FKIE_CVE-2026-98996

Vulnerability from fkie_nvd - Published: 2026-10-23 13:17 - Updated: 2026-10-23 13:17
Severity
Summary
In the Linux kernel, the following vulnerability has been resolved: fs: don't silently unmount busy mounts The propagate_mount_busy() helper exists to decide whether a synchronous can suceed. It does that by looking at the copies of the victims in the mounts its parent propagates to. One of those functions from propagation hell. It skips a copy that has child mounts except for the case of a single child mount covering the copy's root. That algorithm used to be exact. It isn't anymore. Originally only childless copies were unmounted during propagation. And 1064f874abc0 ("mnt: Tuck mounts under others instead of creating shadow/side mounts.") ended up adding covered mounts to both sides in one go. Along came 99b19d16471e ("mnt: In propgate_umount handle visiting mounts in any order") and that started to unmount more than before: [1]: A copy of a mount is unmounted when each of its children is either its overmount or a copy of the victim itself. In the former case the overmount gets reparented. In the latter case the copy of the victim will end up being unmounted. So propagate_umount() ended up being rewritten without adjusting propagate_mount_busy(). And currently propagate_mount_busy() doesn't look at copies of the victim that have two or more children nor a single child that is not the overmount. So any such copy in [1] is not checked for references. The result is that a synchronous unmount succeeds, pulling out a mount that is still in use. A container can run into this with its own mounts. The host shares a tree with the container and the host has a mount in that tree: mount -t tmpfs none /mnt mount --make-shared /mnt mount -t tmpfs none /mnt/a The container's /mnt is a slave copy of the host mount. The container now takes a detached copy of the tree and mounts that copy beneath its propagated copy of /mnt/a and keeps the file descriptor to the tree open: unshare -m --propagation unchanged mount --make-rslave / fd = open_tree(AT_FDCWD, "/mnt", OPEN_TREE_CLONE | AT_RECURSIVE) move_mount(fd, "", AT_FDCWD, "/mnt/a", MOVE_MOUNT_BENEATH) grep /mnt /proc/self/mountinfo 775 355 ... /mnt ... master:646 777 775 ... /mnt/a ... master:646 776 777 ... /mnt/a ... master:647 778 777 ... /mnt/a/a ... master:647 The now attached tree (777) is now mounted beneath the container's copy of /mnt/a. That's also where the host mount sits (as seen from the host mount namespace). So the propagated copy (776) of the host's mount is on top of its root and the copy of that (778) is inside it. Now the host runs: umount /mnt/a and it succeeds. The moved tree tree and the copy inside of it are gone from the container. Now only the propagated copy is left and it got reparented to where it was before: grep /mnt /proc/self/mountinfo 775 355 ... /mnt ... master:646 776 775 ... /mnt/a ... But the file descriptor still refers to the moved tree. So a synchronous umount detached a mount that is still in use. Teach propagate_mount_busy() the same checks that propagate_umount() does. It now checks for a single victim mount without children for which it can assume that MNT_LOCKED is cleared on every copy like propagate_umount() does. The copies form chains. IOW, when a copy itself receives propagation from the victim's parent then the mount at the same mountpoint inside that copy is a copy as well. Whether a mount receives propagation from the victim's can simply be checked by walking its masters. Walk each chain once from its top so we are sure that nested copies stay linear. Check every reference once of every copy that gets unmounted.
References
416baaa9-dc9f-4396-8d5f-8c081fb06d67https://git.kernel.org/stable/c/6857e209015f95be7a8f19a9cd5dac345b9b9859
416baaa9-dc9f-4396-8d5f-8c081fb06d67https://git.kernel.org/stable/c/a210ac218c307513d43e98a08f23c2dfbeeebc2f
416baaa9-dc9f-4396-8d5f-8c081fb06d67https://git.kernel.org/stable/c/bf2c8dec26fb792876b5e9bde7030c45d8be3908
Impacted products
Vendor Product Version
To clipboard 

