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.
Exploitability
AV:LAC:LPR:LUI:NScope
S:CImpact
C:HI:NA:N6.5/CVSS:3.0/AV:L/AC:L/PR:L/UI:N/S:C/C:H/I:N/A:N