The spectre_v2_select_mitigation function in arch/x86/kernel/cpu/bugs.c in the Linux kernel before 4.18.1 does not always fill RSB upon a context switch, which makes it easier for attackers to conduct userspace-userspace spectreRSB attacks.
git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=fdf82a7856b32d905c39afc85e34364491e46346
cdn.kernel.org/pub/linux/kernel/v4.x/ChangeLog-4.18.1
github.com/torvalds/linux/commit/fdf82a7856b32d905c39afc85e34364491e46346
lists.debian.org/debian-lts-announce/2018/10/msg00003.html
usn.ubuntu.com/3775-1/
usn.ubuntu.com/3775-2/
usn.ubuntu.com/3776-1/
usn.ubuntu.com/3776-2/
usn.ubuntu.com/3777-1/
usn.ubuntu.com/3777-2/
usn.ubuntu.com/3777-3/
www.debian.org/security/2018/dsa-4308