{
  "affected": [
    {
      "affectedData": [
        {
          "defaultStatus": "unaffected",
          "product": "Linux",
          "programFiles": [
            "fs/pnode.c"
          ],
          "repo": "https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git",
          "vendor": "Linux",
          "versions": [
            {
              "lessThan": "bf2c8dec26fb792876b5e9bde7030c45d8be3908",
              "status": "affected",
              "version": "99b19d16471e9c3faa85cad38abc9cbbe04c6d55",
              "versionType": "git"
            },
            {
              "lessThan": "a210ac218c307513d43e98a08f23c2dfbeeebc2f",
              "status": "affected",
              "version": "99b19d16471e9c3faa85cad38abc9cbbe04c6d55",
              "versionType": "git"
            },
            {
              "lessThan": "6857e209015f95be7a8f19a9cd5dac345b9b9859",
              "status": "affected",
              "version": "99b19d16471e9c3faa85cad38abc9cbbe04c6d55",
              "versionType": "git"
            },
            {
              "status": "affected",
              "version": "dc5ce95b0252d143497283f7f659840d4f8a79d0",
              "versionType": "git"
            },
            {
              "status": "affected",
              "version": "fdb8f10499924d96b8eb60cc1354b278f6fcf0e9",
              "versionType": "git"
            },
            {
              "status": "affected",
              "version": "bb4fbf094b440a9209ed88c8681960d4b26eec0f",
              "versionType": "git"
            },
            {
              "status": "affected",
              "version": "b43f81ef0babcc07db1eaaece37c989204e4ff00",
              "versionType": "git"
            },
            {
              "status": "affected",
              "version": "81311d1698c0c87c972ba71d3ce44a617b52ea21",
              "versionType": "git"
            },
            {
              "lessThan": "4.2",
              "status": "affected",
              "version": "4.1.43",
              "versionType": "semver"
            },
            {
              "lessThan": "4.5",
              "status": "affected",
              "version": "4.4.78",
              "versionType": "semver"
            },
            {
              "lessThan": "4.10",
              "status": "affected",
              "version": "4.9.39",
              "versionType": "semver"
            },
            {
              "lessThan": "4.12",
              "status": "affected",
              "version": "4.11.12",
              "versionType": "semver"
            },
            {
              "lessThan": "4.13",
              "status": "affected",
              "version": "4.12.3",
              "versionType": "semver"
            }
          ]
        },
        {
          "defaultStatus": "affected",
          "product": "Linux",
          "programFiles": [
            "fs/pnode.c"
          ],
          "repo": "https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git",
          "vendor": "Linux",
          "versions": [
            {
              "status": "affected",
              "version": "4.13"
            },
            {
              "lessThan": "4.13",
              "status": "unaffected",
              "version": "0",
              "versionType": "semver"
            },
            {
              "lessThanOrEqual": "6.18.*",
              "status": "unaffected",
              "version": "6.18.57",
              "versionType": "semver"
            },
            {
              "lessThanOrEqual": "7.2.*",
              "status": "unaffected",
              "version": "7.2.11",
              "versionType": "semver"
            },
            {
              "lessThanOrEqual": "*",
              "status": "unaffected",
              "version": "7.3",
              "versionType": "original_commit_for_fix"
            }
          ]
        }
      ],
      "source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67"
    }
  ],
  "cveTags": [],
  "descriptions": [
    {
      "lang": "en",
      "value": "In the Linux kernel, the following vulnerability has been resolved:\n\nfs: don\u0027t silently unmount busy mounts\n\nThe propagate_mount_busy() helper exists to decide whether a synchronous\ncan suceed. It does that by looking at the copies of the victims in the\nmounts its parent propagates to. One of those functions from propagation\nhell.\n\nIt skips a copy that has child mounts except for the case of a single\nchild mount covering the copy\u0027s root. That algorithm used to be exact.\nIt isn\u0027t anymore.\n\nOriginally only childless copies were unmounted during propagation.\nAnd 1064f874abc0 (\"mnt: Tuck mounts under others instead of creating\nshadow/side mounts.\") ended up adding covered mounts to both sides in\none go.\n\nAlong came 99b19d16471e (\"mnt: In propgate_umount handle visiting mounts\nin any order\") and that started to unmount more than before:\n\n[1]: A copy of a mount is unmounted when each of its children is either\n     its overmount or a copy of the victim itself. In the former case\n     the overmount gets reparented. In the latter case the copy of the\n     victim will end up being unmounted.\n\nSo propagate_umount() ended up being rewritten without adjusting\npropagate_mount_busy(). And currently propagate_mount_busy() doesn\u0027t\nlook at copies of the victim that have two or more children nor a single\nchild that is not the overmount.\n\nSo any such copy in [1] is not checked for\nreferences. The result is that a synchronous unmount succeeds, pulling\nout a mount that is still in use.\n\nA container can run into this with its own mounts. The host shares a\ntree with the container and the host has a mount in that tree:\n\n  mount -t tmpfs none /mnt\n  mount --make-shared /mnt\n  mount -t tmpfs none /mnt/a\n\nThe container\u0027s /mnt is a slave copy of the host mount. The container\nnow takes a detached copy of the tree and mounts that copy beneath its\npropagated copy of /mnt/a and keeps the file descriptor to the tree\nopen:\n\n  unshare -m --propagation unchanged\n  mount --make-rslave /\n  fd = open_tree(AT_FDCWD, \"/mnt\", OPEN_TREE_CLONE | AT_RECURSIVE)\n  move_mount(fd, \"\", AT_FDCWD, \"/mnt/a\", MOVE_MOUNT_BENEATH)\n\n  grep /mnt /proc/self/mountinfo\n  775 355 ... /mnt ...     master:646\n  777 775 ... /mnt/a ...   master:646\n  776 777 ... /mnt/a ...   master:647\n  778 777 ... /mnt/a/a ... master:647\n\nThe now attached tree (777) is now mounted beneath the container\u0027s copy\nof /mnt/a. That\u0027s also where the host mount sits (as seen from the host\nmount namespace). So the propagated copy (776) of the host\u0027s mount is on\ntop of its root and the copy of that (778) is inside it. Now the host\nruns:\n\n  umount /mnt/a\n\nand it succeeds. The moved tree tree and the copy inside of it are gone\nfrom the container. Now only the propagated copy is left and it got\nreparented to where it was before:\n\n  grep /mnt /proc/self/mountinfo\n  775 355 ... /mnt ... master:646\n  776 775 ... /mnt/a ...\n\nBut the file descriptor still refers to the moved tree. So a synchronous\numount detached a mount that is still in use.\n\nTeach propagate_mount_busy() the same checks that propagate_umount()\ndoes. It now checks for a single victim mount without children\nfor which it can assume that MNT_LOCKED is cleared on every copy like\npropagate_umount() does. The copies form chains. IOW, when a copy itself\nreceives propagation from the victim\u0027s parent then the mount at the same\nmountpoint inside that copy is a copy as well. Whether a mount receives\npropagation from the victim\u0027s can simply be checked by walking its\nmasters. Walk each chain once from its top so we are sure that nested\ncopies stay linear. Check every reference once of every copy that gets\nunmounted."
    }
  ],
  "id": "CVE-2026-98996",
  "lastModified": "2026-10-23T13:17:21.770",
  "metrics": {},
  "published": "2026-10-23T13:17:21.770",
  "references": [
    {
      "source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
      "url": "https://git.kernel.org/stable/c/6857e209015f95be7a8f19a9cd5dac345b9b9859"
    },
    {
      "source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
      "url": "https://git.kernel.org/stable/c/a210ac218c307513d43e98a08f23c2dfbeeebc2f"
    },
    {
      "source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
      "url": "https://git.kernel.org/stable/c/bf2c8dec26fb792876b5e9bde7030c45d8be3908"
    }
  ],
  "sourceIdentifier": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
  "vulnStatus": "Received"
}



