---
id: "CVE-2026-99186"
kind: "vulnerability"
aliases: ["CVE-2026-99186"]
title: "Linux kernel: segment override bug lets any user hang CPUs"
severity: "high"
cvss: null
epss: null
exploited: false
status: "open"
category: "infrastructure"
products: []
ecosystems: []
tags: ["infrastructure", "linux-kernel", "multi-user-hosts", "self-hosted", "x86"]
not_affected_if: ["non-x86-hardware", "patched-kernel"]
published_at: "2026-10-28T11:17:39+00:00"
first_seen_at: "2026-10-28T11:32:37+00:00"
updated_at: "2026-10-28T11:35:20+00:00"
sources: [{"source": "nvd", "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-99186", "first_seen_at": "2026-10-28T11:32:37+00:00"}]
---

# Linux kernel: segment override bug lets any user hang CPUs

In the x86 instruction emulation used for UMIP, a loop counter collision makes SGDT, SIDT and SMSW with a CS prefix spin forever in kernel mode. Any local user can tie up CPUs until reboot, and FS or GS prefixed operands are written to the wrong address.

_Summary, check and fix written by a model from the sources; labelled as such._

## How to check

- `uname -r` and compare against the kernel version that contains the upstream fix commit, unknown from the sources
- `grep -i umip /proc/cpuinfo` to see whether the CPU exposes UMIP, which is when the affected emulation path is reachable

## Fix

- Update the Linux kernel to a release that includes the upstream commit renaming the for_each_insn_prefix() iterator in get_seg_reg_override_idx(); exact fixed version unknown from the sources
- Reboot into the patched kernel, since the bug needs a kernel-level fix

## Timeline

- 2026-10-28T11:32:37+00:00 nvd https://nvd.nist.gov/vuln/detail/CVE-2026-99186

## From the sources

In the Linux kernel, the following vulnerability has been resolved:

x86/insn-eval: Fix shadowed segment index in get_seg_reg_override_idx()

Commit 45e1dccc0653 ("x86/insn: Simplify for_each_insn_prefix()") made
for_each_insn_prefix() declare its own loop counter named idx.
get_seg_reg_override_idx() keeps its result in a local of the same name,
so the assignments in the loop body now write the loop counter:

 - the function always returns INAT_SEG_REG_DEFAULT, so FS and GS
   overrides are ignored;

 - a CS override sets the counter to INAT_SEG_REG_CS (2), the increment
   makes it 3, and prefixes.bytes[3] holds the last prefix, 0x2e again,
   so the loop never terminates.

The UMIP emulation reaches this from the #GP handler for user space: an
SGDT, SIDT or SMSW with a memory operand and a CS prefix leaves the task
spinning in the kernel, never returning to user space, with a SIGKILL
that is never delivered.  Any user can thus tie up CPUs until reboot.
With an FS or GS prefix, the emulated result is written to the wrong
address.

Rename the internal macro iterator variable.

[ Lukas' original patch renamed the use site, I changed it to rename the
  iterator in the macro instead, but left Lukas' commit message and
  authorship   - Linus ]

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