# CVE-2026-99016: riscv: futex: untag the user pointer before the atomic access


- **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
- **Fix available:** Yes
- **Published:** 23 Oct 2026
- **Updated:** 24 Oct 2026
- **Also known as:** BELL-CVE-2026-99016, DEBIAN-CVE-2026-99016, GHSA-5mv4-wx47-m6wh, UBUNTU-CVE-2026-99016

## What is CVE-2026-99016?

CVE-2026-99016 is a high-severity vulnerability in Linux, published on 23 Oct 2026. 

### Official description

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

riscv: futex: untag the user pointer before the atomic access

access_ok() checks untagged_addr(uaddr), but arch_futex_atomic_op_inuser()
and futex_atomic_cmpxchg_inatomic() hand the raw uaddr to the inline asm
amoswap/amoadd/.../lr.w+sc.w through the "+m" (*uaddr) operand.  When the
tagged address ABI is enabled (CONFIG_RISCV_ISA_SUPM, prctl
PR_SET_TAGGED_ADDR_CTRL with PMLEN != 0), those two addresses differ: a
user pointer whose top PMLEN bits hold a tag passes access_ok() after
untagging, but the atomic is then performed on the still-tagged raw
address.

Supervisor-mode data accesses are not subject to the U-mode pointer
masking (that is governed by menvcfg.PMM, which the kernel does not set),
so hardware does not strip the tag for the kernel's own access.  With
Sv57 and PMLEN=16 the tag bits overlap the canonical-address bits, so a
tagged pointer can name a canonical kernel virtual address (e.g. in the
linear map) whose untagged form is a valid user address.  An unprivileged
process can thus make FUTEX_WAKE_OP perform an atomic read-modify-write on
an arbitrary kernel address, with the matching FUTEX_OP_CMP_* result
serving as a read oracle.

get_user()/put_user()/raw_copy_{to,from}_user() already untag the pointer
after the access_ok() check; do the same in the futex helpers so the
atomic operates on the address that was actually validated.

## Which versions are affected by CVE-2026-99016?


## How do I fix CVE-2026-99016?

Upgrade to a fixed version.

## Is CVE-2026-99016 being exploited?

There is no known exploitation of CVE-2026-99016 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-99016 and fix it if it is affected.

What it is: CVE-2026-99016 is a high-severity vulnerability in Linux, published on 23 Oct 2026. 
Exploitation: There is no known exploitation of CVE-2026-99016 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-2026-99016/ (Markdown: https://insights.sbomify.com/CVE-2026-99016/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-99016
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-99016
- OSV: https://osv.dev/vulnerability/CVE-2026-99016

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-99016/
Summaries are generated and may contain mistakes; the official description above is the record.