Log in or create an account to share your comment.




Tags
Taxonomy of the tags.




Drag nodes to arrange them. Double-click a related vulnerability to open it, or a source to visit its link. The Physics button on the left tunes the layout and the simulation; the legend at the bottom left hides or shows nodes by kind and by sighting type.

Forecast uses a logistic model when the trend is rising, or an exponential decay model when the trend is falling. Fitted via linearized least squares.

Sightings

Author Source Type Date Other

Nomenclature

  • Seen: The vulnerability was mentioned, discussed, or observed by the user.
  • Confirmed: The vulnerability has been validated from an analyst's perspective.
  • Published Proof of Concept: A public proof of concept is available for this vulnerability.
  • Exploited: The vulnerability was observed as exploited by the user who reported the sighting.
  • Patched: The vulnerability was observed as successfully patched by the user who reported the sighting.
  • Not exploited: The vulnerability was not observed as exploited by the user who reported the sighting.
  • Not confirmed: The user expressed doubt about the validity of the vulnerability.
  • Not patched: The vulnerability was not observed as successfully patched by the user who reported the sighting.
Vulnerability timeline

Sightings and recorded dates, grouped by day · UTC

JSON

Loading timeline…




    Related by attack behaviour

    Vulnerabilities whose description is nearest to this one in the vector space of the CIRCL/vulnerability-attack-technique-biencoder model. This is a similarity search over the bi-encoder space (plain cosine), not a classification, and it has no measured accuracy.