GHSA-VGV8-2MH4-QH7R

Vulnerability from github – Published: 2026-05-08 15:31 – Updated: 2026-05-08 15:31
VLAI?
Details

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) ;-/

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{
  "affected": [],
  "aliases": [
    "CVE-2026-43472"
  ],
  "database_specific": {
    "cwe_ids": [],
    "github_reviewed": false,
    "github_reviewed_at": null,
    "nvd_published_at": "2026-05-08T15:17:00Z",
    "severity": null
  },
  "details": "In the Linux kernel, the following vulnerability has been resolved:\n\nunshare: fix unshare_fs() handling\n\nThere\u0027s an unpleasant corner case in unshare(2), when we have a\nCLONE_NEWNS in flags and current-\u003efs hadn\u0027t been shared at all; in that\ncase copy_mnt_ns() gets passed current-\u003efs instead of a private copy,\nwhich causes interesting warts in proof of correctness]\n\n\u003e I guess if private means fs-\u003eusers == 1, the condition could still be true.\n\nUnfortunately, it\u0027s worse than just a convoluted proof of correctness.\nConsider the case when we have CLONE_NEWCGROUP in addition to CLONE_NEWNS\n(and current-\u003efs-\u003eusers == 1).\n\nWe pass current-\u003efs to copy_mnt_ns(), all right.  Suppose it succeeds and\nflips current-\u003efs-\u003e{pwd,root} to corresponding locations in the new namespace.\nNow we proceed to copy_cgroup_ns(), which fails (e.g. with -ENOMEM).\nWe call put_mnt_ns() on the namespace created by copy_mnt_ns(), it\u0027s\ndestroyed and its mount tree is dissolved, but...  current-\u003efs-\u003eroot and\ncurrent-\u003efs-\u003epwd are both left pointing to now detached mounts.\n\nThey are pinning those, so it\u0027s not a UAF, but it leaves the calling\nprocess with unshare(2) failing with -ENOMEM _and_ leaving it with\npwd and root on detached isolated mounts.  The last part is clearly a bug.\n\nThere is other fun related to that mess (races with pivot_root(), including\nthe one between pivot_root() and fork(), of all things), but this one\nis easy to isolate and fix - treat CLONE_NEWNS as \"allocate a new\nfs_struct even if it hadn\u0027t been shared in the first place\".  Sure, we could\ngo for something like \"if both CLONE_NEWNS *and* one of the things that might\nend up failing after copy_mnt_ns() call in create_new_namespaces() are set,\nforce allocation of new fs_struct\", but let\u0027s keep it simple - the cost\nof copy_fs_struct() is trivial.\n\nAnother benefit is that copy_mnt_ns() with CLONE_NEWNS *always* gets\na freshly allocated fs_struct, yet to be attached to anything.  That\nseriously simplifies the analysis...\n\nFWIW, that bug had been there since the introduction of unshare(2) ;-/",
  "id": "GHSA-vgv8-2mh4-qh7r",
  "modified": "2026-05-08T15:31:30Z",
  "published": "2026-05-08T15:31:30Z",
  "references": [
    {
      "type": "ADVISORY",
      "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-43472"
    },
    {
      "type": "WEB",
      "url": "https://git.kernel.org/stable/c/42e21e74061b0ebbd859839f81acf10efad02a27"
    },
    {
      "type": "WEB",
      "url": "https://git.kernel.org/stable/c/6c4b2243cb6c0755159bd567130d5e12e7b10d9f"
    },
    {
      "type": "WEB",
      "url": "https://git.kernel.org/stable/c/845bf3c6963a52096d0d3866e4a92db77a0c03d8"
    },
    {
      "type": "WEB",
      "url": "https://git.kernel.org/stable/c/aa9ebc084505fb26dd90f4d7a249045aad152043"
    },
    {
      "type": "WEB",
      "url": "https://git.kernel.org/stable/c/af8f4be3b68ac8caa41c8e5ead0eeaf5e85e42d0"
    },
    {
      "type": "WEB",
      "url": "https://git.kernel.org/stable/c/d0d99f60538ddb4a62ccaac2168d8f448965f083"
    },
    {
      "type": "WEB",
      "url": "https://git.kernel.org/stable/c/d3ffc8f13034af895531a02c30b1fe3a34b46432"
    },
    {
      "type": "WEB",
      "url": "https://git.kernel.org/stable/c/d7963d6997fea86a6def242ac36198b86655f912"
    }
  ],
  "schema_version": "1.4.0",
  "severity": []
}


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