---
id: CVE-2026-43472
title: 'unshare: fix unshare_fs() handling'
summary: |-
  In the Linux kernel, the following vulnerability has been resolved:

  unshare: fix unshare_fs() handling

  There's an unpleasant corner case in unshare(2), when we have a
  CLONE_NEWNS in flags and current->fs hadn't been shared at all; in t…
severity: none
vendor: Linux
product: Linux
affected:
  - >-
    Linux >= 741a295130606143edbf9fc740f633dbc1e6225f <
    845bf3c6963a52096d0d3866e4a92db77a0c03d8
  - >-
    Linux >= 741a295130606143edbf9fc740f633dbc1e6225f <
    d3ffc8f13034af895531a02c30b1fe3a34b46432
  - >-
    Linux >= 741a295130606143edbf9fc740f633dbc1e6225f <
    d0d99f60538ddb4a62ccaac2168d8f448965f083
  - >-
    Linux >= 741a295130606143edbf9fc740f633dbc1e6225f <
    d7963d6997fea86a6def242ac36198b86655f912
  - >-
    Linux >= 741a295130606143edbf9fc740f633dbc1e6225f <
    aa9ebc084505fb26dd90f4d7a249045aad152043
  - >-
    Linux >= 741a295130606143edbf9fc740f633dbc1e6225f <
    af8f4be3b68ac8caa41c8e5ead0eeaf5e85e42d0
  - >-
    Linux >= 741a295130606143edbf9fc740f633dbc1e6225f <
    42e21e74061b0ebbd859839f81acf10efad02a27
  - >-
    Linux >= 741a295130606143edbf9fc740f633dbc1e6225f <
    6c4b2243cb6c0755159bd567130d5e12e7b10d9f
  - Linux 2.6.16
ssvc:
  exploitation: none
  automatable: 'no'
  technicalImpact: partial
  timestamp: '2026-11-06T15:39:35.857143Z'
published: '2026-05-08'
updated: '2026-11-06'
sourceUpdated: '2026-11-06T15:42:56.209Z'
source: CVEORG
sourceUrl: 'https://www.cve.org/CVERecord?id=CVE-2026-43472'
references:
  - url: 'https://git.kernel.org/stable/c/845bf3c6963a52096d0d3866e4a92db77a0c03d8'
  - url: 'https://git.kernel.org/stable/c/d3ffc8f13034af895531a02c30b1fe3a34b46432'
  - url: 'https://git.kernel.org/stable/c/d0d99f60538ddb4a62ccaac2168d8f448965f083'
  - url: 'https://git.kernel.org/stable/c/d7963d6997fea86a6def242ac36198b86655f912'
  - url: 'https://git.kernel.org/stable/c/aa9ebc084505fb26dd90f4d7a249045aad152043'
  - url: 'https://git.kernel.org/stable/c/af8f4be3b68ac8caa41c8e5ead0eeaf5e85e42d0'
  - url: 'https://git.kernel.org/stable/c/42e21e74061b0ebbd859839f81acf10efad02a27'
  - url: 'https://git.kernel.org/stable/c/6c4b2243cb6c0755159bd567130d5e12e7b10d9f'
tags:
  - cve.org
epss: 0.00114
epssPercentile: 0.01501
ingestedAt: '2026-11-06T15:54:03.903Z'
---

## Overview

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

unshare: fix unshare_fs() handling

There's an unpleasant corner case in unshare(2), when we have a
CLONE_NEWNS in flags and current->fs hadn't been shared at all; in that
case copy_mnt_ns() gets passed current->fs instead of a private copy,
which causes interesting warts in proof of correctness]

> I guess if private means fs->users == 1, the condition could still be true.

Unfortunately, it's worse than just a convoluted proof of correctness.
Consider the case when we have CLONE_NEWCGROUP in addition to CLONE_NEWNS
(and current->fs->users == 1).

We pass current->fs to copy_mnt_ns(), all right.  Suppose it succeeds and
flips current->fs->{pwd,root} to corresponding locations in the new namespace.
Now we proceed to copy_cgroup_ns(), which fails (e.g. with -ENOMEM).
We call put_mnt_ns() on the namespace created by copy_mnt_ns(), it's
destroyed and its mount tree is dissolved, but...  current->fs->root and
current->fs->pwd are both left pointing to now detached mounts.

They are pinning those, so it's not a UAF, but it leaves the calling
process with unshare(2) failing with -ENOMEM _and_ leaving it with
pwd and root on detached isolated mounts.  The last part is clearly a bug.

There is other fun related to that mess (races with pivot_root(), including
the one between pivot_root() and fork(), of all things), but this one
is easy to isolate and fix - treat CLONE_NEWNS as "allocate a new
fs_struct even if it hadn't been shared in the first place".  Sure, we could
go for something like "if both CLONE_NEWNS *and* one of the things that might
end up failing after copy_mnt_ns() call in create_new_namespaces() are set,
force allocation of new fs_struct", but let's keep it simple - the cost
of copy_fs_struct() is trivial.

Another benefit is that copy_mnt_ns() with CLONE_NEWNS *always* gets
a freshly allocated fs_struct, yet to be attached to anything.  That
seriously simplifies the analysis...

FWIW, that bug had been there since the introduction of unshare(2) ;-/

## Affected

- `Linux >= 741a295130606143edbf9fc740f633dbc1e6225f < 845bf3c6963a52096d0d3866e4a92db77a0c03d8`
- `Linux >= 741a295130606143edbf9fc740f633dbc1e6225f < d3ffc8f13034af895531a02c30b1fe3a34b46432`
- `Linux >= 741a295130606143edbf9fc740f633dbc1e6225f < d0d99f60538ddb4a62ccaac2168d8f448965f083`
- `Linux >= 741a295130606143edbf9fc740f633dbc1e6225f < d7963d6997fea86a6def242ac36198b86655f912`
- `Linux >= 741a295130606143edbf9fc740f633dbc1e6225f < aa9ebc084505fb26dd90f4d7a249045aad152043`
- `Linux >= 741a295130606143edbf9fc740f633dbc1e6225f < af8f4be3b68ac8caa41c8e5ead0eeaf5e85e42d0`
- `Linux >= 741a295130606143edbf9fc740f633dbc1e6225f < 42e21e74061b0ebbd859839f81acf10efad02a27`
- `Linux >= 741a295130606143edbf9fc740f633dbc1e6225f < 6c4b2243cb6c0755159bd567130d5e12e7b10d9f`
- `Linux 2.6.16`

## Remediation

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