# CVE-2026-43472: unshare: fix unshare_fs() handling

Status: published. Published: 2026-05-08. Updated: 2026-11-06.

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) ;-/

## Remediation

- fix: The reporting source identifies this reference as a patch. Check the advisory for affected versions and supported release branches. (source: 416baaa9-dc9f-4396-8d5f-8c081fb06d67) — https://git.kernel.org/stable/c/42e21e74061b0ebbd859839f81acf10efad02a27
- fix: The reporting source identifies this reference as a patch. Check the advisory for affected versions and supported release branches. (source: 416baaa9-dc9f-4396-8d5f-8c081fb06d67) — https://git.kernel.org/stable/c/6c4b2243cb6c0755159bd567130d5e12e7b10d9f
- fix: The reporting source identifies this reference as a patch. Check the advisory for affected versions and supported release branches. (source: 416baaa9-dc9f-4396-8d5f-8c081fb06d67) — https://git.kernel.org/stable/c/845bf3c6963a52096d0d3866e4a92db77a0c03d8
- fix: The reporting source identifies this reference as a patch. Check the advisory for affected versions and supported release branches. (source: 416baaa9-dc9f-4396-8d5f-8c081fb06d67) — https://git.kernel.org/stable/c/aa9ebc084505fb26dd90f4d7a249045aad152043
- fix: The reporting source identifies this reference as a patch. Check the advisory for affected versions and supported release branches. (source: 416baaa9-dc9f-4396-8d5f-8c081fb06d67) — https://git.kernel.org/stable/c/af8f4be3b68ac8caa41c8e5ead0eeaf5e85e42d0
- fix: The reporting source identifies this reference as a patch. Check the advisory for affected versions and supported release branches. (source: 416baaa9-dc9f-4396-8d5f-8c081fb06d67) — https://git.kernel.org/stable/c/d0d99f60538ddb4a62ccaac2168d8f448965f083
- fix: The reporting source identifies this reference as a patch. Check the advisory for affected versions and supported release branches. (source: 416baaa9-dc9f-4396-8d5f-8c081fb06d67) — https://git.kernel.org/stable/c/d3ffc8f13034af895531a02c30b1fe3a34b46432
- fix: The reporting source identifies this reference as a patch. Check the advisory for affected versions and supported release branches. (source: 416baaa9-dc9f-4396-8d5f-8c081fb06d67) — https://git.kernel.org/stable/c/d7963d6997fea86a6def242ac36198b86655f912

## Affected products and versions

- Linux / Linux: ≥ 741a295130606143edbf9fc740f633dbc1e6225f; < 845bf3c6963a52096d0d3866e4a92db77a0c03d8 (affected, git); ≥ 741a295130606143edbf9fc740f633dbc1e6225f; < d3ffc8f13034af895531a02c30b1fe3a34b46432 (affected, git); ≥ 741a295130606143edbf9fc740f633dbc1e6225f; < d0d99f60538ddb4a62ccaac2168d8f448965f083 (affected, git); ≥ 741a295130606143edbf9fc740f633dbc1e6225f; < d7963d6997fea86a6def242ac36198b86655f912 (affected, git); ≥ 741a295130606143edbf9fc740f633dbc1e6225f; < aa9ebc084505fb26dd90f4d7a249045aad152043 (affected, git); ≥ 741a295130606143edbf9fc740f633dbc1e6225f; < af8f4be3b68ac8caa41c8e5ead0eeaf5e85e42d0 (affected, git); ≥ 741a295130606143edbf9fc740f633dbc1e6225f; < 42e21e74061b0ebbd859839f81acf10efad02a27 (affected, git); ≥ 741a295130606143edbf9fc740f633dbc1e6225f; < 6c4b2243cb6c0755159bd567130d5e12e7b10d9f (affected, git). Source: CNA: Linux
- Linux / Linux: 2.6.16 (affected); ≥ 0; < 2.6.16 (unaffected, semver); ≥ 5.10.253; ≤ 5.10.* (unaffected, semver); ≥ 5.15.203; ≤ 5.15.* (unaffected, semver); ≥ 6.1.167; ≤ 6.1.* (unaffected, semver); ≥ 6.6.130; ≤ 6.6.* (unaffected, semver); ≥ 6.12.78; ≤ 6.12.* (unaffected, semver); ≥ 6.18.19; ≤ 6.18.* (unaffected, semver); ≥ 6.19.9; ≤ 6.19.* (unaffected, semver); ≥ 7.0; ≤ * (unaffected, original_commit_for_fix). Source: CNA: Linux

