---
id: CVE-2026-99016
title: 'riscv: futex: untag the user pointer before the atomic access'
summary: |-
  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_inato…
severity: none
vendor: Linux
product: Linux
affected:
  - >-
    Linux >= 2e1743085887ba3f553c2bb472a75a3ff744b242 <
    2e5c80bbc062a8dda33e687ebf7be8f074ee4f7d
  - >-
    Linux >= 2e1743085887ba3f553c2bb472a75a3ff744b242 <
    8e48d8d51ab37b7decfbff3e2844e027a52ad6ea
  - >-
    Linux >= 2e1743085887ba3f553c2bb472a75a3ff744b242 <
    405290be962b32eec842776fbe3bdf81f15b1676
  - Linux 6.13
published: '2026-10-23'
updated: '2026-10-23'
sourceUpdated: '2026-10-23T12:20:47.859Z'
source: CVEORG
sourceUrl: 'https://www.cve.org/CVERecord?id=CVE-2026-99016'
references:
  - url: 'https://git.kernel.org/stable/c/2e5c80bbc062a8dda33e687ebf7be8f074ee4f7d'
  - url: 'https://git.kernel.org/stable/c/8e48d8d51ab37b7decfbff3e2844e027a52ad6ea'
  - url: 'https://git.kernel.org/stable/c/405290be962b32eec842776fbe3bdf81f15b1676'
tags:
  - cve.org
ingestedAt: '2026-10-23T13:24:55.539Z'
---

## Overview

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.

## Affected

- `Linux >= 2e1743085887ba3f553c2bb472a75a3ff744b242 < 2e5c80bbc062a8dda33e687ebf7be8f074ee4f7d`
- `Linux >= 2e1743085887ba3f553c2bb472a75a3ff744b242 < 8e48d8d51ab37b7decfbff3e2844e027a52ad6ea`
- `Linux >= 2e1743085887ba3f553c2bb472a75a3ff744b242 < 405290be962b32eec842776fbe3bdf81f15b1676`
- `Linux 6.13`

## Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.