---
id: "CVE-2026-99016"
kind: "vulnerability"
aliases: ["CVE-2026-99016"]
title: "Linux kernel RISC-V futex: tagged pointer lets unprivileged atomic writes to kernel memory"
severity: "high"
cvss: null
epss: 0.00206
exploited: false
status: "open"
category: "runtime"
products: []
ecosystems: []
tags: ["kernel", "linux", "riscv", "runtime", "self-hosted", "shared-hosts"]
not_affected_if: ["default-config", "non-riscv", "tagged-address-abi-disabled"]
published_at: "2026-10-23T13:17:24+00:00"
first_seen_at: "2026-10-23T21:57:33+00:00"
updated_at: "2026-10-25T12:38:50+00:00"
sources: [{"source": "nvd", "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-99016", "first_seen_at": "2026-10-23T21:57:33+00:00"}, {"source": "epss", "url": "https://api.first.org/data/v1/epss?cve=CVE-2026-99016", "first_seen_at": "2026-10-25T12:38:50+00:00"}]
---

# Linux kernel RISC-V futex: tagged pointer lets unprivileged atomic writes to kernel memory

On RISC-V kernels with tagged address support, the futex helpers perform atomic operations on a raw tagged user pointer instead of the untagged address that passed the access check. A local unprivileged process can make FUTEX_WAKE_OP perform read-modify-write operations on an arbitrary kernel address and read results through the comparison outcome.

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

## How to check

- Check the kernel version with `uname -r` and whether the machine is RISC-V with `uname -m`.
- Check whether tagged address support is built in with `grep CONFIG_RISCV_ISA_SUPM /boot/config-$(uname -r)`.
- Check whether the kernel was built with a fix by comparing against the vendor's kernel changelog for this CVE.

## Fix

- Update the Linux kernel to a release that includes the upstream riscv futex untag fix for CVE-2026-99016; the exact fixed version is unknown from the sources.
- Reboot into the patched kernel and confirm with `uname -r`.

## Timeline

- 2026-10-23T21:57:33+00:00 nvd https://nvd.nist.gov/vuln/detail/CVE-2026-99016
- 2026-10-25T12:38:50+00:00 epss https://api.first.org/data/v1/epss?cve=CVE-2026-99016

## From the sources

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.

---

_Free to read at a human pace. An API with keys and an MCP server are in preparation; early access: hello@infrastack.ai_