## NVD configuration conditions

Affected CPE matches depend on the following AND/OR and negation conditions:

```json
[
  {
    "negate": false,
    "nodes": [
      {
        "cpeMatch": [
          {
            "criteria": "cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*",
            "versionEndExcluding": "5.10.253",
            "versionStartIncluding": "2.6.16.1",
            "vulnerable": true
          },
          {
            "criteria": "cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*",
            "versionEndExcluding": "5.15.203",
            "versionStartIncluding": "5.11",
            "vulnerable": true
          },
          {
            "criteria": "cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*",
            "versionEndExcluding": "6.1.167",
            "versionStartIncluding": "5.16",
            "vulnerable": true
          },
          {
            "criteria": "cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*",
            "versionEndExcluding": "6.6.130",
            "versionStartIncluding": "6.2",
            "vulnerable": true
          },
          {
            "criteria": "cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*",
            "versionEndExcluding": "6.12.78",
            "versionStartIncluding": "6.7",
            "vulnerable": true
          },
          {
            "criteria": "cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*",
            "versionEndExcluding": "6.18.19",
            "versionStartIncluding": "6.13",
            "vulnerable": true
          },
          {
            "criteria": "cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*",
            "versionEndExcluding": "6.19.9",
            "versionStartIncluding": "6.19",
            "vulnerable": true
          },
          {
            "criteria": "cpe:2.3:o:linux:linux_kernel:2.6.16:-:*:*:*:*:*:*",
            "vulnerable": true
          },
          {
            "criteria": "cpe:2.3:o:linux:linux_kernel:2.6.16:rc3:*:*:*:*:*:*",
            "vulnerable": true
          },
          {
            "criteria": "cpe:2.3:o:linux:linux_kernel:2.6.16:rc4:*:*:*:*:*:*",
            "vulnerable": true
          },
          {
            "criteria": "cpe:2.3:o:linux:linux_kernel:2.6.16:rc5:*:*:*:*:*:*",
            "vulnerable": true
          },
          {
            "criteria": "cpe:2.3:o:linux:linux_kernel:2.6.16:rc6:*:*:*:*:*:*",
            "vulnerable": true
          },
          {
            "criteria": "cpe:2.3:o:linux:linux_kernel:2.6.16:rc7:*:*:*:*:*:*",
            "vulnerable": true
          },
          {
            "criteria": "cpe:2.3:o:linux:linux_kernel:7.0:rc1:*:*:*:*:*:*",
            "vulnerable": true
          }
        ],
        "negate": false,
        "operator": "OR"
      }
    ],
    "operator": "OR"
  }
]
```

## Severity and prioritization

- CVSS 3.1: 5.5 (medium); CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H; source: NVD: nvd@nist.gov

- CISA KEV: No listing in collected sources

- EPSS: 0.11% as of 2026-11-06; percentile 1.50%

- SSVC: exploitation none; automatable no; technical impact partial; source ADP: CISA-ADP

- Weaknesses: CWE-908

## Exploit evidence

No exploit evidence in collected sources. This is not proof of no exploitation.

## References

- https://git.kernel.org/stable/c/845bf3c6963a52096d0d3866e4a92db77a0c03d8 (CNA: Linux)

- https://git.kernel.org/stable/c/d3ffc8f13034af895531a02c30b1fe3a34b46432 (CNA: Linux)

- https://git.kernel.org/stable/c/d0d99f60538ddb4a62ccaac2168d8f448965f083 (CNA: Linux)

- https://git.kernel.org/stable/c/d7963d6997fea86a6def242ac36198b86655f912 (CNA: Linux)

- https://git.kernel.org/stable/c/aa9ebc084505fb26dd90f4d7a249045aad152043 (CNA: Linux)

- https://git.kernel.org/stable/c/af8f4be3b68ac8caa41c8e5ead0eeaf5e85e42d0 (CNA: Linux)

- https://git.kernel.org/stable/c/42e21e74061b0ebbd859839f81acf10efad02a27 (CNA: Linux)

- https://git.kernel.org/stable/c/6c4b2243cb6c0755159bd567130d5e12e7b10d9f (CNA: Linux)

- https://git.kernel.org/stable/c/42e21e74061b0ebbd859839f81acf10efad02a27 (416baaa9-dc9f-4396-8d5f-8c081fb06d67; Patch)

- https://git.kernel.org/stable/c/6c4b2243cb6c0755159bd567130d5e12e7b10d9f (416baaa9-dc9f-4396-8d5f-8c081fb06d67; Patch)

- https://git.kernel.org/stable/c/845bf3c6963a52096d0d3866e4a92db77a0c03d8 (416baaa9-dc9f-4396-8d5f-8c081fb06d67; Patch)

- https://git.kernel.org/stable/c/aa9ebc084505fb26dd90f4d7a249045aad152043 (416baaa9-dc9f-4396-8d5f-8c081fb06d67; Patch)

- https://git.kernel.org/stable/c/af8f4be3b68ac8caa41c8e5ead0eeaf5e85e42d0 (416baaa9-dc9f-4396-8d5f-8c081fb06d67; Patch)

- https://git.kernel.org/stable/c/d0d99f60538ddb4a62ccaac2168d8f448965f083 (416baaa9-dc9f-4396-8d5f-8c081fb06d67; Patch)

- https://git.kernel.org/stable/c/d3ffc8f13034af895531a02c30b1fe3a34b46432 (416baaa9-dc9f-4396-8d5f-8c081fb06d67; Patch)

- https://git.kernel.org/stable/c/d7963d6997fea86a6def242ac36198b86655f912 (416baaa9-dc9f-4396-8d5f-8c081fb06d67; Patch)

## Provenance

- CNA: Linux: https://www.cve.org/CVERecord?id=CVE-2026-43472; retrieved 2026-11-06; source updated 2026-11-06

- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-43472; retrieved 2026-11-06; source updated 2026-11-06

- FIRST EPSS: https://api.first.org/data/v1/epss?cve=CVE-2026-43472; retrieved 2026-11-06; source updated Not reported

Revision: 9c7f198ba8aa3330558dfa28b0dcb5602c5e5a85650559af16750cf43e0b61f6

Canonical: https://y2.dev/cve/CVE-2026-43472

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