---
id: "CVE-2026-98990"
kind: "vulnerability"
aliases: ["CVE-2026-98990"]
title: "Linux kernel: busy submounts can be unmounted, breaking in-use mounts"
severity: "unknown"
cvss: null
epss: 0.00209
exploited: false
status: "open"
category: "infrastructure"
products: []
ecosystems: []
tags: ["afs", "cifs", "infrastructure", "linux-kernel", "nfs", "tracefs"]
not_affected_if: ["no-automounted-submounts", "no-tracefs"]
published_at: "2026-10-23T13:17:20+00:00"
first_seen_at: "2026-10-23T21:57:33+00:00"
updated_at: "2026-10-25T12:38:50+00:00"
sources: [{"source": "nvd", "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-98990", "first_seen_at": "2026-10-23T21:57:33+00:00"}, {"source": "epss", "url": "https://api.first.org/data/v1/epss?cve=CVE-2026-98990", "first_seen_at": "2026-10-25T12:38:50+00:00"}]
---

# Linux kernel: busy submounts can be unmounted, breaking in-use mounts

A race in the Linux kernel's mount handling lets a synchronous umount remove a mount that is still in use elsewhere, because the busy check runs before an earlier unmount changes the mount tree. The sources do not say what an attacker gains beyond disrupting mounts such as NFS, AFS, CIFS or tracefs automounts.

_Summary, check and fix written by a model from the sources; labelled as such._

## How to check

- Run `uname -r` and compare against the fixed kernel version in the vendor advisory (unknown from these sources).
- Check whether automounted NFS, AFS, CIFS or tracefs submounts are in use with `findmnt -o TARGET,FSTYPE,PROPAGATION`.

## Fix

- Update the Linux kernel to a release that includes the upstream commit titled 'namespace: check a submount for references right before unmounting it' (version unknown from these sources).

## Timeline

- 2026-10-23T21:57:33+00:00 nvd https://nvd.nist.gov/vuln/detail/CVE-2026-98990
- 2026-10-25T12:38:50+00:00 epss https://api.first.org/data/v1/epss?cve=CVE-2026-98990

## From the sources

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

namespace: check a submount for references right before unmounting it

shrink_submounts() and mark_mounts_for_expiry() first collect all the
mounts they are allowed to unmount and then unmount them.

Whether a mount is busy is decided by propagate_mount_busy() on the
tree. But unmounting the first mount changes the tree that the second
one propagates into. propagate_umount()
moves a surviving overmount off a copy it unmounts and mounts it where
the copy was mounted. If that's where the propagated copy of the second
victim is looked up the overmount becomes a candidate of the second
umount_tree(). It's childless and so trim_one() commits it without
looking at its reference count.

So a synchronous umount pulls out a busy mount that is in use somewhere
else and marks it MNT_SYNC_UMOUNT while it has users:

  umount2(/tmp/plshrink/p1, 0) = 0 errno 0
  cwd is (unreachable)

The same two umounts requested one after the other fail with EBUSY.
This needs a mount with MNT_SHRINKABLE, i.e., automounted submounts of
NFS, AFS and CIFS or the tracefs mount below debugfs. Bind mounts
inherit the flag.

Check each mount right before it is unmounted under the same hold of
namespace_sem and mount_lock as the umount itself. Make sure that the
algorithm stays linear.

---

_Free to read at a human pace. An API with keys and an MCP server are in preparation; early access: hello@infrastack.ai_