CVE-2023-1998: Spectre v2 SMT mitigations problem in Linux kernel

Linux Kernel could allow a local authenticated attacker to obtain sensitive information, caused by a flaw in the spectrev2userselectmitigation function in arch/x86/kernel/cpu/bugs.c in the Spectre Mitigation component . By sending a specially crafted request, an attacker could exploit this vulnerability to obtain sensitive information, and use this information to launch further attacks against the affected system.

— IBM

The Linux kernel allows userspace processes to enable mitigations by calling prctl with PRSETSPECULATIONCTRL which disables the speculation feature as well as by using seccomp [..] The kernel failed to protect applications that attempted to protect against Spectre v2 leaving them open to attack from other processes running on the same physical core in another hyperthread.

Reference: https://github.com/google/security-research/security/advisories/GHSA-mj4w-6495-6crx

Upstream fix: https://github.com/torvalds/linux/commit/6921ed9049bc7457f66c1596c5b78aec0dae4a9d

— Red Hat

The Linux kernel allows userspace processes to enable mitigations by calling prctl with PRSETSPECULATIONCTRL 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.

— MITRE