CVE-2023-1998

What it actually does

The Linux kernel incorrectly clears the IBRS bit on return to userspace when legacy IBRS is active, disabling the implicit cross-thread protection that STIBP would otherwise provide and leaving processes that enabled Spectre-BTI mitigation via prctl(PR_SET_SPECULATION_CTRL) still exposed to branch target injection from sibling threads.

Why it matters to the business

Organizations running affected kernels on cloud VMs or bare-metal systems that rely on legacy IBRS may experience reduced isolation between processes, increasing the risk of information leakage across trust boundaries in multi-tenant or shared-host environments.

How an attack unfolds

An attacker with the ability to run code on the same physical core as a victim process first co-locates with the target, then uses timing or side-channel techniques to perform cross-thread branch target injection against the victim after the kernel has cleared the IBRS bit on userspace return, bypassing the mitigation the victim enabled with prctl.

How to fix it

Upgrade the Linux kernel to commit 457391b0380335d5e9a5babdec90ac53928b23b4 or later; alternatively, enable enhanced IBRS via the kernel command line or rely on eIBRS-capable hardware and updated microcode.

How to confirm the fix worked

Confirm the running kernel contains commit 457391b0380335d5e9a5babdec90ac53928b23b4, verify that prctl(PR_SET_SPECULATION_CTRL) no longer results in IBRS being cleared on userspace return, and test with spectre-BTI validation tools on both bare-metal and virtualized systems.

Typical attack path

1. Attacker obtains code execution on a co-located thread or VM on the same host
2. Victim process enables Spectre-BTI mitigation via prctl or seccomp
3. Kernel returns to userspace and clears the legacy IBRS bit
4. Attacker performs branch target injection to leak data from the victim
5. Data exfiltration completes without triggering the expected mitigation

How to detect it

Examine kernel logs for prctl speculation control calls, review process creation and scheduling events on shared cores, and look for unexpected cross-thread branch predictor activity in performance counter data on systems configured with legacy IBRS.

What your security team should do

• Prioritize kernel upgrades on multi-tenant hosts and cloud instances using legacy IBRS
• Audit boot parameters to prefer enhanced IBRS where hardware supports it
• Monitor for new kernel releases that backport the fix to supported distribution branches