CVE-2023-1998

The Linux kernel allows userspace processes to enable mitigations by
calling prctl with PR_SET_SPECULATION_CTRL which disables the speculation
feature as well as by using seccomp. We had noticed that on VMs of at least
one major cloud provider, the kernel still left the victim process exposed
to attacks in some cases even after enabling the spectre-BTI mitigation
with prctl. The same behavior can be observed on a bare-metal machine when
forcing the mitigation to IBRS on boot command line. This happened because
when plain IBRS was enabled (not enhanced IBRS), the kernel had some logic
that determined that STIBP was not needed. The IBRS bit implicitly protects
against cross-thread branch target injection. However, with legacy IBRS,
the IBRS bit was cleared on returning to userspace, due to performance
reasons, which disabled the implicit STIBP and left userspace threads
vulnerable to cross-thread branch target injection against which STIBP
protects.

Bugs

• <https://bugzilla.redhat.com/show_bug.cgi?id=2187257&gt;
• <https://bugzilla.suse.com/show_bug.cgi?id=1210506&gt;