---
id: "CVE-2026-99084"
kind: "vulnerability"
aliases: ["CVE-2026-99084"]
title: "Linux kernel: readdir lock on nullfs directory lets one user stall others"
severity: "unknown"
cvss: null
epss: 0.00198
exploited: false
status: "open"
category: "runtime"
products: []
ecosystems: []
tags: ["linux-kernel", "mount-namespaces", "multi-user", "runtime"]
not_affected_if: []
published_at: "2026-10-23T13:17:32+00:00"
first_seen_at: "2026-10-23T21:57:34+00:00"
updated_at: "2026-10-25T12:38:50+00:00"
sources: [{"source": "nvd", "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-99084", "first_seen_at": "2026-10-23T21:57:34+00:00"}, {"source": "epss", "url": "https://api.first.org/data/v1/epss?cve=CVE-2026-99084", "first_seen_at": "2026-10-25T12:38:50+00:00"}]
---

# Linux kernel: readdir lock on nullfs directory lets one user stall others

The Linux kernel held the directory inode lock during readdir on an immutable nullfs directory, so a reader stuck on a slow FUSE-backed buffer could block lookups, creates and mounts for every user sharing that directory. The fix adds a FOP_IMMUTABLE flag that skips the lock for directories that never change.

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

## Timeline

- 2026-10-23T21:57:34+00:00 nvd https://nvd.nist.gov/vuln/detail/CVE-2026-99084
- 2026-10-25T12:38:50+00:00 epss https://api.first.org/data/v1/epss?cve=CVE-2026-99084

## From the sources

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

readdir: take no inode lock on an immutable directory

iterate_dir() takes the directory's i_rwsem shared and holds it across
->iterate_shared(). For a directory that never has an entry and is
never removed the lock keeps nothing still, it only orders every reader
and every writer of that inode behind each other.

For the directory of a nullfs instance that matters. The instance of
the initial mount namespace is the root of every empty mount namespace
and the private instance is the root of every kernel thread, so one
inode is shared across users who have nothing else in common. And a
reader can hold the lock for as long as it likes: back the getdents()
buffer with a mapping of a file on a FUSE mount of your own, let the
copy of "." and ".." fault and let the server wait. Queue an exclusive
taker behind it, a mkdir() in that directory goes through start_dirop()
before the read-only mount is reported, and from then on every lookup
that misses the dcache in that directory, every create and every mount
on it waits until the server answers. One user of an empty mount
namespace stalls all the others.

Add FOP_IMMUTABLE for the file operations of a directory that never
changes and is never removed and let iterate_dir() skip the lock for
it. The flag never changes for a file, ->f_pos is protected by
f_pos_lock since directories are FMODE_ATOMIC_POS, IS_DEADDIR can't be
set on such a directory and neither touch_atime() nor fsnotify take
i_rwsem. Set it on the nullfs directory. The placeholder directories of
libfs never have an entry either but their owners remove them, so they
keep the lock.

---

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