# CVE-2024-42240: x86/bhi: Avoid warning in #DB handler due to BHI mitigation

> A local attacker can trigger a kernel warning and potential crash by exploiting a flaw in the BHI mitigation's single-step handling during SYSENTER execution.

- **Severity:** Medium 5.5 (`CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H`)
- **Exploited in the wild:** Not known
- **Login needed:** Any signed-in user
- **Attackable:** Local or same network only
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 7 Aug 2024
- **Updated:** 23 May 2026
- **Also known as:** AZL-47528, AZL-47791, BELL-CVE-2024-42240, DEBIAN-CVE-2024-42240, GHSA-jxv8-jmp2-87p8, UBUNTU-CVE-2024-42240

## What is CVE-2024-42240?

CVE-2024-42240 is a medium-severity denial of service vulnerability in Linux kernel, published on 7 Aug 2024. It can only be exploited from the same machine or local network, with no action from the victim.

### Official description

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

x86/bhi: Avoid warning in #DB handler due to BHI mitigation

When BHI mitigation is enabled, if SYSENTER is invoked with the TF flag set
then entry_SYSENTER_compat() uses CLEAR_BRANCH_HISTORY and calls the
clear_bhb_loop() before the TF flag is cleared. This causes the #DB handler
(exc_debug_kernel()) to issue a warning because single-step is used outside the
entry_SYSENTER_compat() function.

To address this issue, entry_SYSENTER_compat() should use CLEAR_BRANCH_HISTORY
after making sure the TF flag is cleared.

The problem can be reproduced with the following sequence:

  $ cat sysenter_step.c
  int main()
  { asm("pushf; pop %ax; bts $8,%ax; push %ax; popf; sysenter"); }

  $ gcc -o sysenter_step sysenter_step.c

  $ ./sysenter_step
  Segmentation fault (core dumped)

The program is expected to crash, and the #DB handler will issue a warning.

Kernel log:

  WARNING: CPU: 27 PID: 7000 at arch/x86/kernel/traps.c:1009 exc_debug_kernel+0xd2/0x160
  ...
  RIP: 0010:exc_debug_kernel+0xd2/0x160
  ...
  Call Trace:
  <#DB>
   ? show_regs+0x68/0x80
   ? __warn+0x8c/0x140
   ? exc_debug_kernel+0xd2/0x160
   ? report_bug+0x175/0x1a0
   ? handle_bug+0x44/0x90
   ? exc_invalid_op+0x1c/0x70
   ? asm_exc_invalid_op+0x1f/0x30
   ? exc_debug_kernel+0xd2/0x160
   exc_debug+0x43/0x50
   asm_exc_debug+0x1e/0x40
  RIP: 0010:clear_bhb_loop+0x0/0xb0
  ...
  </#DB>
  <TASK>
   ? entry_SYSENTER_compat_after_hwframe+0x6e/0x8d
  </TASK>

  [ bp: Massage commit message. ]

Weakness: CWE-835 (Loop with Unreachable Exit Condition)

## Which versions are affected by CVE-2024-42240?

It affects 1 product.


Products: Linux kernel.

## How do I fix CVE-2024-42240?

Apply the vendor's patch.

## Is CVE-2024-42240 being exploited?

There is no known exploitation of CVE-2024-42240 in the wild and no public exploit code.

## Other vulnerabilities in Linux kernel

- [CVE-2026-53266](https://insights.sbomify.com/CVE-2026-53266/): Linux Kernel Out-of-Bounds Write (high 8.8, exploited in the wild)
- [CVE-2013-6282](https://insights.sbomify.com/CVE-2013-6282/): Linux Kernel Improper Input Validation (high 8.8, exploited in the wild)
- [CVE-2022-0185](https://insights.sbomify.com/CVE-2022-0185/): Linux Kernel Heap-Based Buffer Overflow (high 8.4, exploited in the wild)
- [CVE-2013-2094](https://insights.sbomify.com/CVE-2013-2094/): Linux Kernel Privilege Escalation (high 8.4, exploited in the wild)
- [CVE-2026-53362](https://insights.sbomify.com/CVE-2026-53362/): Linux Kernel Unspecified (high 7.8, exploited in the wild)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2024-42240 and fix it if it is affected.

What it is: CVE-2024-42240 is a medium-severity denial of service vulnerability in Linux kernel, published on 7 Aug 2024. It can only be exploited from the same machine or local network, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2024-42240 in the wild and no public exploit code.

Affected product: Linux kernel. No package-level version data is available, so check the vendor advisory linked from the reference page.

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2024-42240/ (Markdown: https://insights.sbomify.com/CVE-2024-42240/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2024-42240
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2024-42240
- OSV: https://osv.dev/vulnerability/CVE-2024-42240

---
From sbomify Insights: https://insights.sbomify.com/CVE-2024-42240/
Summaries are generated and may contain mistakes; the official description above is the record.