsa-4266
www.exploit-db.com/exploits/45033/