Per https://illumos.topicbox.com/groups/developer/T697a32b688807e56-Mf889eb5783e975446573b373/cve-2026-102916-18491-bhyve-rep-prefix-instr-emulation-fumbles-flags illumos would like to report the following CVE:

CVE-2026-102916 18491 bhyve: rep prefix instr emulation fumbles flags

This was discovered by Emily Albini, and fixed by iximeow, both of Oxide Computer.

Thank you, Dan McDonald, on behalf of illumos security

A reachable assertion in the illumos bhyve instruction emulator allows a guest to panic the host. When emulating a REP-prefixed MOVS or STOS instruction that accesses guest MMIO, vieemulatemovs() and vieemulatestos() in usr/src/uts/intel/io/vmm/vmminstructionemul.c do not clear the VIESREPEAT status flag on the final iteration. For MMIO regions emulated in the kernel (the local APIC, I/O APIC and HPET), the stale flag causes a VERIFY assertion in vieadvancepc() to fail, and the host panics. A privileged user within a guest VM can issue a REP MOVS or REP STOS instruction against the local APIC page to cause a denial of service of the host and every other guest running on it. The flaw has existed since 2020 (illumos-gate commit e0c0d44e), and affects any illumos distribution prior to illumos-gate commit 696ecf8d.