# CVE-2026-89581: bpf, x86: Fix per-CPU address resolution into an extended register

> A Linux kernel BPF JIT bug causes incorrect register usage for per-CPU addresses, leading to kernel panics and potential memory corruption.

- **Severity:** High 7.8 (`CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/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:** 11 Sep 2026
- **Updated:** 19 Oct 2026
- **Also known as:** BELL-CVE-2026-89581, DEBIAN-CVE-2026-89581, GHSA-vx2x-82ww-5r7r, UBUNTU-CVE-2026-89581

## What is CVE-2026-89581?

CVE-2026-89581 is a high-severity memory corruption vulnerability in Linux, published on 11 Sep 2026. 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:

bpf, x86: Fix per-CPU address resolution into an extended register

The destination of the per-CPU address MOV is encoded in ModRM.reg,
which is extended by REX.R, but the REX prefix is built with
add_1mod(), which sets REX.B. REX.B extends ModRM.rm and SIB.base, and
this instruction addresses memory as disp32 with no base, so the bit
has no effect at all and the high register bit is simply lost.

Every is_ereg() destination therefore resolves to the wrong register,
picking whichever one shares the low three bits:

  R5 -> RAX    R7 -> RBP    R8 -> RSI    R9 -> RDI

With BPF_REG_5, whose reg2hex is 0, the emitted

  65 49 03 04 25 <off>	add %gs:<off>,%rax

adds the per-CPU offset to RAX rather than R8. The destination keeps
the unadjusted address and RAX is clobbered, so the program goes on to
dereference a pointer that was never made per-CPU:

  BUG: unable to handle page fault for address: 0000607e386a8894
  RIP: bpf_prog_707837aafd2aa9ae_update_percpu_data+0x93/0xc9
  Call Trace:
   __bpf_prog_test_run_raw_tp+0x2dc/0x7d0
   __flush_smp_call_function_queue+0x1e9/0xc80
  Kernel panic - not syncing: Fatal exception in interrupt

R5 is the mildest of the four, aliasing a scratch register and faulting
at the store. R7 aliases RBP and would corrupt the frame pointer, R8
and R9 alias the argument registers.

Use add_2mod() so the register goes through REX.R, matching how
add_2reg() places it in ModRM.reg and how emit_priv_frame_ptr()
hardcodes 0x4c for the same instruction with R9. Encodings for the
non-extended registers are unchanged.

Problem showed up when trying to resurrect BPF_GCC CI (selftests built
with BPF_GCC).

This has gone unnoticed because clang reloads the address into R1
before each per-CPU access, so the destination is never an extended
register. GCC keeps several per-CPU addresses live at once, and
test_progs-bpf_gcc panics the kernel in global_percpu_data/init, where
the address of a .percpu variable ends up in R5.

## Which versions are affected by CVE-2026-89581?


## How do I fix CVE-2026-89581?

Apply the vendor's patch.

## Is CVE-2026-89581 being exploited?

There is no known exploitation of CVE-2026-89581 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-89581 and fix it if it is affected.

What it is: CVE-2026-89581 is a high-severity memory corruption vulnerability in Linux, published on 11 Sep 2026. 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-2026-89581 in the wild and no public exploit code.

Affected product: Linux. 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-2026-89581/ (Markdown: https://insights.sbomify.com/CVE-2026-89581/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-89581
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-89581
- OSV: https://osv.dev/vulnerability/CVE-2026-89581

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-89581/
Summaries are generated and may contain mistakes; the official description above is the record.